An Introduction to Physics Quiz Answer

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How Things Work: An Introduction to Physics

Offered By ”University of Virginia”

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Instructor: Thomas Bjarnsholt

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Week- 1

Preliminary Assessment

1. You make a sharp left turn in your car and your cellphone slips off the dashboard and out the right passenger window. The cellphone leaves the car because

the car did not exert enough leftward force on the cellphone to make the cellphone accelerate with the car.
the dashboard of the turning car tilted sharply to become a ramp and the downhill ramp force pushed the cellphone out the window.

the car exerted a rightward centrifugal force on the cellphone that pushed the cellphone out the window.
the cellphone’s weight pulled the cellphone out the window.

2. You are pulling your niece uphill on a sled and the sled experiences a small downhill frictional force as it slides uphill on the snow. Your niece is traveling in a straight line at a constant speed. The net force your niece is experiencing is

in the uphill direction.
in the downhill direction.

zero.
in the upward direction.

3. You are rearranging your room and are now sliding your desk across the floor at constant velocity. Which of the following statements about the forces acting on the desk is correct? [Consider only horizontal forces.]

The amount of force that you’re exerting on the desk must be more than the amount of its weight.
The force that you’re exerting on the desk must be equal in amount but opposite in direction to the force that the floor is exerting on it.

The amount of force that you’re exerting on the desk must be more than the amount of force that friction is exerting on it.
The amount of force that you’re exerting on the desk must be equal to the amount of its weight.

4. In the game of shuffleboard, you push plastic disks forward and then release them so that they slide across a level playing surface. Once you release a disk, you are no longer pushing and it gradually skids to a stop. Its final position determines your score. As each disk skids to a stop, what becomes of the kinetic energy it had when you released it? That energy is

still present in the disk, as it must be because kinetic energy is conserved and can’t be created or destroyed.
now gravitational potential energy in the disk.

now elastic potential energy in the disk.
now thermal energy in the disk and playing surface.

5. You are carrying a child on your back as you walk down a hill. The child is traveling straight at a steady speed. In which direction is the force you are exerting on the child?

Downhill (in the direction of your velocity).
Upward (vertical).

Upward and forward (between vertical and horizontal).
Forward (horizontal)

6. You take off your shoes to sneak quietly into your home late at night. Unfortunately, it’s too dark to see the concrete block your friend left on the floor and your big toe collides with it. The block doesn’t move and your foot comes to a complete stop due to the impact. Luckily, you are wearing soft woolen socks because when your foot stops during the impact, your toe transfers

the same momentum, whether or not you are wearing socks, but that transfer takes more time when you are wearing socks.
less momentum to the block than it would have if you had not been wearing socks.

less energy to the block than it would have if you had not been wearing socks.
less velocity to the block than it would have if you had not been wearing socks.

7. A rigid two-blade wind turbine that is experiencing zero net torque

has a constant angular velocity, which may be zero.
is motionless and may be horizontal or tilted.

is motionless and horizontal.
has an angular velocity that is gradually decreasing toward zero.

8. A dog and a cat jump horizontally off a wall at the same moment and soon land on the level horizontal field that extends outward from the base of the wall. The dog weighs twice as much as the cat, but the cat was moving forward horizontally twice as fast as the dog when the two animals left the wall. In this situation,

both animals land at approximately the same time and at approximately the same distance from the wall.
both animals land at approximately the same time, but the cat lands considerably farther from the wall than the dog does.

the dog lands first, but the cat lands considerably farther from the wall than the dog does.
the dog lands first, but both animals land at approximately the same distance from the wall.

9. A skateboarder rides swiftly up the edge of a bowl-shaped surface and leaps into the air. While in the air, the skateboarder flips upside and tosses the skateboard from hand to hand. The skateboarder then rides safely back down the bowl. During the time that the skateboarder and skateboard are not touching anything, one aspect of their motion that is constant is their total (or combined) [note: neglect any effects due to the air]

momentum.
angular velocity.

velocity.
angular momentum.

10. You are tossing popcorn straight up and catching it in your mouth. At the moment each piece of popcorn reaches its peak height, its velocity is

downward and its acceleration is downward.
zero and its acceleration is downward.

zero and its acceleration is zero.
downward and its acceleration is zero.

11. Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because

it barely moves downward as you push downward on it, so you do almost zero work on it.
it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand.

it stops the downward motion of your foot faster and thus absorbs less of your momentum.
it stops the downward motion of your foot faster and thus absorbs more of your momentum.

12. You’re at the lake and watch two children jump off a dock. They jump at the same time and at the same speed, but the boy jumps mostly upward while the girl jumps mostly forward. After they leave the dock,

the boy reaches the water before the girl.
the two children reach the water at the same moment but the girl travels farther from the dock than does the boy.

the girl reaches the water before the boy.
the two children reach the water at the same moment and at the same distance from the dock.

13. It is a beautiful summer day and the residents of a high-rise apartment building are eating dinner on their balconies. A resident accidently knocks an empty glass off a balcony that is about 80 meters (260 feet) above the cement patio. The glass would have smashed on that patio after falling for 4 seconds, however, a quick-witted resident catches the glass after it has fallen for only 2 seconds. How far above the patio was the glass when it was caught?

Approximately 50 meters above the patio.
Approximately 60 meters above the patio.

Approximately 40 meters above the patio.
Approximately 30 meters above the patio.

14. You are at the gym, exercising on a step machine. You have one foot on each of the machine’s pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push upward harder on your feet while moving downward than they do while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?

As that pedal moves downward.
When that pedal is accelerating.

As that pedal moves either upward or down.
As that pedal moves upward.

15. A juggler tosses a club straight up. Disregarding any effects due to the air, what force or forces are acting on the club while it is above the juggler’s hands?

Its weight.
Its weight along with a steadily decreasing upward force.

Its weight along with an upward force that steadily decreases until the club reaches its highest point. After that point, there is only the constant downward force of gravity.
A steadily decreasing upward force from the moment it leaves the juggler’s hands until it reaches its highest point and then a steadily increasing downward force as the club returns toward the jugglers hands.

16. Two children are balanced on a seesaw, but one child weighs twice as much as the other child. The heavier child is sitting half as far from the pivot as is the lighter child. Since the seesaw is balanced, the heavier child is exerting on the seesaw

a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting.
a force that is equal in amount but oppositely directed to the force the lighter child is exerting.

a torque that is less than the torque the lighter child is exerting.
a force that is less than the force the lighter child is exerting.

17. You are standing in the middle of a subway car that is moving forward at constant horizontal velocity when another passenger accidentally spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Because nothing else is touching you, you

remain in the middle of the subway car.
shift toward the front of the subway car (in the direction of its velocity).

shift toward the side of the subway car (perpendicular to the direction of its velocity).
shift toward the back of the subway car (opposite its velocity).

18. You are practicing tennis alone by hitting a tennis ball forward toward a cement wall. Each time the ball hits the wall, it bounces backward at high speed so that you can hit it again. During its bounce, the ball

retains approximately all of its momentum but transfers most of its energy to the wall.
retains approximately all of its energy but transfers more forward momentum than it had to the wall.

retains approximately all of its energy and momentum.
retains approximately all of its energy but transfers most of its momentum to the wall.

19. Your table at a family-style restaurant has a “lazy Susan” in the middle. This large circular platform rotates frictionlessly so that you can “pass” an entree to your friends by placing it on the platform and then rotating the platform. When the server places a new entrée on the platform, it

decreases the platform’s angular velocity.
makes it harder to change the platform’s angular velocity.

makes it easier to change the platform’s angular velocity.
increases the platform’s angular velocity.

20. A cross-country skier is struggling to get up a hill, so you offer to help. As the skier passes you, you reach out with your hand and exert an uphill force of 80 N (18 pounds) on the skier. When you do this, the skier exerts

a downhill force of 80 N on you.
a downhill force of somewhat more than 80 N on you.

no downhill force on you at all.
a downhill force of somewhat less than 80 N on you.

Week- 2

Skating

1. While searching for your keys, you place your cup of coffee on the roof of your parked car. Unfortunately, you forget about the coffee and climb into the car without it. As you start driving the car forward, you hear the coffee hit the ground behind the car. Why didn’t the coffee stay on the roof of the car?

The coffee’s inertia kept it essentially motionless as the car accelerated forward and left the coffee behind.
The car’s roof pushed the coffee backward, in the direction opposite the car’s velocity.

The car’s roof pushed the coffee backward, in the direction opposite the car’s acceleration.
The coffee pushed itself backward as the car pushed itself forward.

2. An African swallow is flying east at 60 mph (about 100 km/h) and a European swallow is flying west at 60 mph (about 100 km/h). These two swallows have _______________.

the same speed and the same velocity.
different speeds and different velocities.

the same speed, but different velocities.
different speeds, but the same velocity.

3. You kick a soccer ball (a football) toward the goal. When the fast-moving ball is midway to the goal and nothing is touching it, why is the ball moving toward the goal? [Ignore any effects due to the air or gravity.]

The force of your kick continues to push the ball forward steadily as the ball moves from you to the goal.
Inertia keeps the ball in motion.

The force of your kick continues to push the ball forward, although that force decreases gradually as the ball moves from you to the goal.
The ball exerts a forward force on itself even after the ball leaves your foot.

4. Which of the following is experiencing zero net force?

The driver of a car that is increasing its forward speed at the start of a race.
A passenger in an elevator that is slowing down after its trip from the ground floor to the 10th floor.

A child who is riding a carousel (a merry-go-round) and is traveling around in a circle at a steady pace.
A water-skier who is being pulled forward by a speedboat and is moving in a straight-line path at a steady speed.

5. You are riding your bicycle forward on a level road, traveling in a straight line at a steady pace. An animal suddenly runs in front of you, so you apply the brakes quickly and stop just in time to avoid hitting the animal. While the brakes are on and you are experiencing a large net force, what is the direction of velocity and acceleration, respectively?

Your velocity is backward and your acceleration is backward.
Your velocity is backward, but your acceleration is forward.
Your velocity is forward, but your acceleration is backward.
Your velocity is forward, but your acceleration is zero.

6. You are coasting forward at constant velocity on your inline skates. Suddenly, another skater pushes you so that the net force you are experiencing points toward your left. While your net force points toward the left, your acceleration _______________.

is zero.
points at an angle between forward and toward your left.
points forward (in the direction of your velocity).
points toward the left (in the direction of your net force).

7. Two skaters are coasting forward across the ice. The skater in red has a greater mass than the skater in blue. You begin pushing the two skaters forward with equal forces. How do they move while you are pushing them? [The correct answer must always be true, no matter how fast the skaters were moving before you began pushing them.]

The skater in red experiences less acceleration than the skater in blue.
The skater in red experiences more acceleration than the skater in blue.
The skater in red moves faster than the skater in blue.
The skater in red moves slower than the skater in blue.

8. In what circumstance can you be accelerating forward and still be moving at a constant velocity?

When you are moving backward and therefore slowing to a stop.
When you are moving toward the side and your path is therefore curving.
When you are experiencing zero net force.
In no circumstance. If you are accelerating, you velocity is changing with time.

9. You are dragging a heavy chair across the floor and that chair is moving toward the east at constant velocity. The net force on the chair __________________.

is zero.
points toward the east.
points downward and eastward (at an angle between the two individual directions).
points upward and eastward (at an angle between the two individual directions).

10. A set of dishes sits motionless on a slippery silk tablecloth. If you pull the tablecloth sideways quickly, it will slide out from under the dishes and leave the dishes almost unaffected. Why won’t that same result occur if you pull the tablecloth sideways slowly?

Inertia lasts only for a short time, so objects at rest stay at rest only for a short time.
The moving tablecloth exerts small forces on the dishes and, given enough time, those forces will overwhelm inertia and cause the dishes to move with the tablecloth.
The tablecloth is slippery only at high speeds. At low speeds, it clings to the dishes like glue.
The tablecloth can hold the dishes in place only when the tablecloth is moving at high speeds.

Week- 3

Falling Balls

1. Which force is your weight? [The force that is your weight and not a force that is equal to your weight.]

The force you exert on a bathroom scale as you stand on that scale.
The force that a bathroom scale exerts on you as you stand on that scale.
The force you exert on a trampoline as you land on that trampoline after jumping high above the trampoline’s surface.
The force that causes you to accelerate downward when you are high above the surface of a trampoline.

2. You visit a bowling alley and examine the bowling balls that are available for use. They all look identical, but some are heavier (have greater weights) than others. How can you identify the heaviest ball? [Neglect any effects due to air]

Hold each ball motionless in your hand and choose the one that requires the largest upward force to keep it from falling.
Drop each ball from rest at the same height and choose the one that reaches the ground in the shortest time.
Drop each ball from rest at the same height and choose the one that reaches the ground in the longest time.
Roll each ball down the alley toward the bowling pins and choose the ball that moves fastest.

3. You are traveling on an intergalactic cruise spaceship in deep, gravity-free space. You find that the ship has a bowling alley! Once again, there are many identical-looking balls available for use. Without gravity, however, they all have the same weight: zero.

While still on the spaceship in deep, gravity-free space, how can you identify the ball that will be heaviest when your cruise ship lands on a planet and gravity is present? [Neglect any effects due to air]

Hold each ball motionless in your hand and choose the one that requires the largest upward force to keep it from falling.
Throw each ball down the alley toward the bowling pins and choose the ball that moves fastest.
Throw each ball down the alley toward the bowling pins and choose the ball that slows down the most (that undergoes the greatest decrease in forward velocity).
Shake each ball rapidly back and forth, and choose the ball that accelerates the least in response to the same force as on Earth.

4. You’ve learned to juggle 4 balls at once here on Earth. During a visit to the moon, where the acceleration due to gravity is about 1/6th its Earth value, you decide to try juggling those same 4 balls. You find that, on the moon, each ball has ________________.

less weight and falls more slowly than on Earth. It undergoes the same acceleration as on Earth when exposed to the same force as on Earth.
less weight and falls more slowly than on Earth. It accelerates more rapidly than on Earth when exposed to the same force as on Earth.
the same weight, but falls more slowly than on Earth. It accelerates more slowly than on Earth when exposed to the same force as on Earth.
the same weight and falls at the same rate as on Earth. It accelerates more slowly than on Earth when exposed to the same force as on Earth.

5. While vacationing on a tropical island, you find the courage to step off a high cliff and fall for 4 seconds before entering the water below. Exactly 2 seconds into your fall, you glance at the cliff face and see a secret treasure embedded in the rock. When you recover from your plunge, you return to the cliff top and find that treasure ______________________.

1/2 the distance down from the cliff top to the water.
1/4 the distance down from the cliff top to the water.
3/4 the distance down from the cliff top to the water.
Slightly above 1/2 the distance down from the cliff top to the water. In other words, about a meter (3 feet) closer to the cliff top than to the water.

6. You are playing basketball and take a shot toward the basket. When the ball is midway to the basket and nothing is touching the ball, what is the direction of the net force on the basketball? [Neglect any effects due to air]

Downward.
Toward the basket.
Toward a point slightly above the basket.
Toward a point slightly below the basket.

7. You throw a handful of different coins up and forward and watch them arc through space. They leave your hand at the same moment and with the same starting velocity. Neglecting any effects due to air, where and when do those coins hit the level ground in front of your feet? [Note: air can significantly affect fast-moving coins. If you want to test your answer experimentally, be careful to minimize those air effects. Also, be safe!]

All the coins hit the ground at the same time and at the same distance from your feet.
The heavier coins hit the ground before the lighter coins, but they all hit the ground at the same distance from your feet.
The heavier coins hit the ground before the lighter coins and they hit the ground farther from your feet than do the lighter coins.
All the coins hit the ground at the same time, but the heavier coins hit the ground farther from your feet than do the lighter coins.

8. You throw a priceless porcelain vase straight up and watch as it rises to peak height and then drops safely back into your hands. Fortunately, the vase’s owner wasn’t watching. What were the vase’s velocity and acceleration at the moment it reached peak height? [Do not test your answer experimentally, unless you take full responsibility for any consequences!]

The vase’s velocity was zero. The vase’s acceleration was the acceleration due to gravity, which is not zero.
The vase’s velocity was the velocity due to gravity, which is not zero. The vase’s acceleration was zero.
The vase’s velocity was zero. The vase’s acceleration was zero.
The vase’s velocity was the velocity due to gravity, which is not zero. The vase’s acceleration was the acceleration due to gravity, which is not zero.

9. When an archer sends an arrow toward a target, the archer must aim the arrow above the target’s bullseye (its center) in order for the arrow to hit that bullseye. If the archer uses a stronger bow and therefore a faster-moving arrow, how will that change how the archer aims the arrow in order to hit the same target’s bullseye? [Neglect any effects due to air]

The archer must still aim above the target’s bullseye, but less far above the bullseye than with the slower-moving arrow.
The archer must aim exactly as before.
The archer must now aim exactly at the target’s bullseye.
The archer must still aim above the target’s bullseye, but farther above the bullseye than with the slower-moving arrow.

10. As you collect plastic bottles for recycling, one of the bottles rolls horizontally off the kitchen counter and bounces on the floor about 1 foot (0.3 meters) outward from the base of the counter. Why didn’t the bottle drop straight down and hit the floor exactly at the base of the counter?

The bottle coasted horizontally outward as it fell vertically.
The counter pushed the bottle horizontally outward as the bottle fell vertically.
The bottle pushed itself horizontally outward as it fell vertically.
The direction of the net force on the falling bottle was at an angle between outward and downward, so the bottle moved outward and downward.

Week- 4

Ramps

1. You are in an ordinary room (both its floor and ceiling are horizontal). You throw a ball directly upward and it bounces off the ceiling. While the ball is touching the ceiling, in which direction is the ceiling’s support force on the ball?

The ceiling’s support force on the ball is directed downward.
The ceiling’s support force on the ball is directed upward.
The ceiling’s support force on the ball is directed downward while the ball is moving upward and is directed upward while the ball is moving downward.
The ceiling’s support force on the ball is directed upward while the ball is moving upward and is directed downward while the ball is moving downward.

2. You are playing volleyball and your teammate has just hit the ball forward — toward your opponents. To increase the ball’s forward speed, you push it with a forward force of 200 newton (45 pounds-force). What force, if any, does the ball exert on you?

Zero force.
A backward force less than 200 newtons, but not zero.
A backward force greater than 200 newtons.
A backward force of 200 newtons.

3. When you stand and remain motionless on a bathroom scale, what force is the scale exerting on your feet?

An upward support force greater than your weight, to keep you from falling.
An upward support force equal in amount to your weight.
An upward support force less than your weight, to allow the scale to read your weight.
Zero force, so that the scale can read your weight.

4. As a ball bounces on the floor, the floor exerts an upward support force on the ball. Can the amount of that upward support force on the ball be different from the ball’s weight?

Yes. It can be greater than the ball’s weight and it can be less than the ball’s weight.
Yes. It can be greater than the ball’s weight. It cannot be less than the ball’s weight.
Yes. It can be less than the ball’s weight. It cannot be greater than the ball’s weight.
No. It can only be equal in amount to the ball’s weight.

5. You are using a string to lift a heavy picnic basket up to your treehouse. Alas, the string isn’t strong enough for the job. The picnic basket becomes motionless, even though you are moving the portion of string you are holding upward, and the string breaks. Breaking the string required energy and that energy was provided by

the picnic basket.
both you and the picnic basket.
you.
the string’s elastic potential energy.

6. You are shopping in a store and want to go upward from the second floor to the third floor. You can make that trip using an escalator, an elevator, or a staircase. Which method of going from the second floor to the third floor will increase your gravitational potential energy the most?

Using the escalator will increase your gravitational potential energy more than the other methods.
Uing the staircase will increase your gravitational potential energy more than the other methods.
Using the elevator will increase your gravitational potential energy more than the other methods.
All three methods will increase your gravitational potential energy by the same amount.

7. A downhill skier is descending a snow-covered mountain. The skier steps off of a level region of the mountain and onto a steep slope. The skier begins to accelerate rapidly downhill on the slope. What force is causing the skier to accelerate downhill?

The downhill ramp force that is the sum of the skier’s weight and the support force exerted on the skier by the snow-covered slope.
The skier’s weight.
The support force exerted on the skier by the snow-covered slope.
The support force exerted on the snow-covered slope by the skier.

8. You have shopping cart full of groceries and that cart is on a ramp. You are exerting an uphill force on the cart, so that the net force on the cart is zero. What energy transfer is occurring?

If the cart is moving, you are transferring energy to the cart.
If the cart is moving uphill, you are transferring energy to the cart. If the cart is moving downhill, the cart is transferring energy to you.
You are transferring energy to the cart, whether the cart is moving or motionless.
No energy is being transferred between you and the cart.

9. You are trying to lift a heavy file cabinet into the back of a truck. The file cabinet weighs 200 pounds (about 900 newtons) and you must raise it 2 feet (about 0.6 meters) upward. The file cabinet has wheels, so it rolls freely. You create a ramp using rigid boards that are 8 feet long and successfully push the wheeled file cabinet up the ramp and into the truck. What force did you exert on the file cabinet to keep it moving up the ramp at constant velocity? [Assume the ramp was smooth, straight, and exactly 8 feet long, and neglect any imperfections, such as friction, air resistance, or rotation of the cabinet’s wheels.]

You exerted a force of 25 pounds directed uphill along the ramp.
You exerted a force of 50 pounds directed uphill along the ramp.
You exerted a force of 100 pounds directed uphill along the ramp.
You exerted a force of 200 pounds directed uphill along the ramp.

10. You talking on your cellphone and you accidentally ride your bicycle off the road. You realize that you are going to collide with either a tree or a garbage can, so you must choose which object to hit. The tree will not move at all (exactly zero distance, which is an idealization) if you hit the tree, but the garbage can will move if you hit the garbage can. How will your choice of object affect the energy you transfer to that object when you hit it? [Fortunately, you are going slowly, so you won’t be injured regardless of your choice.]

You will transfer energy regardless of your choice, but you will transfer more energy if you hit the garbage can than if you hit the tree.
You will transfer energy regardless of your choice, but you will transfer more energy if you hit the tree than if you hit the garbage can.
You will transfer energy if you hit the garbage can, but you will not transfer energy if you hit the tree.
Regardless of which object you hit, the amount of energy you transfer to the object will be the same.

Week- 5

Seesaws

1. A toy top is a disk-shaped object with a sharp point and a thin stem projecting from its bottom and top, respectively. When you twist the stem hard, the top begins to spin rapidly. When you then set the top’s point on the ground and let go of it, it continues to spin about a vertical axis for a very long time. What keeps the top spinning?

The top has rotational inertia.
You continue to twist the top, even though you are no longer touching it.
Gravity twists the top and keeps it spinning.
The ground exerts an upward support force on the top that keeps the top spinning.

2. To start a motionless toy top spinning, you twist it. What determines the direction in which the top spins?

In the Northern Hemisphere, the top spins clockwise. In the Southern Hemisphere, the top spins counterclockwise.
In the Northern Hemisphere, the top spins counterclockwise. In the Southern Hemisphere, the top spins clockwise.
The torque you exert on the toy top has a direction and the top undergoes angular acceleration in the direction of the torque you exert on it.
How you set the top on the ground determines the direction in which the top spins.

3. You are traveling through deep space in a large spaceship and everything in the ship is weightless. The ship is experiencing zero net force and it coasts forward. However, in preparation for docking at a space station, the ship is rotating slowly. You notice that one location in the coasting ship moves at constant velocity, even as the rest of the ship rotates about that location. What is this special location in the ship?

The ship’s center of gravity.
The ship’s center of mass.
The ship’s center of balance.
The ship’s geometrical center.

4. A tall luxury hotel has a rotating restaurant at its top. The disk-shaped floor of the restaurant rotates slowly about the center of the restaurant and completes one full rotation every 30 minutes. When the restaurant opens each day, the manager turns on the motors that make the restaurant spin, but it takes several minutes for the restaurant to begin spinning at its full angular velocity. Why doesn’t the restaurant reach full speed immediately?

The restaurant’s angular velocity is proportional to the net torque exerted on it. The motors take time to warm up and the net torque they produce on the restaurant increases steadily for the first few minutes of operation.
The restaurant’s angular velocity is proportional to its rotational mass. The motors gradually increase the restaurant’s rotational mass over the first few minutes of operation.
The restaurant’s angular velocity is proportional to 1 divided by its rotational mass. The motors gradually decrease the restaurant’s rotational mass over the first few minutes of operation.
The restaurant’s angular acceleration is proportional to the net torque exerted on it. The motors produce a net torque on the restaurant and it immediately undergoes angular acceleration. But it takes time for the angular-accelerating restaurant to reach its full angular velocity.

5. A modern bicycle has two pedals mounted on a rotating device known as a crank. Pushing down on one pedal with your foot produces a torque on the crank, about its pivot, except in which situation(s)?

When the pedal is directly in front of the pivot, any motion of the pedal will cause it to travel backward—toward the rear of the bicycle. A force exerted on the pedal in that position will produce zero torque on the crank.
When the pedal is vertically above or below the pivot, your force on the pedal is directed along the lever arm from the pivot to your force. A force that is parallel to the lever arm produces zero torque.
When the pedal is directly in back of the pivot, any motion of the pedal will cause it to travel forward—toward the front of the bicycle. A force exerted on the pedal in that position will produce zero torque on the crank.
When the pedal is moving as fast as you can move your foot, a force you exert on that pedal will produce zero torque on the crank.

6. Arm-wrestling is a simple game that two people can play. The players sit across from one another at a table, place their right elbows together on the tabletop and clasp their right hands together. When the competition starts, each person tries to twist the pair of arms counterclockwise from that person’s perspective until those arms touch the table. It’s a rotational problem, with the elbows acting as the pivot and the two players trying to rotate the pair of arms in opposite directions. Suppose you are arm-wrestling with a friend and you are winning. Compare the torque you are exerting on your friend to the torque that your friend is exerting on you.

Those two torques are equal in amount but opposite in direction.
The torque you are exerting on your friend is greater in amount than the torque your friend is exerting on you.
The torque your friend is exerting on you is greater in amount than the torque you are exerting on your friend.
Those two torques are equal in amount and direction.

7. You are arm-wrestling another friend and find that you are almost perfectly matched. Your pair of arms is vertical and motionless, even though you are both trying hard to win. To begin winning, you want that pair of arms to rotate counterclockwise from your perspective. What must you do to make that happen?

The torque you exert on your friend’s arm must be greater in amount than the torque your friend exerts on your arm.
The angular acceleration of your arm must be greater than the angular acceleration of your friend’s arm.
The angular velocity of your arm must be greater than the angular velocity of your friend’s arm.
The counterclockwise torque you exert on the pair of arms must be greater in amount than the clockwise torque your friend is exerting on that pair.

8. You are making pizza and are spinning a ball of pizza dough in midair to make a larger and larger disk. As the diameter of the disk increases, you find it more difficult to change the disk’s angular velocity. Why?

The disk’s mass increases with its diameter, although its rotational mass remains unchanged.
Both the disk’s mass and rotational mass increase with its diameter.
The disk’s rotational mass increases with its diameter, although its mass remains unchanged.
The disk’s angular acceleration increases with its diameter, but its angular velocity remains unchanged.

9. Your car has a flat tire and you are using an automobile jack to lift the corner of the car so that you can change the tire. The jack involves a lever and you lift the corner of the car upward by pushing the handle of the lever downward. You notice that as the handle moves downward 10 inches, the corner of the car moves upward only 0.5 inches. Assuming that the jack is not wasting any energy, compare the downward force you exert on the jack handle to the upward force that the jack exerts on the car.

The jack’s upward force on the car is 20 times as large as your downward force on the jack handle.
The jack’s upward force on the car is 10 times as large as your downward force on the jack handle.
The jack’s upward force on the car is 0.5 times as large as your downward force on the jack handle.
Your downward force on the jack handle is 20 times as large as the jack’s upward force on the car.

10. Tower cranes are frequently seen in cities, where they are used to construct tall buildings. In a tower crane, a huge metal beam sits atop a vertical metal tower. The beam extends outward from the tower in two directions and it pivots about the top of the tower. A lifting cable hangs from one end of the beam and heavy weights hang from the other end of the beam. Since the lifting cable end of the beam is the only end that seems to do anything, what useful purpose does the weight-end of the beam serve?

The weight-end of the beam makes the beam more responsive to torques so that the crane operator can make it undergo more rapid angular accelerations about its pivot.
The weight-end of the beam ensures that the beam is approximately balanced about its pivot and experiences approximately zero torque due to gravity.
The beam pivots about its geometrical center, so it needs both ends in order for that geometrical center to located above the tower.
The weight-end of the beam places the beam’s center of gravity at that end and thus makes the beam more stable.

Week- 6

Wheels

1. You are walking at constant velocity on a sidewalk that slopes gently uphill. What force(s) is the sidewalk exerting on you?

A frictional force directed uphill, parallel to the sidewalk’s surface, and a support force directed perpendicular to the sidewalk’s surface.
A ramp force directed downhill.
A support force directed perpendicular to the sidewalk’s surface, but no frictional force.
Zero force, since you are at constant velocity.

2. For millennia, people have ground grain into flour by placing a thin layer of grain between two surfaces that slide across one another. Those two surfaces are usually stacked on top of one another and the surface above the grain is that of a very heavy object, such as a millstone. When motionless, the heavy millstone will exert large support forces on the grain and thereby crush that grain to some extent, but it won’t make fine flour. Spinning the millstone while keeping the surface below the grain motionless, however, will grind the grain as fine as you like. What role does the millstone’s weight play in this grinding process?

The millstone’s weight reduces the effects of friction so that it is easier to keep the millstone turning steadily.
The millstone’s great weight keeps the grain from bunching up as the millstone rotates.
The heavy millstone continues to exert large support forces on the grain, even as the millstone spins, so that the support forces continue to crush the grain into flour. Frictional forces are unavoidable nuisances that only make the grinding process more difficult.
The grinding process uses both support forces to crush the grain and sliding frictional forces to wear the grain. Since frictional forces are approximately proportional to support forces, the millstone’s weight enhances both the crushing and the wearing processes.

3. Walking on ice can be treacherous, but some techniques are safer than others. In general, you are less likely to slip and fall if you lower your foot vertically into place on the ice as you take each step than you are if you slide your foot horizontally into place on the ice as you take each step. Why is the vertical landing method of walking on ice usually safer than the horizontal sliding method?

The vertical landing method increases the support forces the ice exerts on you and larger support forces can prevent you from slipping sideways and falling.
The static frictional forces you can obtain from the ice using the vertical landing method are larger, and more effective at preventing sideways slips, than the sliding friction forces you are likely to obtain from the ice using the horizontal sliding method.
The vertical landing method increases your mass and makes it harder for you to accelerate sideways and slip.
The vertical landing method increases your weight and makes it harder for you to accelerate sideways and slip.

4. Most automobiles have mechanical brakes on all four of their wheels. Each of these brakes consists of two surfaces—one surface that rotates with the wheel and one surface that doesn’t rotate. When you put your foot on the brake pedal in such an automobile, those two surfaces begin to slide across one another. The harder you press on the brake pedal, the more tightly those surfaces are pressed against one another. Why do the brakes permit the two surfaces to slide across one another, rather than locking those two surfaces together so that they don’t slide across one another?

The purpose of the brakes is to waste the moving automobile’s kinetic energy, using sliding friction, and thereby slowing the automobile safely. Locking the brakes would result in static friction in the brakes and prevent the brakes themselves from wasting more than a tiny fraction of the automobile’s kinetic energy.
The purpose of the brakes is to waste the moving automobile’s kinetic energy and thereby slowing the automobile safely. While either static or sliding friction between the brake surfaces wastes energy, static friction offers more control as it wastes energy than sliding friction does as it wastes energy.
Locking the brakes would stop the automobile instantly.
Locking the brakes would stop the automobile much faster than unlocked brakes, but not instantly.

5. The harder you press on an automobile’s brake pedal, the greater the support forces that the two surfaces in each brake exert on one another. Why does this increase in support forces in the brakes result in more rapid deceleration (acceleration opposite its velocity) of the automobile?

As the backward support force that the brake exerts on the car increases, so does the car’s backward acceleration.
As those support forces increase, the backward force the car exerts on itself increases and the car slows more rapidly.
The sliding frictional forces the two brake surfaces exert on one another are approximately proportional to their support forces on one another. As the support forces increase, the sliding frictional forces also increase; they waste the automobile’s kinetic energy faster, so that it slows more rapidly.
As those support forces increase, the car’s mass increases and its speed must decrease as a result.

6. The front wheel of your bicycle spins freely on its axle and rotates at almost constant angular velocity if nothing outside the bicycle exerts a torque on it. Suppose the front wheel is motionless as you stand next to your bicycle. You get on the bicycle and pedal forward. As the bicycle begins to move forward, why does its front wheel begin to rotate?

The ground exerts a forward static frictional force on the bottom of the front wheel to help the bicycle accelerate forward. That frictional force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.
The ground exerts an upward support force on the bottom of the front wheel to prevent that wheel from falling into the ground. That support force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.
The ground exerts a backward static frictional force on the bottom of the front wheel to prevent that wheel from sliding across the ground. That frictional force, exerted at a lever arm from the wheel’s center of rotation, produces the torque that causes the wheel to begin rotating.
The force you exert on a pedal, exerted at a lever arm from the front wheel’s center of rotation, produces the torque that causes the front wheel to begin rotating.

7. When you drive a car on a level (horizontal) road that is slippery with ice, you usually have no problems except when you try to speed up, slow down, or turn. Why does the icy road make those three actions hazardous? [neglect any effects due to air]

Each action involves a horizontal acceleration and requires a horizontal force. The only forces that level pavement can exert on the car are support forces and the brittle, fragile ice reduces those support forces.
When the car is moving at constant velocity, its wheels are experiencing static friction with the pavement. When it is speeding up, slowing down, or turning, its wheels are experiencing sliding friction. Since ice exerts zero sliding friction, those three actions are not possible.
When the car is moving at constant velocity, its wheels are experiencing sliding friction with the pavement. When it is speeding up, slowing down, or turning, its wheels are experiencing static friction. Since ice exerts zero static friction, those three actions are not possible.
Each action involves a horizontal acceleration and requires a horizontal force. The only forces that level pavement can exert on the car are frictional forces and the slippery ice reduces those frictional forces.

8. Pedaling your bicycle provides power to its rear wheel and propels your bicycle forward. What force(s) is principally responsible for the bicycle’s forward acceleration as you pedal your bicycle forward from rest on a level (horizontal) road?

The pavement exerts a forward support force on the bottom of the rear wheel.
The pavement exerts a forward frictional force on the bottom of the rear wheel.
The pavement exerts forward frictional forces on the bottoms of the front and rear wheels.
The pavement exerts forward support forces on the bottoms of the front and rear wheels.

9. If you throw a ball directly upward at 10 meters-per-second (22 mph), it will rise upward about 5 meters before coming momentarily to a stop. If you throw that ball directly upward at 20 meters-per-second (44 mph), how far will it rise upward before coming momentarily to a stop?

About 10 meters.
About 5 meters.
About 20 meters.
About 40 meters.

10. Earth’s oceans exhibit tides, so that the average water level near most shores rises and falls at roughly six-hour intervals. A somewhat simplistic explanation for the tides is that the moon’s and sun’s gravity slightly distort Earth’s oceans, creating high and low regions, and the Earth’s rotation causes those high and low regions to shift on the Earth’s surface. There are a few tidal generating plants on Earth that use energy in the tides to generate electricity. Where does nearly all of the energy in the tides originally come from?

The kinetic energy in the Earth rotational motion.
The gravitational potential energy in the Earth’s oceans.
The kinetic energy in the Earth’s oceans.
The gravitational potential energy in the Earth itself.

Week- 7

Bumper Cars

1. A meteor is streaking toward city hall and will hit the building in a few second. As it moves through the sky, what physical quantities is the meteor carrying with it? [Ignore any effects due to air or Earth’s gravity]

Energy, momentum directed toward city hall, but no force.
Energy, momentum directed toward city hall, and an enormous force directed toward city hall.
Energy, an enormous force directed toward city hall, but no momentum.
Energy, but no momentum and no force.

2. You have a midnight craving for ice cream and are walking quickly through your pitch-black apartment when you collide with the wall. You come to a complete stop. Fortunately, your interior decorator mounted a thick woolen tapestry (wall-hanging) on the concrete wall and that soft tapestry saves you from injury. Compare the momentum you transferred while coming to a stop on the tapestry-covered wall to the momentum you would have transferred if you had come to a stop on the bare concrete wall.

You would have transferred the same momentum in either case, but in stopping on the tapestry-covered wall you transferred that momentum with a larger force over a shorter period of time.
You transferred more momentum while stopping on the tapestry-covered wall than you would have transferred while stopping on the bare wall.
You transferred less momentum while stopping on the tapestry-covered wall than you would have transferred while stopping on the bare wall.
You would have transferred the same momentum in either case, but in stopping on the tapestry-covered wall you transferred that momentum with a smaller force over a longer period of time.

3. A car traveling at 60 mph (100 km/h) veers off the road and hits a tree. The car immediately comes to a complete stop. Fortunately, the airbag inflates and the driver comes to a stop in the airbag instead of coming to a stop on the steering wheel. Hitting the airbag rather than the steering wheel saves the driver’s life because the driver

carries less force with her before colliding with the airbag than she would have carried with her if there were no airbag.
transfers more momentum to the airbag than she would have transferred to the steering wheel.
transfers all of her momentum to whatever stops her, but that transfer is slower and involves a smaller force when she hits the airbag.
transfers less momentum to the airbag than she would have transferred to the steering wheel.

4. A diver stands upright at the edge of the 10 meter platform at the Olympics. The diver jumps off the platform, folds into a ball shape, completes 3.5 somersaults, unfold out of the ball shape, and plunges head-first into the water. Compare the diver’s angular momentum about the diver’s center of mass at three different moments while that diver is not touching anything: (a) before folding into a ball shape, (b) while ball-shaped, and (c) after unfolding out of the ball shape [Note that the diver’s weight, which acts at the diver’s center of gravity, produces zero torque on the diver about the diver’s center of mass. Ignore any effects due to the air.]

The diver’s angular momentum is greatest at moment (b), as the diver is completing somersaults.
The diver’s angular momentum is the same at all three moments.
The diver’s angular momentum is greatest at moment (a), before the diver folds into a ball shape.
The diver’s angular momentum is greatest at moment (c), after the diver unfolds out of the ball shape.

5. The chef at a pizza restaurant tosses a spinning disk of pizza dough into the air. As the disk stretches outward in midair and its diameter increases, what happens to the disk’s angular momentum and angular velocity about the disk’s center of mass? [Note that the disk’s weight, which acts at the disk’s center of gravity, produces zero torque on the disk about the disk’s center of mass. Ignore any effects due to the air.]

The disk’s angular velocity is constant, but the disk’s angular momentum increases.
The disk’s angular momentum is constant, but the disk’s angular velocity increases.
The disk’s angular momentum is constant, but the disk’s angular velocity decreases.
The disk’s angular momentum and angular velocity are both constant.

6. You are riding on a large carousel at an amusement park and you are enjoying the moving scenery as the carousel spins about its center of rotation. The ride comes to an end and the carousel gradually slows to a stop. Why does it take so long for the carousel to stop rotating?

The spinning carousel carries a large amount of angular momentum and the angular impulse needed to remove that angular momentum with a reasonable torque requires a long time.
The spinning carousel has a large angular velocity and changing a large angular velocity requires a long time.
The spinning carousel has a large rotational mass and changing a large rotational mass requires a long time.
The spinning carousel has a large angular impulse and changing a large angular impulse requires a long time.

7. A “lazy susan” is a disk-shaped rotating platform that a restaurant places at the center of a large dining table. Dishes of food are placed on the lazy susan and diners can rotate the lazy susan by hand to bring various dishes closer to them. A large torque exerted for a short time makes the motionless platform begin rotating rapidly, but that dangerous technique risks tipping over some of the food dishes. How can you make the same motionless platform and dishes begin rotating just as rapidly, but with a smaller, safer torque?

Do the same angular impulse as the dangerous technique, but by exerting a smaller torque for a smaller time.
Do the same angular impulse as the dangerous technique, but by exerting the smaller torque for a larger rotational mass.
Do the same angular impulse as the dangerous technique, but by exerting the smaller torque for a smaller rotational mass.
Do the same angular impulse as the dangerous technique, but by exerting a smaller torque for a longer time.

8. To win a big prize at the fair or festival, all you have to do is toss a basketball into a bucket located about 10 feet (3 meters) in front of you and have the basketball remain in the bucket. The rigid bucket cannot move and it opens toward you. However, the bucket is tilted upward just enough that the basketball will remain in it if someone places the basketball in the bucket by hand. You try a dozen times to get the basketball to stay in the bucket, but it keeps bouncing back out of the bucket. Why is it so difficult for the basketball to come to rest in the bucket?

To stop moving, the basketball must transfer both energy and momentum to the bucket and, while it transfers momentum easily to the bucket, it transfers almost zero energy to the bucket.
To stop moving, the basketball must transfer both energy and momentum to the bucket and, while it transfers energy easily to the bucket, it transfers almost zero momentum to the bucket.
To stop moving, the basketball must transfer both energy and momentum to the bucket and it transfers almost zero energy and almost zero momentum to the bucket.
To stop moving, the basketball must transform its energy into momentum. During its impact with the bucket, there is not enough time to complete that transformation.

9. You have just added a massive stone sculpture to your modern art collection. Unfortunately, the people who delivered the sculpture accidentally set it on its side. What barbarians! To tip the sculpture onto its proper base, you transfer as much momentum as you can to the highest point on the sculpture. You accomplish this transfer (successfully, I might add) by running full speed toward the sculpture and

hitting the highest point on the sculpture with the softest part of your body so that you come to a complete stop.
hitting the highest point on the sculpture with the hardest part of your body so that you come to a complete stop.
hitting the highest point on the sculpture with your feet as you jump against it so that you end up reversing your velocity.
continuing past the sculpture without touching it (you evidently lost your nerve).

10. Your dynamic sculpture combines magnets, springs, and elastic bands with a variety of moving parts, including levers, pulleys, and pendulums. When someone jostles those parts, a complicated series of motions occurs and predicting how things will proceed seems nearly impossible. You point out, however, that there is a simple rule governing the motion of each part at any moment in time. That rule is that each part

continues in motion at constant velocity.
moves when it experiences a net force and stops when the net force on it is zero.
has constant energy, momentum, and angular momentum.
accelerates in the direction that reduces its total potential energy as quickly as possible.

Week- 8

Final Assessment

1. Two puppies are fighting over a toy. Each puppy grips one end of that toy in its mouth and pulls. Suddenly, the puppy on the right pulls especially hard on the toy and moves its end of the toy to the right. The other puppy manages to keep its end of the toy from moving. Alas, the toy breaks and the game ends. Breaking the toy required energy and that energy was provided

only by the puppy on the right.
by both puppies, but most was provided by the puppy on the right.
only by the puppy on the left.
in equal amounts by the two puppies.

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2. You make a sharp left turn in your car and your cellphone slips off the dashboard and out the right passenger window. The cellphone leaves the car because

the car did not exert enough leftward force on the cellphone to make the cellphone accelerate with the car.
the cellphone’s weight pulled the cellphone out the window.
the dashboard of the turning car tilted sharply to become a ramp and the downhill ramp force pushed the cellphone out the window.
the car exerted a rightward centrifugal force on the cellphone that pushed the cellphone out the window.

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3. You are pulling your niece uphill on a sled and the sled experiences a small downhill frictional force as it slides uphill on the snow. Your niece is traveling in a straight line at a constant speed. The net force your niece is experiencing is

in the uphill direction.
in the upward direction.
zero.
in the downhill direction.

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4. You are rearranging your room and are now sliding your desk across the floor at constant velocity. Which of the following statements about the forces acting on the desk is correct? [Consider only horizontal forces]

The force that you’re exerting on the desk must be equal in amount but opposite in direction to the force that the floor is exerting on it.
The amount of force that you’re exerting on the desk must be more than the amount of force that friction is exerting on it.
The amount of force that you’re exerting on the desk must be equal to the amount of its weight.
The amount of force that you’re exerting on the desk must be more than the amount of its weight.

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5. In the game of shuffleboard, you push plastic disks forward and then release them so that they slide across a level playing surface. Once you release a disk, you are no longer pushing and it gradually skids to a stop. Its final position determines your score. As each disk skids to a stop, what becomes of the kinetic energy it had when you released it? That energy is

now elastic potential energy in the disk.
still present in the disk, as it must be because kinetic energy is conserved and can’t be created or destroyed.
now thermal energy in the disk and playing surface.
now gravitational potential energy in the disk.

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6. You are carrying a child on your back as you walk down a hill. The child is traveling straight at a steady speed. In which direction is the force you are exerting on the child?

Forward (horizontal)
Upward (vertical).
Downhill (in the direction of your velocity).
Upward and forward (between vertical and horizontal).

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7. You take off your shoes to sneak quietly into your home late at night. Unfortunately, it’s too dark to see the concrete block your friend left on the floor and your big toe collides with it. The block doesn’t move and your foot comes to a complete stop due to the impact. Luckily, you are wearing soft woolen socks because when your foot stops during the impact, your toe transfers

less momentum to the block than it would have if you had not been wearing socks.
less velocity to the block than it would have if you had not been wearing socks.
the same momentum, whether or not you are wearing socks, but that transfer takes more time when you are wearing socks.
less energy to the block than it would have if you had not been wearing socks.

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8. A rigid two-blade wind turbine that is experiencing zero net torque

is motionless and horizontal.
is motionless and may be horizontal or tilted.
has a constant angular velocity, which may be zero.
has an angular velocity that is gradually decreasing toward zero.

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9. A dog and a cat jump horizontally off a wall at the same moment and soon land on the level horizontal field that extends outward from the base of the wall. The dog weighs twice as much as the cat, but the cat was moving forward horizontally twice as fast as the dog when the two animals left the wall. In this situation,

the dog lands first, but the cat lands considerably farther from the wall than the dog does.
both animals land at approximately the same time, but the cat lands considerably farther from the wall than the dog does.
the dog lands first, but both animals land at approximately the same distance from the wall.
both animals land at approximately the same time and at approximately the same distance from the wall.

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10. A skateboarder rides swiftly up the edge of a bowl-shaped surface and leaps into the air. While in the air, the skateboarder flips upside and tosses the skateboard from hand to hand. The skateboarder then rides safely back down the bowl. During the time that the skateboarder and skateboard are not touching anything, one aspect of their motion that is constant is their total (or combined) [note: neglect any effects due to the air]

momentum.
angular velocity.
velocity.
angular momentum.

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11. You are tossing popcorn straight up and catching it in your mouth. At the moment each piece of popcorn reaches its peak height, its velocity is

zero and its acceleration is downward.
downward and its acceleration is zero.
downward and its acceleration is downward.
zero and its acceleration is zero.

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12. Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because

it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand.
it barely moves downward as you push downward on it, so you do almost zero work on it.
it stops the downward motion of your foot faster and thus absorbs less of your momentum.
it stops the downward motion of your foot faster and thus absorbs more of your momentum.

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13. You’re at the lake and watch two children jump off a dock. They jump at the same time and at the same speed, but the boy jumps mostly upward while the girl jumps mostly forward. After they leave the dock,

the boy reaches the water before the girl.
the two children reach the water at the same moment and but the girl travels farther from the dock than does the boy.
the girl reaches the water before the boy.
the two children reach the water at the same moment and at the same distance from the dock.

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14. It is a beautiful summer day and the residents of a high-rise apartment building are eating dinner on their balconies. A resident accidently knocks an empty glass off a balcony that is about 80 meters (260 feet) above the cement patio. The glass would have smashed on that patio after falling for 4 seconds, however, a quick-witted resident catches the glass after it has fallen for only 2 seconds. How far above the patio was the glass when it was caught?

Approximately 40 meters above the patio.
Approximately 30 meters above the patio.
Approximately 60 meters above the patio.
Approximately 50 meters above the patio.

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15. You are at the gym, exercising on a step machine. You have one foot on each of the machine’s pedals and you move those pedals up and down as you step. The pedals always push upward on your feet, but they push upward harder on your feet while moving downward than they do while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?

When that pedal is accelerating.
As that pedal moves downward.
As that pedal moves upward.
As that pedal moves either upward or down.

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16. A juggler tosses a club straight up. Disregarding any effects due to the air, what force or forces are acting on the club while it is above the juggler’s hands?

Its weight.
Its weight along with a steadily decreasing upward force.
Its weight along with an upward force that steadily decreases until the club reaches its highest point. After that point, there is only the constant downward force of gravity.
A steadily decreasing upward force from the moment it leaves the juggler’s hands until it reaches its highest point and then a steadily increasing downward force as the club returns toward the jugglers hands.

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17. Two children are balanced on a seesaw, but one child weighs twice as much as the other child. The heavier child is sitting half as far from the pivot as is the lighter child. Since the seesaw is balanced, the heavier child is exerting on the seesaw

a torque that is less than the torque the lighter child is exerting.
a force that is equal in amount but oppositely directed to the force the lighter child is exerting.
a torque that is equal in amount but oppositely directed to the torque the lighter child is exerting.
a force that is less than the force the lighter child is exerting.

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18. You are standing in the middle of a subway car that is moving forward at constant velocity when another passenger accidently spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Because nothing else is touching you, you

remain in the middle of the subway car.
shift toward the side of the subway car (perpendicular to the direction of its velocity).
shift toward the back of the subway car (opposite its velocity).
shift toward the front of the subway car (in the direction of its velocity).

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19. You are practicing tennis alone by hitting a tennis ball forward toward a cement wall. Each time the ball hits the wall, it bounces backward at high speed so that you can hit it again. During its bounce, the ball

retains approximately all of its momentum but transfers most of its energy to the wall.
retains approximately all of its energy and momentum.
retains approximately all of its energy but transfers most of its momentum to the wall.
retains approximately all of its energy but transfers more forward momentum than it had to the wall.

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20. Your table at a family-style restaurant has a “lazy Susan” in the middle. This large circular platform rotates frictionlessly so that you can “pass” an entree to your friends by placing it on the platform and then rotating the platform. When the server places a new entrée on the platform, it

decreases the platform’s angular velocity.
makes it easier to change the platform’s angular velocity.
makes it harder to change the platform’s angular velocity.
increases the platform’s angular velocity.

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21. A cross-country skier is struggling to get up a hill, so you offer to help. As the skier passes you, you reach out with your hand and exert an uphill force of 80 N (18 pounds) on the skier. When you do this, the skier exerts