Midterm Examination 1 Solutions

Problem 1:

Why does a beanbag bounce poorly when dropped on a concrete sidewalk and bounce well when dropped on a trampoline?

(A) During their collision, the beanbag transfers much less momentum to the soft trampoline than to the hard sidewalk. The beanbag's remaining momentum causes the beanbag to bounce upward from the trampoline. [5.7% picked]

(B) When the beanbag collides with the hard sidewalk, the beanbag is softer, and receives and wastes nearly all of the collision energy. When the beanbag collides with the soft trampoline, the trampoline is softer, and receives and returns nearly all of the collision energy. [46.3% picked]

(C) During their collision, the beanbag transfers much less energy to the soft trampoline than to the hard sidewalk. The beanbag's remaining energy causes the beanbag to bounce upward from the trampoline. [8.5% picked]

(D) Hard surfaces do not store and return energy well, so objects do not bounce well from hard concrete sidewalks. Soft surfaces store and return energy efficiently, so objects bounce well from soft trampolines. [39.4% picked]

Answer: (B) When the beanbag collides with the hard sidewalk, the beanbag is softer, and receives and wastes nearly all of the collision energy. When the beanbag collides with the soft trampoline, the trampoline is softer, and receives and returns nearly all of the collision energy. [46.3% picked]

Why: Video Discussion

Problem 2:

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 harder while moving downward than while moving upward. When during this exercise is your foot transferring energy to the pedal that it is touching?

(A) As that pedal moves upward. [2.0% picked]

(B) As that pedal moves either upward or down. [4.9% picked]

(C) As that pedal moves downward. [90.7% picked]

(D) When that pedal is accelerating. [2.4% picked]

Answer: (C) As that pedal moves downward. [90.7% picked]

Why: Video Discussion

Problem 3:

As you run up the marble steps of the Supreme Court building to begin arguing your case, your right foot skids to a stop on one of the steps. During that skid, what work is being done by your right foot and the step?

(A) The step does zero work on your foot and your foot does positive work on the step. [3.7% picked]

(B) The step does positive work on your foot and your foot does zero work on the step. [7.7% picked]

(C) The step does negative work on your foot and your foot does zero work on the step. [84.6% picked]

(D) The step does zero work on your foot and your foot does negative work on the step. [4.1% picked]

Answer: (C) The step does negative work on your foot and your foot does zero work on the step. [84.6% picked]

Why: Video Discussion

Problem 4:

Your shopping cart contains a bag of groceries and you are pushing that cart up a straight ramp at a steady speed. In what direction is the shopping cart's force on that bag of groceries?

(A) The cart is pushing the bag directly upward. [85.8% picked]

(B) The cart is pushing the bag directly forward horizontally. [1.6% picked]

(C) The cart is pushing the bag forward and upward, toward a point somewhat above the top of the ramp. [10.2% picked]

(D) The cart is pushing the bag uphill, toward the top of the ramp. [2.4% picked]

Answer: (A) The cart is pushing the bag directly upward. [85.8% picked]

Why: Video Discussion

Problem 5:

You are trying to weigh a huge tomato you grew in your garden, but your spring scale has lost the needle on its dial. You can successfully weigh your tomato, using the dial-less spring scale, but you will also need a calculator, a 1-pound bag of sugar, and

(A) a 1-kilogram container of oatmeal. [4.1% picked]

(B) a 1-liter bottle of water. [1.6% picked]

(C) a 2-pound box of cookies. [5.3% picked]

(D) a ruler. [88.2% picked]

Answer: (D) a ruler. [88.2% picked]

Why: Video Discussion

Problem 6:

You drop a tennis ball from rest onto a rigid, motionless granite counter. The ball bounces straight upward and peaks at 50% of its original height. Before you let it drop, the ball had gravitational potential energy due to its height above the counter. What has become of that gravitational potential energy at the moment the ball reaches peak height on its rebound?

(A) 50% is now gravitational potential energy in the ball and 50% is thermal energy in the ball. [48.4% picked]

(B) 25% is now gravitational potential energy in the ball and 75% is thermal energy in the ball. [4.9% picked]

(C) 50% is now gravitational potential energy in the ball and 50% is elastic potential energy in the counter. [10.2% picked]

(D) 50% is now gravitational potential energy in the ball and 50% is kinetic energy in the ball. [36.6% picked]

Answer: (A) 50% is now gravitational potential energy in the ball and 50% is thermal energy in the ball. [48.4% picked]

Why: Video Discussion

Problem 7:

To win a stuffed animal at the state fair, you simply need to drop a glass marble onto a stationary glass plate and have the marble come to rest on that plate. Unfortunately, when the marble hits the plate, it bounces upward because the marble

(A) retains essentially all of its energy but transfers a large amount of momentum to the plate. [93.1% picked]

(B) retains essentially all of its energy and momentum. [2.0% picked]

(C) transfers a large amount of momentum and energy to the plate. [0.4% picked]

(D) retains essentially all of its momentum but transfers a large amount of energy to the plate. [4.5% picked]

Answer: (A) retains essentially all of its energy but transfers a large amount of momentum to the plate. [93.1% picked]

Why: Video Discussion

Problem 8:

You drop a beanbag (a bag full of beans) onto four different surfaces: a hard floor, another beanbag, a feather pillow, and a foam pillow. The floor is the hardest surface, the feather pillow is the softest surface. The beanbag bounces upward the least from the second beanbag and it bounces upward most from the foam pillow. To which surface does the beanbag transfer the most momentum during its bounce?

(A) To the foam pillow. [53.3% picked]

(B) To the other beanbag. [20.3% picked]

(C) To the feather pillow. [6.9% picked]

(D) To the floor. [19.5% picked]

Answer: (A) To the foam pillow. [53.3% picked]

Why: Video Discussion

Problem 9:

A rabbit runs by and your dog begins to pull hard on her leash. Despite your best efforts, both of you begin to accelerate toward the rabbit. Compare the amount of force that the dog exerts on the leash to the amount of force the leash exerts on the dog.

(A) The amount of force the dog exerts on the leash is greater than the amount of force the leash exerts on the dog because the leash exerts zero force on the dog. [2.0% picked]

(B) The amount of force the dog exerts on the leash is less than the amount of force the leash exerts on the dog. [1.2% picked]

(C) The two forces are equal in amount. [88.2% picked]

(D) The amount of force the dog exerts on the leash is greater than the amount of force the leash exerts on the dog, but neither force is zero. [8.5% picked]

Answer: (C) The two forces are equal in amount. [88.2% picked]

Why: Video Discussion

Problem 10:

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

(A) the string's elastic potential energy. [2.8% picked]

(B) you. [79.3% picked]

(C) the picnic basket. [11.0% picked]

(D) both you and the picnic basket. [6.5% picked]

Answer: (B) you. [79.3% picked]

Why: Video Discussion

Problem 11:

You are riding a train that is moving straight ahead at high speed. The train comes to a curve in the tracks and begins to turn toward the left. You find yourself pressed against the right side of your seat. What force, if any, is pushing you toward the right?

(A) A friction force from the seat is pushing you toward the right. [2.0% picked]

(B) There is no force pushing you toward the right. [78.0% picked]

(C) The force of your momentum is pushing you toward the right. [17.9% picked]

(D) A support force from the seat is pushing you toward the right. [2.0% picked]

Answer: (B) There is no force pushing you toward the right. [78.0% picked]

Why: Video Discussion

Problem 12:

A player has tossed a lacrosse ball to a distant teammate and that ball is in the air, exactly midway between the two players. Neglecting any effects due to the air itself, what net force is the ball experiencing?

(A) A net force pointing up and forward, toward a point somewhat above the receiving player's head. [1.6% picked]

(B) Zero net force. [22.4% picked]

(C) A net force pointing directly forward horizontally. [3.3% picked]

(D) A net force pointing directly downward. [72.8% picked]

Answer: (D) A net force pointing directly downward. [72.8% picked]

Why: Video Discussion

Problem 13:

A large mobile (a sculpture supported by a single cord) hangs from the ceiling of an art museum. Since the cord supports the mobile at its center of gravity and mass, that the mobile is balanced and experiences zero net torque. The mobile is

(A) oscillating back and forth about equilibrium. [3.3% picked]

(B) motionless, but it may be horizontal or tilted at any angle. [7.3% picked]

(C) moving with constant angular momentum, which may be zero. [77.2% picked]

(D) motionless and horizontal. [11.4% picked]

Answer: (C) moving with constant angular momentum, which may be zero. [77.2% picked]

Why: Video Discussion

Problem 14:

Two coins, a quarter and a dime, roll off a level table side-by-side at the same time and in the same direction. The coins soon hit the level floor below the table. The two coins were traveling at the same horizontal speed when they left the table. In this situation,

(A) the quarter hits the floor at approximately the same time as the dime and the quarter hits at approximately the same distance from the table as does the dime. [90.2% picked]

(B) the quarter hits the floor much sooner than the dime and the quarter hits much farther from the table than does the dime. [0.8% picked]

(C) the quarter hits the floor much sooner than the dime and the quarter hits at approximately the same distance from the table as does the dime. [2.0% picked]

(D) the quarter hits the floor at approximately the same time as the dime and the quarter hits much farther from the table than does the dime. [6.9% picked]

Answer: (A) the quarter hits the floor at approximately the same time as the dime and the quarter hits at approximately the same distance from the table as does the dime. [90.2% picked]

Why: Video Discussion

Problem 15:

You toss your lucky nickel directly upward and watch it rise to its peak height. At the moment that nickel reaches that peak height, its velocity is

(A) downward and its acceleration is downward. [1.2% picked]

(B) downward and its acceleration is zero. [1.2% picked]

(C) zero and its acceleration is downward. [93.9% picked]

(D) zero and its acceleration is zero. [3.7% picked]

Answer: (C) zero and its acceleration is downward. [93.9% picked]

Why: Video Discussion

Problem 16:

A driver in heavy traffic is texting and accidently collides with the stopped car in front of her. Her car and everything in it suddenly come to a complete stop. The passengers in the rear seat are stopped by their seat belts and shoulder straps, which stretch as they stop the passengers. As compared to rigid restraints, stretchable belts and straps remove the same momentum from each passenger, but they transfer that momentum using

(A) a larger force for a shorter time. [2.8% picked]

(B) a larger force for a smaller distance. [2.4% picked]

(C) a smaller force for a longer time. [82.1% picked]

(D) a smaller force for a greater distance. [12.6% picked]

Answer: (C) a smaller force for a longer time. [82.1% picked]

Why: Video Discussion

Problem 17:

You're at the lake and watch two children jump off a dock at the same time. They both kick equally hard during their jumps, but one child jumps mostly upward while the other child jumps mostly forward. After they leave the dock,

(A) the two children reach the water at the same moment and but the child who jumps forward travels farther from the dock than does the other child. [9.8% picked]

(B) the child who jumps upward reaches the water before the child who jumps forward. [2.0% picked]

(C) the two children reach the water at the same moment and at the same distance from the dock. [0.8% picked]

(D) the child who jumps forward reaches the water before the child who jumps upward. [87.0% picked]

Answer: (D) the child who jumps forward reaches the water before the child who jumps upward. [87.0% picked]

Why: Video Discussion

Problem 18:

You have a cart attached to your bicycle and two children are sitting motionless in that cart at the bottom of a hill. You bicycle up to the top of the hill via the gradual path and stop so that the three of you can enjoy the view. You then bicycle down the hill via a steep path and return to exactly where you started and are motionless again. What overall work did you and the bicycle do on the cart and children during the roundtrip?

(A) A positive amount of work because the uphill slope was more gradual than the downhill slope. [3.3% picked]

(B) Zero overall work. [70.7% picked]

(C) A negative amount of work because the uphill slope was more gradual than the downhill slope. [2.4% picked]

(D) A positive amount of work because you followed two different paths, one up the hill and one down the hill. The slopes of those two paths didn't matter. [23.6% picked]

Answer: (B) Zero overall work. [70.7% picked]

Why: Video Discussion

Problem 19:

A high school pitcher can throw a 50 mile-per-hour fastball, but it takes a phenomenal professional pitcher to throw a 100 mile-per-hour fastball. Compared to the 50 mile-per-hour fastball, the 100 mile-per-hour fastball carries

(A) 4 times as much kinetic energy. [74.4% picked]

(B) 4 times as much force. [11.0% picked]

(C) 2 times as much kinetic energy. [8.5% picked]

(D) 2 times as much force. [6.1% picked]

Answer: (A) 4 times as much kinetic energy. [74.4% picked]

Why: Video Discussion

Problem 20:

A pickup truck is stopped at a red light and you are standing in the back of that truck, wearing rollerskates. The light turns green and the truck heads forward. Why do you roll out the back of the truck?

(A) You experience a backward force due to sliding friction. [2.0% picked]

(B) You experience a backward force due to static friction. [1.2% picked]

(C) You experience a backward force due to your weight. [0.0% picked]

(D) Inertia. [96.7% picked]

Answer: (D) Inertia. [96.7% picked]

Why: Video Discussion

Problem 21:

You're making pizza for a dinner party and you toss a spinning disk of dough into the air to stretch it outward. While nothing is touching the disk, what aspect of the disk is constant? [neglect effects due to air]

(A) Its angular momentum is constant. [83.7% picked]

(B) Its angular velocity is constant. [12.2% picked]

(C) Its velocity is constant. [0.4% picked]

(D) Its momentum is constant. [3.3% picked]

Answer: (A) Its angular momentum is constant. [83.7% picked]

Why: Video Discussion

Problem 22:

You take a toboggan (a flat-bottom vehicle for sliding down slippery hills) to the top of an icy hill. Before sliding down the hill, however, you connect a long spring from the toboggan to a tree part way down the slope on the left. When you get on the toboggan and it begins to move frictionlessly on the hill's surface, how does the toboggan move?

(A) It accelerates in the direction that reduces its gravitational potential energy as quickly as possible. [31.3% picked]

(B) Its velocity is in the direction that reduces its gravitational potential energy as quickly as possible. [4.1% picked]

(C) It accelerates in the direction that reduces its total potential energy as quickly as possible. [53.7% picked]

(D) It accelerates directly toward the tree. [10.6% picked]

Answer: (C) It accelerates in the direction that reduces its total potential energy as quickly as possible. [53.7% picked]

Why: Video Discussion

Problem 23:

When you are riding a pogo stick, you are basically bouncing on a spring. When you stand motionless on the pogo stick, the spring compresses to become 1 inch shorter than its equilibrium length. Suppose you are riding the pogo stick and, at this moment, its spring is 2 inches shorter than its equilibrium length. At this moment,

(A) your velocity is downward. [8.1% picked]

(B) you are accelerating upward. [72.8% picked]

(C) you are accelerating downward. [18.7% picked]

(D) your velocity is upward. [0.4% picked]

Answer: (B) you are accelerating upward. [72.8% picked]

Why: Video Discussion

Problem 24:

Before its launch, the Mars rover Curiosity had a mass of 899 kilograms and it weighed 1982 pounds. After landing on Mars, Curiousity's mass

(A) remains unchanged, but its weight is different. [99.6% picked]

(B) is different, but its weight remains unchanged. [0.0% picked]

(C) remains unchanged and its weight remains unchanged. [0.0% picked]

(D) is different and its weight is different. [0.4% picked]

Answer: (A) remains unchanged, but its weight is different. [99.6% picked]

Why: Video Discussion

Problem 25:

You are carrying an unopned 10-pound bag of chocolate chips as you ride your skateboard at constant velocity along a horizontal road. While you hold up the bag at a constant height, you are doing

(A) zero work on the bag and it is doing zero work on you. [91.1% picked]

(B) negative work on the bag and it is doing (positive) work on you. [0.0% picked]

(C) (positive) work on the bag and it is doing (positive) work on you. [0.8% picked]

(D) (positive) work on the bag and it is doing negative work on you. [8.1% picked]

Answer: (A) zero work on the bag and it is doing zero work on you. [91.1% picked]

Why: Video Discussion

Problem 26:

You stand atop a vertical cliff, with the ocean about 80 meters (260 feet) below you. You throw a heavy rock horizontally off the cliff and it falls for 4 seconds before splashing into the water at a point about 20 meters (65 feet) from the base of the cliff. 2 seconds after the rock left your hand, approximately where is the rock located?

(A) 60 meters above the ocean and about 10 meters horizontally from the cliff. [55.7% picked]

(B) 60 meters above the ocean and about 5 meters horizontally from the cliff. [30.1% picked]

(C) 40 meters above the ocean and about 5 meters horizontally from the cliff. [4.1% picked]

(D) 40 meters above the ocean and about 10 meters horizontally from the cliff. [9.8% picked]

Answer: (A) 60 meters above the ocean and about 10 meters horizontally from the cliff. [55.7% picked]

Why: Video Discussion

Problem 27:

Your little nephew is visiting and is having fun diving onto your bed. He leaps off your dresser and begins to fall toward the bed's surface. At what moment does he reach peak downward speed?

(A) At the moment he first touches the surface of the bed. [8.9% picked]

(B) At the moment he reaches equilibrium on the bed for the first time. [56.1% picked]

(C) At the moment he is denting the surface of the bed as deeply as possible for the first time. [5.7% picked]

(D) At the moment just before he first touches the surface of the bed. [29.3% picked]

Answer: (B) At the moment he reaches equilibrium on the bed for the first time. [56.1% picked]

Why: Video Discussion

Problem 28:

A xylophone is a musical instrument consisting of metal bars that you strike with a ball on the end of stick. As the ball bounces from a bar, that bar begins to emit a musical tone. The instrument gently supports the bar at two special points along the bar's length. Suppose you remove the bar from the instrument and strike the bar at one of those special points. Why does the bar emit almost no sound?

(A) Striking the bar's center of percussion cannot change its momentum or angular momentum. [7.3% picked]

(B) Striking either of the bar's two centers of mass does not cause the bar to accelerate. [11.8% picked]

(C) Striking either of the bar's two centers of mass transfers zero momentum to the bar. [9.3% picked]

(D) Since the bar's vibrational nodes do not move during the bar's vibration, striking one of those nodes won't cause that vibration. [71.1% picked]

Answer: (D) Since the bar's vibrational nodes do not move during the bar's vibration, striking one of those nodes won't cause that vibration. [71.1% picked]

Why: Video Discussion

Problem 29:

A girl who is skating straight across perfectly slippery ice picks up a hockey puck that was resting motionless on the ice. As a result of acquiring that hockey puck, the girl

(A) slows down because her initial momentum is now distributed over more mass. [93.9% picked]

(B) retains exactly the same forward velocity because velocity is conserved. [1.6% picked]

(C) speeds up because her forward momentum has increased. [1.6% picked]

(D) retains exactly the same forward velocity because only her overall weight has increased. [2.8% picked]

Answer: (A) slows down because her initial momentum is now distributed over more mass. [93.9% picked]

Why: Video Discussion

Problem 30:

You're having trouble loosening a rusty bolt with a small wrench, so you borrow a large wrench from your neighbor. Exerting only a modest force on the handle of this new wrench easily unscrews the bolt. The large wrench helps because it

(A) has a large acceleration and a large mass, so the force it produces is large, according to the equation F=ma. [0.4% picked]

(B) has a large rotational mass so that it develops a great deal of angular momentum when you exert a force on it. [9.3% picked]

(C) has a large mass so that its inertia allows you to overcome the bolt's velocity and accelerate it around in a circle. [0.4% picked]

(D) allows you to exert your force far from the center of rotation, so that you produce a large torque on the bolt. [89.8% picked]

Answer: (D) allows you to exert your force far from the center of rotation, so that you produce a large torque on the bolt. [89.8% picked]

Why: Video Discussion