Midterm Examination 1 Solutions

Problem 1:

A gymnast is performing a somersault in midair. While she is not touching anything, which of the following physical quantities is not changing with time? [neglect any effects due to the air]

(A) The gymnast's angular momentum [81.1% picked]

(B) The gymnast's angular velocity [2.6% picked]

(C) The gymnast's rotational mass [14.9% picked]

(D) The gymnast's angular position [1.3% picked]

Answer: (A) The gymnast's angular momentum [81.1% picked]

Why: YouTube Video

Problem 2:

You are so excited that you high-five a massive bronze statue. Your hand was moving at 1 meter-per-second before hitting the statue's hand, but comes to a complete stop after the impact. If the statue had been wearing a soft wool mitten and your hand had come to a complete stop on that mitten, how would that have affected the momentum your hand transferred to the statue (and mitten) during this impact?

(A) Your hand would have transferred more momentum. [0.9% picked]

(B) Your hand would have transferred the same momentum, but with a smaller force over a longer time. [89.0% picked]

(C) Your hand would have transferred the same momentum, but with a smaller force over a larger distance. [7.0% picked]

(D) Your hand would have transferred less momentum. [3.1% picked]

Answer: (B) Your hand would have transferred the same momentum, but with a smaller force over a longer time. [89.0% picked]

Why: YouTube Video

Problem 3:

A blue car is stuck in the mud and a red car is trying to pull it out with a bungee cord that connects the red car to the blue car. As the red car moves forward, the bungee cord stretches and then breaks, but the blue car never moves at all. Which car(s) provided the energy that broke the bungee cord?

(A) Only the blue car. [0.9% picked]

(B) Neither car provided the energy that broke the bungee cord. That energy came from the chemical potential energy in the cord itself. [0.0% picked]

(C) Both the red car and the blue car. [6.1% picked]

(D) Only the red car. [93.0% picked]

Answer: (D) Only the red car. [93.0% picked]

Why: YouTube Video

Problem 4:

When you drop a bouncy ball on a hard cement sidewalk, the ball bounces almost to its original height. When you drop that same ball on a soft sand beach, the ball barely bounces at all. Why does the ball bounce so differently from the two surfaces?

(A) Balls bounce well from hard surfaces, but they do not bounce well from soft surfaces. [3.5% picked]

(B) Cement has more mass than sand, so it can store more of the ball's momentum. Sand can only store a small amount of the ball's momentum. [1.8% picked]

(C) When it bounces on cement, the ball receives most of the collision energy, stores it well, and rebounds strongly. When the ball bounces on sand, the sand receives most of the collision energy and wastes it as thermal energy. [91.2% picked]

(D) The bouncy ball carries a large downward force as it falls and it transfers that force to the sidewalk or sand when it hits them. The sidewalk can tolerate that force and cause the ball to bounce, but the sand cannot. [3.5% picked]

Answer: (C) When it bounces on cement, the ball receives most of the collision energy, stores it well, and rebounds strongly. When the ball bounces on sand, the sand receives most of the collision energy and wastes it as thermal energy. [91.2% picked]

Why: YouTube Video

Problem 5:

When its wireless key is nearby, your new car starts at the push of a button. You accidently place the key on the car's roof, start the car, and drive forward. The key slides off the back of the car and falls to the ground. Why did the key do that?

(A) The car pushed the key backward with a force of sliding friction, so the key accelerated backward and fell of the roof. [17.5% picked]

(B) The key experienced too little forward force to accelerate forward with the car, so the car left it behind. [74.1% picked]

(C) The car pushed the key backward with a force of static friction, so the key accelerated backward and fell off the roof. [5.3% picked]

(D) Because of momentum, the key pushed itself toward the back of the roof. Its kinetic energy increased until it fell off the roof. [3.1% picked]

Answer: (B) The key experienced too little forward force to accelerate forward with the car, so the car left it behind. [74.1% picked]

Why: YouTube Video

Problem 6:

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) There is no force pushing you toward the right. [64.5% picked]

(B) A friction force from the seat is pushing you toward the right. [1.3% picked]

(C) A support force from the seat is pushing you toward the right. [3.5% picked]

(D) The force of your momentum is pushing you toward the right. [30.7% picked]

Answer: (A) There is no force pushing you toward the right. [64.5% picked]

Why: YouTube Video

Problem 7:

You are making pizza for a party and you throw a spinning disk of pizza dough into the air. What happens to the disk's angular momentum and angular velocity as the disk stretches and its diameter increases? [Assume nothing is touching the spinning disk and neglect any effects due to the air]

(A) The disk's angular momentum decreases, but its angular velocity remains unchanged. [7.0% picked]

(B) The disk's angular momentum is unchanged, but its angular velocity increases. [8.3% picked]

(C) The disk's angular momentum increases, but its angular velocity remains unchanged. [5.7% picked]

(D) The disk's angular momentum is unchanged, but its angular velocity decreases. [78.9% picked]

Answer: (D) The disk's angular momentum is unchanged, but its angular velocity decreases. [78.9% picked]

Why: YouTube Video

Problem 8:

When 1 one-pound bird rests motionless on a horizontal tree branch, the branch bends downward 1 inch from its empty position. When 4 one-pound birds rest motionless on that same branch, how far does the branch bend downward from its empty position? [Assume the branch behaves like a spring]

(A) 8 inches [3.1% picked]

(B) 4 inches [91.6% picked]

(C) 16 inches [2.6% picked]

(D) 2 inches [2.6% picked]

Answer: (B) 4 inches [91.6% picked]

Why: YouTube Video

Problem 9:

You are standing on a balcony, 80 meters above the sidewalk, when you accidently bump a flowerpot. The flowerpot begins to fall and it will hit the sidewalk in 4 seconds if you don't act swiftly. A childhood accident left you with super-speed, so you race downstairs and catch the flowerpot after it has been falling for only 2 seconds. How far above the sidewalk is the flowerpot when you catch it?

(A) 40 meters. [10.1% picked]

(B) 60 meters. [76.3% picked]

(C) 20 meters. [13.2% picked]

(D) 10 meters. [0.4% picked]

Answer: (B) 60 meters. [76.3% picked]

Why: YouTube Video

Problem 10:

When you walk onto a springboard at the diving center and settle motionless, the board will have bent downward 4 inches from its empty position. You do some gentle bouncing and soon it is bent downward 8 inches from its empty position. What is your acceleration when the board is bent downward 8 inches?

(A) Zero. [10.5% picked]

(B) About 5 meters/second^{2} in the downward direction. [5.7% picked]

(C) About 10 meters/second^{2} in the upward direction. [71.5% picked]

(D) About 5 meters/second^{2} in the upward direction. [12.3% picked]

Answer: (C) About 10 meters/second^{2} in the upward direction. [71.5% picked]

Why: YouTube Video

Problem 11:

You get on your bicycle near the top of a long, smooth hill. After a moment of motionlessness, you start rolling downhill faster and faster. You did not use the pedals, so what caused the bicycle wheels to start turning?

(A) The ground exerted uphill frictional forces on the bottoms of the bicycle wheels. [54.6% picked]

(B) The ground exerted downhill frictional forces on the bottoms of the bicycle wheels. [32.2% picked]

(C) The bicycle exerted uphill forces on the centers of the bicycle wheels. [0.0% picked]

(D) The bicycle exerted downhill forces on the centers of the bicycle wheels. [13.2% picked]

Answer: (A) The ground exerted uphill frictional forces on the bottoms of the bicycle wheels. [54.6% picked]

Why: YouTube Video

Problem 12:

A seesaw is balanced, with its center of gravity located exactly at its pivot. The seesaw is

(A) motionless, but it may be horizontal or tilted at any angle. [11.9% picked]

(B) motionless and horizontal. [22.5% picked]

(C) oscillating back and forth about equilibrium. [4.4% picked]

(D) moving with constant angular momentum, which may be zero. [61.2% picked]

Answer: (D) moving with constant angular momentum, which may be zero. [61.2% picked]

Why: YouTube Video

Problem 13:

You want to look your best during the royal wedding. As part of your training, you have a book balanced on your head as you walk horizontally at constant velocity. You are doing

(A) zero work on the book and the book is doing zero work on you. [82.0% picked]

(B) (positive) work on the book and the book is doing negative work on you. [14.9% picked]

(C) negative work on the book and the book is doing (positive) work on you. [1.8% picked]

(D) (positive) work on the book and the book is doing (positive) work on you. [1.3% picked]

Answer: (A) zero work on the book and the book is doing zero work on you. [82.0% picked]

Why: YouTube Video

Problem 14:

You let go of a rope and drop straight down onto a trampoline. You bounce. During your initial descent from the rope, you accelerate downward until the moment when you

(A) begin to touch the trampoline. [4.4% picked]

(B) reach equilibrium in the trampoline (you dent the trampoline enough that it supports your weight). [78.9% picked]

(C) reach the bottom of your bounce (you dent the trampoline as deeply as possible). [15.8% picked]

(D) are about to touch the trampoline. [0.9% picked]

Answer: (B) reach equilibrium in the trampoline (you dent the trampoline enough that it supports your weight). [78.9% picked]

Why: YouTube Video

Problem 15:

A vertical flagpole is at equilibrium in calm air and it stands perfectly straight and motionless. A sudden gust of wind causes the flagpole to sway back and forth about equilibrium. As the flagpole sways, when is its speed greatest and when is its acceleration greatest?

(A) Its speed is greatest just before it is farthest from equilibrium and its acceleration is greatest just before it reaches equilibrium. [7.0% picked]

(B) Its speed is greatest when it is at equilibrium and its acceleration is greatest when it is farthest from equilibrium. [78.9% picked]

(C) Its speed is greatest when it is farthest from equilibrium and its acceleration is greatest when it is at equilibrium. [6.6% picked]

(D) Its speed is greatest just before it reaches equilibrium and its acceleration is greatest just before it is farthest from equilibrium. [7.5% picked]

Answer: (B) Its speed is greatest when it is at equilibrium and its acceleration is greatest when it is farthest from equilibrium. [78.9% picked]

Why: YouTube Video

Problem 16:

A hammer's weight acts in the downward direction, yet you can use the hammer to pound a nail into the ceiling. Why?

(A) Your downward weight is so much greater than that of the hammer that the net force on the hammer is upward as it strikes the nail. [1.3% picked]

(B) The nail's downward weight is so much smaller than that of the hammer that the overall force due to gravity on the nail is upward and the nail moves upward into the ceiling. [4.4% picked]

(C) The upward-moving hammer transfers its upward momentum to the nail by way of a huge upward force exerted on the nail for a short time. [93.9% picked]

(D) Your downward weight is so much greater than that of the nail that you dominate the hammer's motion. [0.4% picked]

Answer: (C) The upward-moving hammer transfers its upward momentum to the nail by way of a huge upward force exerted on the nail for a short time. [93.9% picked]

Why: YouTube Video

Problem 17:

You trying to get into a pirate's treasure chest, but are have trouble turning the rusty old lock. What must happen for the motionless lock to begin rotating in the direction that opens it?

(A) You must do an angular impulse on the lock, exerting a torque on the lock for a time. [9.7% picked]

(B) The torque you exert on the lock in the opening direction must be greater in amount than the torque the chest exerts on the lock in the opposite direction. [55.5% picked]

(C) The torque you exert on the lock in the opening direction must be greater than the torque the lock exerts on you in the opposite direction. [33.9% picked]

(D) The force you exert on the lock must act at the lock's pivot. [0.9% picked]

Answer: (B) The torque you exert on the lock in the opening direction must be greater in amount than the torque the chest exerts on the lock in the opposite direction. [55.5% picked]

Why: YouTube Video

Problem 18:

A bartender is new to the job and slides two mugs of root beer toward the end of the bar much too fast. The mugs slide off the bar side-by-side at the same instant and soon hit the floor. At the moment the two mugs slid off the bar, one mug weighed twice as much as the other, but the lighter mug was moving twice as fast as the heavier mug. What happened?

(A) The lighter mug hit the floor before the heavier mug hit the floor. [4.4% picked]

(B) The two mugs hit the floor at the same time and at the same distance from the bar. [10.1% picked]

(C) The heavier mug hit the floor before the lighter mug hit the floor. [1.3% picked]

(D) The two mugs hit the floor at the same time, but the lighter mug hit the floor about twice as far from the bar as did the heavier mug. [84.2% picked]

Answer: (D) The two mugs hit the floor at the same time, but the lighter mug hit the floor about twice as far from the bar as did the heavier mug. [84.2% picked]

Why: YouTube Video

Problem 19:

You accidently drop a raw egg on a massive granite countertop and the egg shatters. During its impact, did the egg transfer significant energy or momentum to that countertop?

(A) The egg transferred significant momentum but not significant energy. [61.4% picked]

(B) The egg did not transfer significant energy or significant momentum. [11.8% picked]

(C) The egg transferred significant energy and significant momentum. [15.4% picked]

(D) The egg transferred significant energy but not significant momentum. [11.4% picked]

Answer: (A) The egg transferred significant momentum but not significant energy. [61.4% picked]

Why: YouTube Video

Problem 20:

You are pushing a book away from you and it is sliding across your desk at a constant horizontal velocity. How does your force on the book compare with the frictional force that the desk exerts on the book? [Consider only horizontal forces]

(A) The desk's force is larger in amount than your force. [1.3% picked]

(B) Your force is larger in amount than the desk's force. [6.6% picked]

(C) The two forces are equal in amount but opposite in direction. [86.0% picked]

(D) The desk's frictional force is zero because the book's velocity is constant. [6.1% picked]

Answer: (C) The two forces are equal in amount but opposite in direction. [86.0% picked]

Why: YouTube Video

Problem 21:

You stand motionless on a spring scale and it is correctly reporting your weight. You suddenly jump upward. During that jump, while you are still in contact with the scale and you are accelerating upward, what force is the scale exerting on you?

(A) An upward force that is less in amount than your weight. [19.3% picked]

(B) An upward force that is greater in amount than your weight. [70.6% picked]

(C) Zero force. [2.2% picked]

(D) An upward force that is equal in amount to your weight. [7.9% picked]

Answer: (B) An upward force that is greater in amount than your weight. [70.6% picked]

Why: YouTube Video

Problem 22:

A speedboat is pulling a water skier across the water and that skier is traveling in a straight path at a steady speed. The net force experienced by the water skier

(A) is zero. [93.4% picked]

(B) points in the forward horizontal direction. [4.4% picked]

(C) points in the upward vertical direction. [0.9% picked]

(D) points in the backward horizontal direction. [1.3% picked]

Answer: (A) is zero. [93.4% picked]

Why: YouTube Video

Problem 23:

You are in San Francisco, riding a cable car up a steep hill. The car's speed is steady and it is moving in a constant uphill direction. What force is the cable car exerting on you?

(A) The car is pushing you upward (vertically) with a force that is greater in amount than your weight. [9.2% picked]

(B) The car is pushing you uphill with a force that is less in amount than your weight. [0.4% picked]

(C) The car is pushing you uphill with a force equal in amount to your weight. [16.2% picked]

(D) The car is pushing you upward (vertically) with a force equal in amount to your weight. [74.1% picked]

Answer: (D) The car is pushing you upward (vertically) with a force equal in amount to your weight. [74.1% picked]

Why: YouTube Video

Problem 24:

After you serve a volleyball, the ball is traveling in an arc over the net. While nothing is touching the ball, which of the following quantities is constant? [Neglect effects due to the air.]

(A) The ball's net force [50.0% picked]

(B) The ball's kinetic energy [5.7% picked]

(C) The ball's potential energy [1.3% picked]

(D) The ball's momentum [43.0% picked]

Answer: (A) The ball's net force [50.0% picked]

Why: YouTube Video

Problem 25:

Suppose a soccer ball is approaching your leg at 20 km/h and your leg is moving toward the soccer ball at 20 km/h. What is the fastest the soccer ball can possibly travel after it hits your leg? [You can assume perfect (ideal) bouncing behaviors for the ball and your leg.]

(A) 20 km/h [14.0% picked]

(B) 80 km/h [3.9% picked]

(C) 40 km/h [66.7% picked]

(D) 60 km/h [15.4% picked]

Answer: (D) 60 km/h [15.4% picked]

Why: YouTube Video

Problem 26:

To win a prize at the fair, you must knock a massive block off a shelf by hitting that block with a ball. Assuming all the balls have the same mass and you throw them with equal forward speeds, which of these four balls will transfer the most forward momentum to the block?

(A) A water-filled ball that breaks when it hits the block and sends its water in all directions equally. [0.0% picked]

(B) A sticky ball that remains attached to the block after hitting it. [4.4% picked]

(C) A bouncy ball that rebounds toward you at the same speed you threw it. [89.0% picked]

(D) A dead ball that comes to a complete stop after hitting the block and then falls onto the shelf. [6.6% picked]

Answer: (C) A bouncy ball that rebounds toward you at the same speed you threw it. [89.0% picked]

Why: YouTube Video

Problem 27:

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 child who jumps forward reaches the water before the child who jumps upward. [85.5% picked]

(B) the child who jumps upward reaches the water before the child who jumps forward. [0.4% picked]

(C) 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. [14.0% picked]

(D) the two children reach the water at the same moment and at the same distance from the dock. [0.0% picked]

Answer: (A) the child who jumps forward reaches the water before the child who jumps upward. [85.5% picked]

Why: YouTube Video

Problem 28:

A box of groceries sits motionless on a long wooden board. As you slowly lift one end of the board, so that it becomes a ramp, but the box remains in place. You keep increasing the steepness of the ramp until suddenly the box begins to slide down the ramp. Why did the box begin to move?

(A) The downhill force the box exerted on the board exceeded the uphill force the board exerted on the box. [8.3% picked]

(B) The board's support force on the box decreased to zero, so the box began to move. [2.6% picked]

(C) The increasing downhill ramp force on the box exceeded the maximum uphill force of static friction the board could exert on the box. [83.3% picked]

(D) The downhill force on the box exceeded the box's mass, so the box began to accelerate. [5.7% picked]

Answer: (C) The increasing downhill ramp force on the box exceeded the maximum uphill force of static friction the board could exert on the box. [83.3% picked]

Why: YouTube Video

Problem 29:

The smooth, flat surface of a ramp is tilted so that one end of the ramp is higher than the other. When a ball is on that ramp, the ball's

(A) velocity points uphill along the ramp. [1.3% picked]

(B) velocity points downhill along the ramp. [28.9% picked]

(C) acceleration points downhill along the ramp. [65.8% picked]

(D) acceleration points uphill along the ramp. [3.9% picked]

Answer: (C) acceleration points downhill along the ramp. [65.8% picked]

Why: YouTube Video

Problem 30:

You and your friend each carry a 1-liter bottle of water in your right hand from the first floor to the fifth floor. The two bottles started and finished motionless and side-by-side. If you walked up the stairs to the fifth floor and your friend took the elevator, which of you did more work on your water bottle?

(A) You did more work on your water bottle than your friend did on their water bottle. [4.8% picked]

(B) Your friend did more work on their water bottle than you did on your water bottle. [0.0% picked]

(C) Each of you did the same work on your water bottle. [94.3% picked]

(D) The person who reached the fifth floor first did more work on their water bottle than the other person did. [0.9% picked]

Answer: (C) Each of you did the same work on your water bottle. [94.3% picked]

Why: YouTube Video