Midterm Examination 2 Solutions

Problem 1:

When you pour milk from a pitcher into a bowl, the flow is likely to become turbulent. When you pour honey from a jar into a bowl, however, the flow is likely to remain laminar. Compared to milk, honey is more likely to exhibit laminar flow because honey's larger

(A) weight tends to keep the flow orderly. [0.4% picked]

(B) inertia tends to keep the flow orderly. [0.0% picked]

(C) viscosity tends to keep the flow orderly. [97.8% picked]

(D) density tends to keep the flow orderly. [1.7% picked]

Answer: (C) viscosity tends to keep the flow orderly. [97.8% picked]

Why: TBA

Problem 2:

When a ray of sunlight passes through a darkened room, you see dust particles seemingly suspended in the air. What upward forces are primarily responsible for keeping those dust particles from falling to the ground?

(A) Pressure drag forces— the drag forces associated with unbalanced air pressure. [11.8% picked]

(B) Viscous drag forces—the drag forces associated with air's viscosity. [72.9% picked]

(C) Lift forces. [1.7% picked]

(D) Buoyant forces. [13.5% picked]

Answer: (B) Viscous drag forces—the drag forces associated with air's viscosity. [72.9% picked]

Why: TBA

Problem 3:

You are driving forward in a sports car with the windows closed. A squirrel runs across the road and you slam on the brakes. As the car slows to a stop, missing the squirrel, the air pressure inside the car is

(A) lower near the front of the car than near the back of the car. [4.8% picked]

(B) higher near the front of the car than near the back of the car. [89.5% picked]

(C) zero. [1.3% picked]

(D) the same near the front of the car as near the back of the car. [4.4% picked]

Answer: (B) higher near the front of the car than near the back of the car. [89.5% picked]

Why: TBA

Problem 4:

As part of your new circus routine, you jump off a high platform onto a trampoline. You bounce up and down spectacularly before eventually settling at equilibrium on the trampoline. When during your descent from the high platform do you first stop accelerating downward?

(A) When you touch the trampoline for the first time. [2.2% picked]

(B) When you pass through equilibrium on the trampoline for the first time. [87.3% picked]

(C) When you reach your lowest point on the trampoline in the middle of your first bounce. [10.0% picked]

(D) Just before you touch the trampoline for the first time. [0.4% picked]

Answer: (B) When you pass through equilibrium on the trampoline for the first time. [87.3% picked]

Why: TBA

Problem 5:

When you try to balance a pencil on its sharpened point, it invariably tips over. That's because the upright pencil

(A) has no equilibrium and is always accelerating. [0.4% picked]

(B) cannot be motionless. [0.4% picked]

(C) cannot have zero angular velocity. [0.0% picked]

(D) has an unstable equilibrium and it accelerates away from that equilibrium after the slightest disturbance. [99.1% picked]

Answer: (D) has an unstable equilibrium and it accelerates away from that equilibrium after the slightest disturbance. [99.1% picked]

Why: TBA

Problem 6:

You're filming a movie and you arrange for two unoccupied cars to drive horizontally off a tall cliff. The Mercedes sedan weighs twice as much as the Mini Cooper, but the Mini Cooper is traveling at twice the velocity of the Mercedes when the two cars careen off the cliff side-by-side. In this situation, the two cars hit the level ground below the cliff

(A) at approximately the same time and at the same distance from the cliff. [12.7% picked]

(B) at approximately the same distance from the cliff, but the Mercedes sedan hits much sooner than the Mini Cooper. [0.0% picked]

(C) at approximately the same time, but the Mini Cooper hits considerably farther from the cliff than the Mercedes sedan. [86.9% picked]

(D) at approximately the same time, but the Mercedes sedan hits considerably farther from the cliff than the Mini Cooper. [0.4% picked]

Answer: (C) at approximately the same time, but the Mini Cooper hits considerably farther from the cliff than the Mercedes sedan. [86.9% picked]

Why: TBA

Problem 7:

As a rocket takes off from its launch pad, a plume of flaming exhaust gas squirts out of its engine and scorches the launch pad. What is exerting the upward force on the rocket that causes the rocket to accelerate upward?

(A) The launch pad is pushing the rocket upward. [1.8% picked]

(B) The entire Earth is pushing the rocket upward. [1.8% picked]

(C) The exhaust gas is pushing the rocket upward. [93.9% picked]

(D) The air is pushing the rocket upward. [2.6% picked]

Answer: (C) The exhaust gas is pushing the rocket upward. [93.9% picked]

Why: TBA

Problem 8:

You are at a barbeque and the bottle of hot sauce is almost empty. You put the cap on the bottle and swing it rapidly in a circle with its cap end pointing outward. The remaining sauce collects just inside the cap and you are able to extract enough to spice up your vanilla ice cream. This technique works because

(A) the force of the hot sauce's momentum pushes it toward the cap end of the bottle. [1.7% picked]

(B) viscous forces push the hot sauce toward the cap end of the bottle. [0.9% picked]

(C) centrifugal force pushes the hot sauce toward the cap end of the bottle. [8.7% picked]

(D) the bottle's rapid inward acceleration leaves the hot sauce behind so that it drifts toward the cap end of the bottle. [88.6% picked]

Answer: (D) the bottle's rapid inward acceleration leaves the hot sauce behind so that it drifts toward the cap end of the bottle. [88.6% picked]

Why: TBA

Problem 9:

A satellite is traveling around the earth in a circular orbit. It briefly fires its rocket engine to increase its speed in the forward direction; it is suddenly going faster but its direction of travel didn't change. As a result of this speed increase, the orbiting satellite's average distance from the center of the earth

(A) increases. [68.6% picked]

(B) remains unchanged, but the orbit is no longer circular. [20.1% picked]

(C) decreases. [1.3% picked]

(D) remains unchanged and the orbit continues to be circular. [10.0% picked]

Answer: (A) increases. [68.6% picked]

Why: TBA

Problem 10:

You are arm-wrestling a friend. While you are gradually winning the competition, compare the torque that your arm exerts on your friend's arm to the torque your friend's arm exerts on your arm.

(A) Those two torques are equal in amount but opposite in direction. [64.6% picked]

(B) The torque you exert on your friend is greater in amount than the torque your friend exerts on you. [31.0% picked]

(C) The two torques are equal. [3.9% picked]

(D) The torque you exert on your friend is less in amount than the torque your friend exerts on you. [0.4% picked]

Answer: (A) Those two torques are equal in amount but opposite in direction. [64.6% picked]

Why: TBA

Problem 11:

A gymnasium has a large ventilation duct near its ceiling. When air that was moving at uniform velocity and pressure through a straight section of duct enters a bend in the duct and it curves, its pressure is greatest at the [neglect any effects due to viscosity or turbulence]

(A) inside of the bend and its speed is greatest at the outside of the bend. [5.2% picked]

(B) outside of the bend and its speed is greatest at the inside of the bend. [90.0% picked]

(C) inside of the bend and its speed is greatest at the inside of the bend. [0.9% picked]

(D) outside of the bend and its speed is greatest at the outside of the bend. [3.9% picked]

Answer: (B) outside of the bend and its speed is greatest at the inside of the bend. [90.0% picked]

Why: TBA

Problem 12:

A scuba tank is a metal bottle that can be filled with air and used to supply that air to a person who is underwater. Suppose you fill a scuba tank with air until the particle density in the tank is 100 times the particle density of the surrounding air. At room temperature, what are the density and pressure of the air inside that tank?

(A) The density is about 100 times atmospheric density and the pressure is the same as atmospheric pressure. [6.6% picked]

(B) The density is the same as atmospheric density and the pressure is about 100 times atmospheric pressure. [6.1% picked]

(C) The density is about 100 times atmospheric density and the pressure is about 100 times atmospheric pressure. [85.6% picked]

(D) The density is the same as atmospheric density and the pressure is the same as atmospheric pressure. [1.7% picked]

Answer: (C) The density is about 100 times atmospheric density and the pressure is about 100 times atmospheric pressure. [85.6% picked]

Why: TBA

Problem 13:

On a hot afternoon, you seal your empty plastic water bottle and thus trap the hot air inside it. Later in the day, the bottle and its contents are much cooler and the elastic sides of the bottle are now bent inward. The bottle's volume has decreased. Based on these changes, you can be certain that the air pressure inside the bottle has

(A) decreased and the density inside the bottle has stayed the same. [8.3% picked]

(B) decreased and the density inside the bottle has increased. [81.2% picked]

(C) decreased and the density inside the bottle has decreased. [6.6% picked]

(D) stayed the same and the density inside the bottle has increased. [3.9% picked]

Answer: (B) decreased and the density inside the bottle has increased. [81.2% picked]

Why: TBA

Problem 14:

At a baseball game, a pitcher throws a fastball toward the catcher. When the ball is half way to home plate and no one is touching it, where on the ball is the air pressure highest?

(A) At the top of the ball. [1.3% picked]

(B) At the front of the ball—the side nearest home plate. [94.3% picked]

(C) At the back of the ball—the side nearest the pitcher. [3.1% picked]

(D) At the bottom of the ball. [1.3% picked]

Answer: (B) At the front of the ball—the side nearest home plate. [94.3% picked]

Why: TBA

Problem 15:

You are trying to set the world's record for drinking water through the tallest drinking straw. What could you do to have the best chance of setting this record?

(A) Use the narrowest drinking straw you can find. [3.9% picked]

(B) Use the widest drinking straw you can fit in your mouth. [0.4% picked]

(C) Choose a good-weather day when the atmospheric pressure is as large as possible. [92.6% picked]

(D) Choose a bad-weather day when the atmospheric pressure is as small as possible. [3.1% picked]

Answer: (C) Choose a good-weather day when the atmospheric pressure is as large as possible. [92.6% picked]

Why: TBA

Problem 16:

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

(A) it pushes up on your foot just as hard as your foot pushes on it, unlike the soft dry sand. [5.2% picked]

(B) it stops the downward motion of your foot faster and thus absorbs less of your momentum. [14.4% picked]

(C) its water content gives it more mass and that prevents it from absorbing energy. [0.9% picked]

(D) it barely moves downward as you push downward on it, so you do almost zero work on it. [79.5% picked]

Answer: (D) it barely moves downward as you push downward on it, so you do almost zero work on it. [79.5% picked]

Why: TBA

Problem 17:

You drop an unopened metal can of soup and it lands bottom-first on the floor. When you inspect the can, you see that it has dented outward in places. Where did it dent outward and why?

(A) The floor pushed the bottom of the can upward extremely hard on impact and the momentum of that upward force dented the can outward near its top. [3.9% picked]

(B) The soup bounced upward after the can hit the floor and when the soup hit the top of the can, it dented the top of the can outward. [2.2% picked]

(C) The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom. [93.4% picked]

(D) The impact between the floor and the bottom of the can buckled the can outward near its middle (halfway between its top and bottom). [0.4% picked]

Answer: (C) The soup pressure near the bottom of the can increased dramatically as the soup transferred its momentum suddenly to the floor and that pressure dented the can outward near its bottom. [93.4% picked]

Why: TBA

Problem 18:

Firefighters are battling a fire on the 10th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the water steadily up to the 8th floor. They carry the nozzle end of that fire hose to the top of a 4-story-tall ladder and again point the nozzle upward. Their fire hose can now shoot the water steadily up to which floor?

(A) The 6th floor. [0.4% picked]

(B) The 12th floor. [1.3% picked]

(C) The 8th floor. [96.1% picked]

(D) The 10th floor. [2.2% picked]

Answer: (C) The 8th floor. [96.1% picked]

Why: TBA

Problem 19:

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 support force from the seat is pushing you toward the right. [0.9% picked]

(B) There is no force pushing you toward the right. [97.4% picked]

(C) The force of your momentum is pushing you toward the right. [1.7% picked]

(D) A friction force from the seat is pushing you toward the right. [0.0% picked]

Answer: (B) There is no force pushing you toward the right. [97.4% picked]

Why: TBA

Problem 20:

To win a prize at the fair, you must toss a basketball into a stiff fruit basket and have the ball remain in that basket. The basket is tipped toward you and nailed to the wall. Its lower lip has just enough uphill slope to keep a stationary ball from rolling out. Each time you throw the ball into the basket, the ball bounces and rolls onto the floor. The conserved quantity that the basketball is unable to get rid of in order to stay in the basket is

(A) potential energy. [1.3% picked]

(B) kinetic energy. [11.4% picked]

(C) energy. [67.7% picked]

(D) momentum. [19.7% picked]

Answer: (C) energy. [67.7% picked]

Why: TBA

Problem 21:

Because of bad planning during the design and construction of a high-rise apartment building, all 50 floors of the building receive their water from a single pipe. That pipe is fed from a water tank located on the building's roof. On opening day, residents on various floors begin taking showers and have different experiences. They quickly discover that the total energy per liter in the spraying water is (neglecting any effects of viscosity and friction)

(A) the same on all floors, but the speed of the spraying water is smaller on higher floors. [88.6% picked]

(B) greater on higher floors. [1.3% picked]

(C) the same on all floors, but the speed of the spraying water is greater on higher floors. [7.0% picked]

(D) smaller on higher floors. [3.1% picked]

Answer: (A) the same on all floors, but the speed of the spraying water is smaller on higher floors. [88.6% picked]

Why: TBA

Problem 22:

You and your friend are riding a carousel. Your horses are side-by-side and your friend is closer to the center of the carousel than you are. As the carousel turns steadily, which of the following is true?

(A) You and your friend experience equal inward accelerations, but your friend's speed is larger than your speed. [4.8% picked]

(B) You and your friend experience equal inward accelerations, but your speed is larger than your friend's speed. [6.6% picked]

(C) Your friend experiences a larger inward acceleration than you do. [9.2% picked]

(D) You experience a larger inward acceleration than your friend does. [79.5% picked]

Answer: (D) You experience a larger inward acceleration than your friend does. [79.5% picked]

Why: TBA

Problem 23:

A tennis ball is often hit with topspin (its top surface moves forward as it spins), so that it deflects the passing air upward. How does this ball move?

(A) It accelerates downward faster than it would if it were simply falling. [96.1% picked]

(B) It accelerates downward more slowly than it would if it were simply falling. [1.7% picked]

(C) It accelerates upward. [1.3% picked]

(D) It does not accelerate vertically. [0.9% picked]

Answer: (A) It accelerates downward faster than it would if it were simply falling. [96.1% picked]

Why: TBA

Problem 24:

You are designing a chair for a museum. While your chair will be a work of art, it must also be practical and it mustn't tip over easily. To ensure that it stays upright, you design it so that its

(A) angular velocity always remains constant if you tip it. [1.3% picked]

(B) velocity always remains constant if you tip it. [0.0% picked]

(C) center of gravity always rises if you tip it. [74.2% picked]

(D) center of gravity always descends if you tip it. [24.5% picked]

Answer: (C) center of gravity always rises if you tip it. [74.2% picked]

Why: TBA

Problem 25:

Suppose a soccer ball is approaching your leg at 30 km/h and your leg is moving toward the soccer ball at 30 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) 60 km/h [7.0% picked]

(B) 30 km/h [2.2% picked]

(C) 120 km/h [0.9% picked]

(D) 90 km/h [90.0% picked]

Answer: (D) 90 km/h [90.0% picked]

Why: TBA

Problem 26:

A carbon dioxide molecule has more mass than the average air molecule. You blow two large soap bubbles, one filled with ordinary air and one filled with carbon dioxide. The bubbles look identical and they are both at room temperature. How do the bubbles compare?

(A) They have equal weights, they contain the same number of gas particles, and their pressures are equal, but the carbon dioxide bubble experiences less buoyant force than does the air bubble. [3.5% picked]

(B) They experience the same buoyant force, they have equal weights, and they contain the same number of gas particles, but the carbon dioxide bubble has a higher pressure inside it than does the air bubble. [3.5% picked]

(C) They contain the same number of gas particles, their pressures are equal, and they experience the same buoyant force, but the carbon dioxide bubble weighs more than the air bubble. [88.2% picked]

(D) Their pressures are equal, they experience the same buoyant force, and they have equal weights, but the carbon dioxide bubbles contains fewer gas particles than the air bubble. [4.8% picked]

Answer: (C) They contain the same number of gas particles, their pressures are equal, and they experience the same buoyant force, but the carbon dioxide bubble weighs more than the air bubble. [88.2% picked]

Why: TBA

Problem 27:

You place a stiff ruler on a pencil and then stack coins on the two ends of the ruler until it is balanced. The ruler and its coins are then able to remain motionless on the pencil pivot. What did you have to do to achieve this balanced condition?

(A) The weights of the coins on each side of the ruler had to be equal. [0.4% picked]

(B) The forces that the coins on each side of the ruler exerted on the ruler had to be equal. [4.4% picked]

(C) The overall center of gravity of the ruler and its coins had to coincide with the overall center of mass of the ruler and its coins. [20.1% picked]

(D) The overall center of gravity of the ruler and its coins had to be directly above the pencil pivot. [75.1% picked]

Answer: (D) The overall center of gravity of the ruler and its coins had to be directly above the pencil pivot. [75.1% picked]

Why: TBA

Problem 28:

At launch, a rocket bound for the moon is mostly fuel. What purpose does all that fuel serve?

(A) It carries away an enormous amount of backward momentum and gives the remaining spaceship an enormous amount of forward momentum. [92.1% picked]

(B) It produces the vibrations that accelerate the spaceship forward at equilibrium. [0.0% picked]

(C) It gradually reduces the weight of the rocket as it burns, so that gravity pushes the remaining spaceship upward. [2.6% picked]

(D) It provides the enormous energy required to heat the surrounding air so that that hot air pushes the spaceship forward. [5.2% picked]

Answer: (A) It carries away an enormous amount of backward momentum and gives the remaining spaceship an enormous amount of forward momentum. [92.1% picked]

Why: TBA

Problem 29:

You are pulling a wagon forward on a level sidewalk and the wagon is experiencing no other horizontal force. As you exert your forward force on the wagon, the wagon exerts a backward force on you. When do those two forces cancel so that the wagon does not accelerate?

(A) When you and the wagon have equal velocities. [3.5% picked]

(B) They never cancel because they act on different objects. [74.2% picked]

(C) When you and the wagon have equal masses. [0.0% picked]

(D) When those two forces are equal in amount but opposite in direction so that they sum to zero. [22.3% picked]

Answer: (B) They never cancel because they act on different objects. [74.2% picked]

Why: TBA

Problem 30:

As a soccer ball travels from the kicker to the goal, air approaches the front of the ball and they interact. The ball loses much of its forward momentum during its flight because

(A) the air on the sides of the ball is higher than atmospheric pressure and it squeezes the ball so that the ball slows down. [0.9% picked]

(B) the air at the front of the ball is higher than atmospheric pressure while the air behind the ball is approximately atmospheric pressure. [83.3% picked]

(C) the air at the front of the ball is lower than atmospheric pressure while the air behind the ball is higher than atmospheric pressure. [7.0% picked]

(D) the air on the sides of the ball is lower than atmospheric pressure and it sucks the ball so that the ball slows down. [8.8% picked]

Answer: (B) the air at the front of the ball is higher than atmospheric pressure while the air behind the ball is approximately atmospheric pressure. [83.3% picked]

Why: TBA