Midterm Examination 2 Solutions

Problem 1:

Why must a sprinter lean forward as she picks up speed at the start of a race?

(A) She must lean in the direction of her velocity in order to increase that forward velocity. [48.8% picked]

(B) By leaning forward, she can use her weight to make her accelerate forward at the acceleration due to gravity. [17.5% picked]

(C) Her forward lean moves her closer to the finish line so that she arrives sooner. [0.4% picked]

(D) If she stood upright as the ground pushed her feet forward, the inertia of her upper body would cause her to tip over backward. [33.3% picked]

Answer: (D) If she stood upright as the ground pushed her feet forward, the inertia of her upper body would cause her to tip over backward. [33.3% picked]

Why: TBA

Problem 2:

An astronaut is orbiting the Earth in a spaceship. In which direction is the astronaut accelerating?

(A) The orbiting astronaut's acceleration is zero, so the acceleration has no direction. [0.4% picked]

(B) Toward the center of the Earth. [98.8% picked]

(C) Backward, in the direction opposite the astronaut's velocity. [0.4% picked]

(D) Forward, in the direction of the astronaut's velocity. [0.4% picked]

Answer: (B) Toward the center of the Earth. [98.8% picked]

Why: TBA

Problem 3:

At the grocery store, you select a melon with a mass of 2 kilograms and weigh it on a spring scale. The scale reports a weight of about 20 newtons. You then take the melon and scale to Planet X, where the acceleration due to gravity is only half the Earth's acceleration due to gravity. After setting up the spring scale, you again weigh the melon. What are the melon's mass and weight on Planet X?

(A) The melon's mass is 1 kilogram and its weight is about 20 newtons. [5.8% picked]

(B) The melon's mass is 2 kilograms and its weight is about 10 newtons. [72.1% picked]

(C) The melon's mass is 2 kilograms and its weight is about 20 newtons. [17.1% picked]

(D) The melon's mass is 1 kilogram and its weight is about 10 newtons. [5.0% picked]

Answer: (B) The melon's mass is 2 kilograms and its weight is about 10 newtons. [72.1% picked]

Why: TBA

Problem 4:

Water flows smoothly off the edge of a cliff and travels in the arc of a waterfall. As the water descends without hitting anything, what energy transformation is taking place?

(A) The water is converting kinetic energy into gravitational potential energy. [5.8% picked]

(B) The water is converting pressure potential energy into kinetic energy. [10.0% picked]

(C) The water is converting gravitational potential energy into kinetic energy. [84.2% picked]

(D) The water is converting kinetic energy into pressure potential energy. [0.0% picked]

Answer: (C) The water is converting gravitational potential energy into kinetic energy. [84.2% picked]

Why: TBA

Problem 5:

You are pulling some children in a wagon and are about to climb a hill. There are two paths up the hill, each of which has a constant slope as it winds around the hill to the top. The steeper path is half as long as the less steep path. Compare the uphill force you must exert on the wagon to keep it moving at a steady pace up each path. [neglect friction and any effects due to the air.]

(A) You must exert twice as much uphill force on the wagon when going up the steeper path. [84.2% picked]

(B) You must exert the same amount of uphill force on the wagon when going up either path. [4.2% picked]

(C) You must exert half as much uphill force on the wagon when going up the steeper path. [1.2% picked]

(D) You must exert four times as much uphill force on the wagon when going up the steeper path. [10.4% picked]

Answer: (A) You must exert twice as much uphill force on the wagon when going up the steeper path. [84.2% picked]

Why: TBA

Problem 6:

When the air moving passed an airplane flows over the top surface of the airplane's wing, that air speeds up significantly. One important consequence of the air's increase in speed is that the air's

(A) viscosity decreases. [22.9% picked]

(B) pressure decreases. [66.7% picked]

(C) pressure increases. [4.6% picked]

(D) viscosity increases. [5.8% picked]

Answer: (B) pressure decreases. [66.7% picked]

Why: TBA

Problem 7:

Two coins slide horizontally off a tabletop side-by-side at the same instant and begin to fall. The US quarter was moving twice as fast as the US dime when the two coins left the tabletop. Where and when do the coins hit the level floor beneath the table? [neglect any effects due to the air.]

(A) The two coins hit the ground at the same time, but the quarter lands about twice as far from the table as does the dime. [96.7% picked]

(B) The two coins hit the ground at the same time and land about the same distance from the table. [1.7% picked]

(C) The quarter hits the ground before the dime, but the two coins land about the same distance from the table. [0.4% picked]

(D) The dime hits the ground before the quarter, but the quarter lands about twice as far from the table as does the dime. [1.2% picked]

Answer: (A) The two coins hit the ground at the same time, but the quarter lands about twice as far from the table as does the dime. [96.7% picked]

Why: TBA

Problem 8:

You are driving Northward on a highway when you come to a smooth 90° turn toward the right. You maintain a constant speed around the turn and are soon driving Eastward. During the middle of the turn, in which direction were you accelerating?

(A) You were accelerating toward the South. [25.4% picked]

(B) You were accelerating toward the East. [10.0% picked]

(C) You were accelerating toward the Southeast. [57.1% picked]

(D) You were accelerating toward the Northeast. [7.5% picked]

Answer: (C) You were accelerating toward the Southeast. [57.1% picked]

Why: TBA

Problem 9:

In which of the following four situations are you accelerating? When you are

(A) traveling in an elevator that is moving upward at constant speed toward the top floor. [2.5% picked]

(B) bicycling around a circular path at a constant speed. [79.0% picked]

(C) sliding down a straight vertical pole at a steady speed. [12.6% picked]

(D) riding an escalator and are moving at constant velocity toward the second-floor exit. [5.9% picked]

Answer: (B) bicycling around a circular path at a constant speed. [79.0% picked]

Why: TBA

Problem 10:

A wooden ball floats motionless on the water in a large bowl. Only half of the ball is in the water. You pour some salad oil into the bowl and that oil floats on the surface of the water. What fraction of the ball is in the water now?

(A) More than half of the ball is in the water. [10.4% picked]

(B) There is not enough information in the question to determine an answer. [8.8% picked]

(C) Less than half of the ball is in the water. [77.1% picked]

(D) Half of the ball is in the water. [3.8% picked]

Answer: (C) Less than half of the ball is in the water. [77.1% picked]

Why: TBA

Problem 11:

As water in steady state flows straight up in a vertical pipe with a uniform diameter, that water

(A) transforms its gravitational potential energy into pressure potential energy. [3.8% picked]

(B) transforms its pressure potential energy into gravitational potential energy. [79.1% picked]

(C) transforms its gravitational potential energy into kinetic energy. [7.5% picked]

(D) transforms its kinetic energy into gravitational potential energy. [9.6% picked]

Answer: (B) transforms its pressure potential energy into gravitational potential energy. [79.1% picked]

Why: TBA

Problem 12:

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) greater on higher floors. [7.9% picked]

(B) the same on all floors, but the speed of the spraying water is smaller on higher floors. [70.0% picked]

(C) smaller on higher floors. [9.6% picked]

(D) the same on all floors, but the speed of the spraying water is greater on higher floors. [12.5% picked]

Answer: (B) the same on all floors, but the speed of the spraying water is smaller on higher floors. [70.0% picked]

Why: TBA

Problem 13:

A baseball pitcher throws the baseball so that it spins rapidly as it moves toward the catcher. The ball's top surface is spinning toward the pitcher and its bottom surface is spinning toward the catcher. What vertical force(s) is the ball experiencing as it travels through the air?

(A) Its weight downward. [10.4% picked]

(B) Its weight downward, a lift force downward, and a buoyant force upward. [16.2% picked]

(C) Its weight downward, a lift force upward, and a buoyant force upward. [72.5% picked]

(D) Its weight downward and a buoyant force upward. [0.8% picked]

Answer: (C) Its weight downward, a lift force upward, and a buoyant force upward. [72.5% picked]

Why: TBA

Problem 14:

You are rocking back and forth in a rocking chair. At what point in the rocking motion do you and the chair have the least total potential energy?

(A) When you and the chair are at equilibrium. [77.9% picked]

(B) When you are leaning forward as far as possible and you are at your lowest point above the ground. [14.6% picked]

(C) When you are leaning back as far as possible and the chair's back is at its lowest point above the ground. [2.1% picked]

(D) When the chair's seat is exactly horizontal. [5.4% picked]

Answer: (A) When you and the chair are at equilibrium. [77.9% picked]

Why: TBA

Problem 15:

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

(A) Your downward weight is so much greater than that of the nail that you dominate the hammer's motion. [1.2% picked]

(B) 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 period of time. [96.2% picked]

(C) 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. [1.7% picked]

(D) 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. [0.8% picked]

Answer: (B) 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 period of time. [96.2% picked]

Why: TBA

Problem 16:

You have just coasted down a hill on a bicycle and the pavement is curving upward as you move along it at constant speed. What is your apparent weight?

(A) Your apparent weight is downward and less than your actual weight. [5.0% picked]

(B) Your apparent weight is upward and less than the amount of your actual weight. [9.6% picked]

(C) Your apparent weight is downward and equal to your actual weight. [23.3% picked]

(D) Your apparent weight is downward and greater than your actual weight. [62.1% picked]

Answer: (D) Your apparent weight is downward and greater than your actual weight. [62.1% picked]

Why: TBA

Problem 17:

Space-bound rockets are launched from ground-based launch pads and their torch-like exhaust plumes scorch the ground during the launches. If there were no ground or other horizontal surface beneath a rocket at launch, how would that change affect the upward thrust force on the rocket?

(A) The upward thrust force would be only half as strong. [15.4% picked]

(B) It would have almost no effect on the upward thrust force. [77.9% picked]

(C) The upward thrust force would be only a quarter as strong. [4.6% picked]

(D) The upward thrust force would be zero. [2.1% picked]

Answer: (B) It would have almost no effect on the upward thrust force. [77.9% picked]

Why: TBA

Problem 18:

Water is flowing through a horizontal pipe in laminar, steady-state flow. Halfway along the pipe, its diameter decreases and the water must continue on through that narrow second half. Compared to when it was in the wide first half of the pipe, water in the narrow second half of the pipe is moving

(A) faster and its pressure is smaller. [61.2% picked]

(B) slower and its pressure is smaller. [22.1% picked]

(C) slower and its pressure is larger. [15.0% picked]

(D) faster and its pressure is larger. [1.7% picked]

Answer: (A) faster and its pressure is smaller. [61.2% picked]

Why: TBA

Problem 19:

Two shiny plastic balloons are identical in every way except that one contains air and the other contains helium. Which one of the following statements about the two balloons is true?

(A) They have the same particle density, different densities, and different weights. [91.2% picked]

(B) They have the same particle density, the same density, and different weights. [2.5% picked]

(C) They have different particle densities, the same density, and different weights. [1.7% picked]

(D) They have different particle densities, different densities, and different weights. [4.6% picked]

Answer: (A) They have the same particle density, different densities, and different weights. [91.2% picked]

Why: TBA

Problem 20:

Water is flowing through a hose that is lying on a slippery surface. Most of the hose is straight, but it bends 90° to the right just before its open end (see figure). You increase the water flowing through the hose and the bent portion of the hose begins to slide to the left (opposite the end). What pushes this portion of the hose toward the left?

(A) The water that is flowing toward the bend at constant velocity. [4.6% picked]

(B) The increased water pressure inside the outer edge of the bend. [67.5% picked]

(C) The decreased water pressure inside the outer edge of the bend. [26.2% picked]

(D) The water that is flowing out of the open end at constant velocity. [1.7% picked]

Answer: (B) The increased water pressure inside the outer edge of the bend. [67.5% picked]

Why: TBA

Problem 21:

A player kicks a soccer ball directly toward the goal. As the ball moves through the air, what part(s) of the ball experiences the greatest air pressure?

(A) The upper part of the ball experiences the greatest air pressure. [0.4% picked]

(B) The part of the ball farthest from the goal experiences the greatest air pressure. [14.6% picked]

(C) The part of the ball closest to the goal experiences the greatest air pressure. [83.3% picked]

(D) The parts of the ball closest and farthest from the goal both experience the greatest air pressure. [1.7% picked]

Answer: (C) The part of the ball closest to the goal experiences the greatest air pressure. [83.3% picked]

Why: TBA

Problem 22:

When you and your friend arrive at the ball, people gasp in delight and accidently start the chandelier swinging back and forth. When during that swinging motion is the chandelier moving fastest?

(A) When it is passing through equilibrium. [87.4% picked]

(B) At the end of its swing closest the door through which you entered. [1.3% picked]

(C) At either end of its swing.. [1.7% picked]

(D) At the end of its swing farthest from the door through which you entered. [9.6% picked]

Answer: (A) When it is passing through equilibrium. [87.4% picked]

Why: TBA

Problem 23:

You're visiting Candyland, where the rivers contain honey instead of water. There are rocks made of rock candy in one of the rivers, but even though the honey flows quickly past those rocks, there is no turbulence. Rafting on the honey rivers is boring. A river of water would exhibit turbulent "white water," so why doesn't a river of honey exhibit turbulence?

(A) The honey's large viscosity orders the flow and keeps it laminar. [75.8% picked]

(B) The honey's large momentum orders the flow and keeps it laminar. [4.2% picked]

(C) The honey's large force orders the flow and keeps it laminar. [12.1% picked]

(D) The honey's large density orders the flow and keeps it laminar. [7.9% picked]

Answer: (A) The honey's large viscosity orders the flow and keeps it laminar. [75.8% picked]

Why: TBA

Problem 24:

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 8th floor. [80.0% picked]

(B) The 12th floor. [10.4% picked]

(C) The 10th floor. [6.2% picked]

(D) The 6th floor. [3.3% picked]

Answer: (A) The 8th floor. [80.0% picked]

Why: TBA

Problem 25:

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

(A) It gradually reduces the weight of the rocket as it burns, so that gravity pushes the remaining spaceship upward. [1.7% picked]

(B) It provides the enormous energy required to heat the surrounding air so that that hot air pushes the spaceship forward. [15.4% picked]

(C) It produces the vibrations that accelerate the spaceship forward at equilibrium. [7.5% picked]

(D) It carries away an enormous amount of backward momentum and gives the remaining spaceship an enormous amount of forward momentum. [75.4% picked]

Answer: (D) It carries away an enormous amount of backward momentum and gives the remaining spaceship an enormous amount of forward momentum. [75.4% picked]

Why: TBA

Problem 26:

You remove a plastic container and its lid from the hot dishwasher. After snapping the lid on the container, you put the sealed container of air on the counter to cool. When you examine the container later, you find that its lid has bowed inward significantly, reducing the container's volume. How has the cooling process affected the air pressure and air density in the container?

(A) The air's pressure has increased and its density has decreased. [0.4% picked]

(B) The air's pressure has decreased and its density has increased. [42.1% picked]

(C) The air's pressure has increased and its density has increased. [54.6% picked]

(D) The air's pressure has decreased and its density has decreased. [2.9% picked]

Answer: (B) The air's pressure has decreased and its density has increased. [42.1% picked]

Why: TBA

Problem 27:

Your friends have purchased an old house that has significant mineral accumulation inside its pipes. Every pipe in the house is now effectively half the diameter it was when new. For example, a pipe that had a 1-inch diameter opening when new now has a 0.5-inch diameter opening. If it took 10 minutes to fill the bathtub when the house was new, how long does it now take to fill the bathtub?

(A) 20 minutes [0.8% picked]

(B) 160 minutes [85.8% picked]

(C) 80 minutes [8.8% picked]

(D) 40 minutes [4.6% picked]

Answer: (B) 160 minutes [85.8% picked]

Why: TBA

Problem 28:

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

(A) Pressure drag forces— the drag forces associated with unbalanced air pressure. [15.8% picked]

(B) Viscous drag forces—the drag forces associated with air's viscosity. [77.5% picked]

(C) Lift forces. [5.4% picked]

(D) Buoyant forces. [1.2% picked]

Answer: (B) Viscous drag forces—the drag forces associated with air's viscosity. [77.5% picked]

Why: TBA

Problem 29:

An airplane is cruising horizontally at 30,000 feet and maintains constant velocity. How does the airplane avoid falling?

(A) Because the airplane is so high, it is weightless and does not fall. [4.2% picked]

(B) The airplane experiences an air resistance force equal in amount to the airplane's weight, so the airplane doesn't fall. [2.5% picked]

(C) The airplane pushes downward on the passing airstream with a force equal to the airplane's weight and the airstream exerts an equal but opposite force upward on the airplane. [86.7% picked]

(D) The airplane displaces air and consequently experiences an upward buoyant force equal in amount to the airplane's weight. [6.7% picked]

Answer: (C) The airplane pushes downward on the passing airstream with a force equal to the airplane's weight and the airstream exerts an equal but opposite force upward on the airplane. [86.7% picked]

Why: TBA

Problem 30:

Water is flowing gently out of the end of a garden hose. You block off most of the hose's opening with your thumb and now the water sprays out at high speed because you

(A) are doing work on the water with your thumb and greatly increasing its ordered energy. [15.8% picked]

(B) are keeping air from getting into the hose and reducing the water pressure. [12.5% picked]

(C) have reduced the water's speed in the hose so that it wastes less of its ordered energy. [70.0% picked]

(D) are compressing the water and increasing its density. [1.7% picked]

Answer: (C) have reduced the water's speed in the hose so that it wastes less of its ordered energy. [70.0% picked]

Why: TBA