Physics 1050 - How Things Work - Fall 2012 Final Examination Solutions

Final Examination Solutions

Video solutions will be posted eventually.

Problem 1:

A builder outsourced the design and construction of a high-rise apartment building, as a result, all 50 floors 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) smaller on higher floors. [4.9% picked]

(B) greater on higher floors. [0.8% picked]

(D) the same on all floors, but the speed of the spraying water is smaller on higher floors. [89.7% picked]

Answer: (D) the same on all floors, but the speed of the spraying water is smaller on higher floors. [89.7% picked]

Why: TBA

Problem 2:

An astronaut is orbiting the Earth inside the International Space Station and she is hovering in the middle of the cabin. The net force on that astronaut is

(A) zero and she is not accelerating. [15.2% picked]

(B) zero, but her velocity is changing as her path bends around the Earth. [10.3% picked]

(D) equal to her weight and she is accelerating downward (toward the center of the Earth). [66.3% picked]

Answer: (D) equal to her weight and she is accelerating downward (toward the center of the Earth). [66.3% picked]

Why: TBA

Problem 3:

An automobile engine "knocks" when the gas in its cylinder ignites spontaneously rather than waiting for the sparkplug to ignite it. During which of the 4 strokes can this premature ignition occur?

(A) The compression stroke (when the gas in the cylinder is being compressed). [77.0% picked]

(B) The power stroke (when the gas in the cylinder is pushing the piston out of the cylinder). [11.1% picked]

(D) The intake stroke (when gas is being added to the cylinder). [7.4% picked]

Answer: (A) The compression stroke (when the gas in the cylinder is being compressed). [77.0% picked]

Why: TBA

Problem 4:

You add sugar to the coffee in your mug and put the lid on. The mug is thermally insulated and sealed, so nothing goes in or out. At first, the mug contains pure coffee and undissolved sugar. Soon, the mug contains sweetened coffee and no undissolved sugar. Compare the entropies (disorders) of these two situations.

(A) They have the same entropy because entropy is conserved. [2.5% picked]

(B) They have the same entropy because coffee's entropy is independent of its purity. [1.2% picked]

(D) The mug of pure coffee and undissolved sugar has more entropy. [5.8% picked]

Answer: (C) The mug of sweetened coffee has more entropy. [90.5% picked]

Why: TBA

Problem 5:

You're at a lovely outdoor café in New Orleans during the summer and the ice in your glass of water is gradually melting. That glass contained 75% ice and 25% water when it was first poured but now contains 25% ice and 75% water. How has the temperature of the water in that glass changed since it was poured?

(A) The temperature of the water has increased by 2 °F. [2.1% picked]

(B) The temperature of the water has remained constant. [95.1% picked]

(D) The temperature of the water has increased by 8 °F. [0.0% picked]

Answer: (B) The temperature of the water has remained constant. [95.1% picked]

Why: TBA

Problem 6:

Near sea level, water normally boils at about 212 °F (100 °C). However, a microwave oven can heat water in a glass container above that temperature if

(A) the glass container does not conduct electricity. [3.3% picked]

(B) the water is very pure. [1.2% picked]

(D) nothing helps form the initial steam bubbles. [92.2% picked]

Answer: (D) nothing helps form the initial steam bubbles. [92.2% picked]

Why: TBA

Problem 7:

A pitcher throws a spinning baseball toward home plate and the baseball curves toward the pitcher's right. Why did the baseball curve?

(A) The spinning baseball had angular momentum toward the left and the ball reacted to this angular momentum by pushing itself toward the right. [5.3% picked]

(B) The spinning baseball deflected the passing air toward the pitcher's left. [90.1% picked]

(C) The pitcher pushed the spinning baseball toward the right and the pitcher's rightward force caused the baseball to accelerator toward the right. [0.8% picked]

(D) The spinning baseball had angular momentum toward the right and that angular momentum bent its path toward the right. [3.7% picked]

Answer: (B) The spinning baseball deflected the passing air toward the pitcher's left. [90.1% picked]

Why: TBA

Problem 8:

You are filling a plastic party balloon with helium. At first, the under-filled balloon doesn't float. But as you keep filling it with helium, it eventually begins floating because

(A) the average pressure of the helium-filled balloon increases as it fills and its pressure eventually becomes greater than that of the air it displaces. [1.6% picked]

(B) the average density of the helium-filled bag decreases as it fills and its density eventually becomes less than that of the air it displaces. [90.5% picked]

(C) the helium-filled bag's weight decreases as you put more lightweight helium particles inside it and eventually it becomes weightless. [2.5% picked]

(D) the upward buoyant force on a balloon full of helium is larger than the buoyant force on an identical balloon full of air. [5.3% picked]

Answer: (B) the average density of the helium-filled bag decreases as it fills and its density eventually becomes less than that of the air it displaces. [90.5% picked]

Why: TBA

Problem 9:

You are riding a bus that is moving straight ahead at high speed. The bus comes to a curve in the road 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. [1.2% picked]

(B) There is no force pushing you toward the right. [90.1% picked]

(D) The force of your momentum is pushing you toward the right. [8.6% picked]

Answer: (B) There is no force pushing you toward the right. [90.1% picked]

Why: TBA

Problem 10:

Although you meant to sign up for "semester abroad," you accidently signed up for "semester aboard." You are now aboard a tiny, perfectly insulated spaceship in deep space. What a mistake! As you play solitaire aboard your tiny ship,

(A) the energy of the ship and its contents is decreasing. [2.5% picked]

(B) the entropy (disorder) of the ship and its contents is increasing. [79.8% picked]

(D) the energy of the ship and its contents is increasing. [5.8% picked]

Answer: (B) the entropy (disorder) of the ship and its contents is increasing. [79.8% picked]

Why: TBA

Problem 11:

If you try to cook vegetables with 100 °C air, it takes a long time. But if you cook those same vegetables with 100 °C steam, they cook quickly. This is because the steam

(A) causes moisture inside the vegetables to boil and transfer heat to the vegetables. [4.1% picked]

(B) condenses on the colder vegetables and absorbs a large amount of heat from the vegetables. [1.2% picked]

(D) condenses on the colder vegetables and releases a large amount of heat to the vegetables. [94.2% picked]

Answer: (D) condenses on the colder vegetables and releases a large amount of heat to the vegetables. [94.2% picked]

Why: TBA

Problem 12:

Two identical rooms at identical temperatures contain identical collections of atoms. In one room that collection consists of moist air and a small empty cup and in the other room that collection consists of dry air and a small cup of water. Which of the two rooms has the greater entropy (disorder)?

(A) The room containing dry air and the full bowl has the greater entropy. [2.5% picked]

(B) The two rooms have equal entropies because they have identical temperatures. [1.6% picked]

(D) The two rooms have equal entropies because they contain identical collections of atoms. [1.2% picked]

Answer: (C) The room containing moist air and an empty bowl has the greater entropy. [94.7% picked]

Why: TBA

Problem 13:

Since a water molecule weighs less than the average air molecule, humid air is less dense than dry air. Suppose that the humidity of the air increases significantly during an orchestra concert. How will the resulting decrease in air density affect the wind instruments (e.g., flutes, clarinets, oboes)?

(A) Their volumes (amplitudes of vibration) will increase. [9.1% picked]

(B) Their pitches (frequencies of vibration) will increase. [57.2% picked]

(D) Their pitches (frequencies of vibration) will decrease. [25.1% picked]

Answer: (B) Their pitches (frequencies of vibration) will increase. [57.2% picked]

Why: TBA

Problem 14:

You are rearranging the game room in your mansion and are now pushing the pool table forward across the floor. The floor is exerting a backward frictional force of 200 newtons on the table as the table moves directly forward at a constant speed. What force are you exerting on the table?

(A) A forward force of more than 200 newtons. [1.6% picked]

(B) You are exerting zero force on the table because the table is coasting. [3.3% picked]

(D) A forward force of less than 200 newtons. [0.4% picked]

Answer: (C) A forward force of 200 newtons. [94.2% picked]

Why: TBA

Problem 15:

To catch a football successfully, you should allow the ball to push your hands in the direction of its travel. Allowing your hands to move with the football is crucial because it allows you to

(A) extract energy from the football. [84.4% picked]

(B) extract momentum from the football. [12.8% picked]

(D) add energy to the football. [1.6% picked]

Answer: (A) extract energy from the football. [84.4% picked]

Why: TBA

Problem 16:

Two squirrels are fighting over a tasty acorn. Each squirrel is pulling on one side of the acorn and they are not making any progress. At last, the female squirrel holds her side of the acorn motionless as the male squirrel tears his half of the acorn away. Tearing the acorn in half required energy and that energy was provided by

(A) both squirrels. [1.6% picked]

(B) neither squirrel. It was instead provided by chemical potential energy in the acorn itself. [0.0% picked]

(D) the female squirrel. [1.2% picked]

Answer: (C) the male squirrel. [97.1% picked]

Why: TBA

Problem 17:

As air flows around a fast-moving ball, that air has difficulty maintaining laminar flow from the middle of the ball to the back of the ball. During the air's passage from the ball's middle to its back, the pressure along each streamline

(A) increases and the airflow speeds up. [5.3% picked]

(B) decreases and the airflow slows down. [10.7% picked]

(D) decreases and the airflow speeds up. [23.0% picked]

Answer: (C) increases and the airflow slows down. [60.9% picked]

Why: TBA

Problem 18:

After a volleyball is served and is heading through the air and over the net, it gradually slows down. The primary reason for this slowing is that the air pressure

(A) at the front of the ball is equal to atmospheric pressure, while the pressure behind the ball is less than atmospheric. [3.3% picked]

(B) at the front and back of the ball is greater than atmospheric pressure, while the pressure on the sides of the ball is less than atmospheric. [5.8% picked]

(C) at the front and back of the ball is less than atmospheric pressure, while the pressure on the sides of the ball is more than atmospheric. [2.1% picked]

(D) at the front of the ball is greater than atmospheric pressure, while the pressure behind the ball is approximately atmospheric. [88.9% picked]

Answer: (D) at the front of the ball is greater than atmospheric pressure, while the pressure behind the ball is approximately atmospheric. [88.9% picked]

Why: TBA

Problem 19:

It's a hot summer day and you turn on the air conditioner to cool your room. As the air conditioner operates, what happens to the energy in the indoor air, the outdoor air, and at the electric company? [Note: the electric company's energy changes when you consume electric power.]

(A) The energies of the outdoor air and the electric company decrease, and the energy of the indoor air increases. [10.3% picked]

(B) The energies of the indoor air and the outdoor air increase, and the energy of the electric company decreases. [9.5% picked]

(C) The energies of the indoor air and the outdoor air decrease, and the energy of the electric company increases. [9.1% picked]

(D) The energies of the indoor air and the electric company decrease, and the energy of the outdoor air increases. [70.4% picked]

Answer: (D) The energies of the indoor air and the electric company decrease, and the energy of the outdoor air increases. [70.4% picked]

Why: TBA

Problem 20:

To put a satellite in orbit around the earth, a rocket has two important tasks. First, the rocket must lift the satellite above the Earth's atmosphere. Second, the rocket must

(A) give the satellite such a large horizontal speed that the falling satellite arcs endlessly around the Earth rather than descending to the ground. [92.6% picked]

(B) give the satellite such a large upward vertical speed that the falling satellite cannot descend to the ground. [0.8% picked]

(C) push upward on the satellite with a force that balances the satellite's downward weight so that the satellite can coast at constant velocity. [4.9% picked]

(D) give the satellite such a large sideways acceleration that the satellite's apparent weight is zero. [1.6% picked]

Answer: (A) give the satellite such a large horizontal speed that the falling satellite arcs endlessly around the Earth rather than descending to the ground. [92.6% picked]

Why: TBA

Problem 21:

Which of the following systems is definitely not a harmonic oscillator?

(A) A chandelier swinging back and forth on its support cable. [2.9% picked]

(B) A melon bouncing gently up and down in the basket of a grocery store spring scale. [6.2% picked]

(D) A metal flagpole bending back and forth about its vertical equilibrium. [4.5% picked]

Answer: (C) A bouncy ball bouncing up and down above a granite tabletop. [86.4% picked]

Why: TBA

Problem 22:

You are kayaking in Alaska. You reach forward with your paddle, lower the paddle into the water, and then pull the paddle backward. You accelerate forward. What force acts on the paddle to cause you to accelerate forward?

(A) The water exerts a drag force on the paddle, pushing the paddle in the forward direction. [86.0% picked]

(B) The water exerts a buoyant force on the paddle, pushing the paddle in the forward direction. [3.7% picked]

(D) The water exerts a lift force on the paddle, pushing the paddle in the forward direction. [4.1% picked]

Answer: (A) The water exerts a drag force on the paddle, pushing the paddle in the forward direction. [86.0% picked]

Why: TBA

Problem 23:

Your French fries are getting cold as you wait for your server to bring you a new bottle of ketchup. In frustration, you decide to get the last bits of ketchup out of the old bottle by swinging it rapidly in a circle with the cap-end of the bottle pointing away from you. This technique works because

(A) the force of the ketchup's momentum pushes it toward the cap-end of the bottle. [2.1% picked]

(B) centrifugal force pushes the ketchup toward the cap-end of the bottle. [2.5% picked]

(D) the bottle's rapid inward acceleration leaves the ketchup behind so that it drifts toward the cap-end of the bottle. [93.8% picked]

Answer: (D) the bottle's rapid inward acceleration leaves the ketchup behind so that it drifts toward the cap-end of the bottle. [93.8% picked]

Why: TBA

Problem 24:

If you pluck a guitar string at different points along its length, it will produce somewhat different sounds. What is different about the string when you pluck it at different points?

(A) The string's fundamental vibrational frequency depends on its amplitude of vibration and that amplitude depends on where you pluck the string. [28.4% picked]

(B) Where you pluck the string divides the string into two parts that vibrate separately, so where you pluck affects those two partial strings. [21.4% picked]

(D) The string can vibrate simultaneously in its fundamental and harmonic vibrational modes, and the mixture of those different vibrations depends on where you pluck it. [35.8% picked]

Answer: (D) The string can vibrate simultaneously in its fundamental and harmonic vibrational modes, and the mixture of those different vibrations depends on where you pluck it. [35.8% picked]

Why: TBA

Problem 25:

You accidently leave the lid off a container of peach sorbet when you place it in a frost-free freezer. Peach sorbet is sugar, water, and peach juice that have been frozen into tiny particles of peach ice. Six months later, you retrieve the container and find it almost empty because

(A) the peach ice has melted periodically and some of it has evaporated before it could refreeze. [7.0% picked]

(B) the peach ice has melted periodically and some of it has dripped into the bottom of the freezer. [0.4% picked]

(D) water molecules have gone directly from solid ice to gaseous water vapor. [92.6% picked]

Answer: (D) water molecules have gone directly from solid ice to gaseous water vapor. [92.6% picked]

Why: TBA

Problem 26:

A child is playing on a swing. As long as he doesn't swing too high, the time it takes him to complete one full oscillation will be independent of

(A) the weight of the child, but not of the amplitude of the child's motion. [7.8% picked]

(B) the amplitude of the child's motion, but not of the weight of the child. [14.4% picked]

(D) both the length of the ropes supporting the swing and the amplitude of the child's motion. [5.3% picked]

Answer: (C) both the amplitude of the child's motion and of the weight of the child. [72.4% picked]

Why: TBA

Problem 27:

It's a cold day and you have run out of wood for your fireplace. You place an electric hotplate in the fireplace and turn it on. For safety, you open the chimney flue to let air above the hot plate flow up the chimney and out of the house. The hotplate glows red hot. Heat will flow from the hotplate to people sitting across the room from the fireplace

(A) primarily via convection and radiation, with almost no heat flowing via conduction. [6.2% picked]

(B) equally poorly via conduction, convection, and radiation, and those people will feel cold. [2.5% picked]

(D) primarily via radiation, with almost no heat flowing via conduction or convection. [90.5% picked]

Answer: (D) primarily via radiation, with almost no heat flowing via conduction or convection. [90.5% picked]

Why: TBA

Problem 28:

Two bars of soap slide off of a horizontal counter at the same time, side by side. The green bar of soap weighs twice as much as the orange bar of soap, but the orange bar has twice the velocity of the green bar when the two first lose contact with the counter. In this situation,

(A) the green bar hits the floor first, but the two bars land at about the same distance from the counter. [0.4% picked]

(B) both bars hit the floor at approximately the same time, but the green bar lands considerably farther from the counter than the orange bar does. [2.5% picked]

(C) the green bar hits the floor first and it lands considerably closer to the counter than the orange bar does. [1.2% picked]

(D) both bars hit the floor at approximately the same time, but the orange bar lands considerably farther from the counter than the green bar does. [95.9% picked]

Answer: (D) both bars hit the floor at approximately the same time, but the orange bar lands considerably farther from the counter than the green bar does. [95.9% picked]

Why: TBA

Problem 29:

You place three nonflammable objects among the red-hot coals of a roaring fire. The objects are identical in shape and size, but one object is black, another is white, and the third is shiny silver. After a few minutes, all three objects are at the same temperature: 1800 °C. They remain solid and are now glowing with thermal radiation. Which one is glowing most brightly?

(A) The object that original appeared black. [95.9% picked]

(B) The object that originally appeared white. [0.4% picked]

(D) They are all glowing with equal brightness. [2.1% picked]

Answer: (A) The object that original appeared black. [95.9% picked]

Why: TBA

Problem 30:

As an orchestra warms up, the wind instruments (e.g., flutes, clarinets, oboes) and the air inside them actually become hotter. These instruments have openings and are not sealed. How does this warming process affect the average pressure and density of the air inside the wind instruments?

(A) The average pressure does not change, but the average density decreases. [48.1% picked]

(B) The average pressure increases, but the average density does not change. [39.9% picked]

(D) The average pressure does not change, but the average density increases. [8.2% picked]

Answer: (A) The average pressure does not change, but the average density decreases. [48.1% picked]

Why: TBA

Problem 31:

Your luxury apartment has a dining room table that is so enormous, it has a "lazy Susan" in the middle. This large circular platform rotates frictionlessly so that you can "pass" the marmalade jar to your guests by placing that jar on the platform and then rotating the platform. When you replace the empty jar with a full jar from the pantry, the full jar definitely

(A) increases the platform's angular velocity. [8.2% picked]

(B) decreases the platform's angular velocity. [25.9% picked]

(D) makes it harder to change the platform's angular velocity. [65.0% picked]

Answer: (D) makes it harder to change the platform's angular velocity. [65.0% picked]

Why: TBA

Problem 32:

A mother lifts her toddler upward while that toddler clutches a toy rabbit. Who is doing work on whom?

(A) There is no work being done. [0.8% picked]

(B) The mother is doing work on the toddler, but there is no work done on the rabbit. [9.5% picked]

(D) The mother is doing work on both the toddler and on the rabbit. [11.5% picked]

Answer: (C) The mother is doing work on the toddler and the toddler is doing work on the rabbit. [78.2% picked]

Why: TBA

Problem 33:

When water rushing through a fire hose flows around a bend in that hose, its pressure changes. The water pressure

(A) increases near the inside of the bend and decreases near the outside of the bend. [7.0% picked]

(B) decreases near the inside of the bend and increases near the outside of the bend. [92.6% picked]

(D) decreases near the inside of the bend and decreases near the outside of the bend. [0.4% picked]

Answer: (B) decreases near the inside of the bend and increases near the outside of the bend. [92.6% picked]

Why: TBA

Problem 34:

In a famous ice skating trick, a skater begins spinning with her arms out and then pulls her arms tightly against her body. She is then spinning extremely fast on the tip of one skate. Her angular velocity increases because her angular momentum

(A) increases but her rotational mass remains constant. [16.9% picked]

(B) is constant but her rotational mass decreases. [77.4% picked]

(D) is constant but her momentum decreases. [0.0% picked]

Answer: (B) is constant but her rotational mass decreases. [77.4% picked]

Why: TBA

Problem 35:

When you ride a bicycle, the ground exerts an upward support force on the wheel. When you turn the bicycle, the ground also exerts a horizontal frictional force on the wheel that causes you to accelerate sideways. Leaning the bicycle toward the inside of a turn keeps you from flipping over because the overall force that the ground exerts on the wheel

(A) points exactly at right angles to your combined center of mass and exerts a torque on you that keeps you from falling over. [14.4% picked]

(B) is exactly zero and causes no acceleration of your combined center of mass. [4.9% picked]

(D) points directly toward your combined center of mass and produces zero torque. [76.5% picked]

Answer: (D) points directly toward your combined center of mass and produces zero torque. [76.5% picked]

Why: TBA

Problem 36:

You are walking a dog on a leash when a squirrel runs by. Suddenly, you are all accelerating forward. If the dog is exerting a 50 newton forward force on the leash, the force that the leash is exerting on the dog is

(A) zero. [3.3% picked]

(B) a 50 newton backward force. [81.9% picked]

(D) more than a 50 newton backward force. [0.8% picked]

Answer: (B) a 50 newton backward force. [81.9% picked]

Why: TBA

Problem 37:

A seemingly "haunted" house emits creaking sounds as it cools off at night. Since sound carries energy, the house must obtain that energy from somewhere (other than ghosts and goblins). While this energy begins as thermal energy, the sound doesn't appear until the outside temperature drops well below the temperature of the house because

(A) the house's thermal energy increases most rapidly after dark. [1.2% picked]

(B) the house's thermal energy can only become work and sound as heat flows from the hotter house to the colder outside air. [90.9% picked]

(C) building materials become too soft during the day to vibrate with enough efficiency to convert thermal energy into sound energy. [4.5% picked]

(D) sound, like heat, can only flow from hotter objects to colder objects. [3.3% picked]

Answer: (B) the house's thermal energy can only become work and sound as heat flows from the hotter house to the colder outside air. [90.9% picked]

Why: TBA

Problem 38:

As the water stream from a fountain rises upward in the open air, its gravitational potential energy increases,

(A) its kinetic energy decreases, but its pressure potential energy remains constant. [35.0% picked]

(B) both its pressure potential energy and its kinetic energy decrease. [21.4% picked]

(D) its pressure potential energy decreases, but its kinetic energy remains constant. [42.0% picked]

Answer: (A) its kinetic energy decreases, but its pressure potential energy remains constant. [35.0% picked]

Why: TBA

Problem 39:

A skateboarder is being pulled up a gradual hill by a bicyclist and they are both moving at constant velocity. The net force on the skateboarder is

(A) directed downhill and the net force on the bicyclist is directed uphill. [1.2% picked]

(B) directed uphill and the net force on the bicyclist is directed downhill. [5.8% picked]

(D) directed uphill and the net force on the bicyclist is directed uphill. [6.6% picked]

Answer: (C) zero and the net force on the bicyclist is zero. [86.4% picked]

Why: TBA

Problem 40:

A pendulum consists of a metal ball hanging on a cable from a fixed point on the ceiling. To increase the period of that pendulum (the time it takes to complete one full cycle of motion), you should

(A) lengthen the cable. [83.5% picked]

(B) decrease the mass of the ball. [2.9% picked]

(D) increase the mass of the ball. [4.5% picked]

Answer: (A) lengthen the cable. [83.5% picked]

Why: TBA

Problem 41:

If you blow across the top of a half-full bottle of mineral water you can produce a clear tone. If you take a drink of water to reduce the amount of liquid in the bottle and try this again, the pitch (frequency) of the sound produced will

(A) increase. [9.5% picked]

(B) stay the same, regardless of how hard you blow. [0.8% picked]

(D) stay the same, but only if you blow exactly as hard as before. [0.4% picked]

Answer: (C) decrease. [89.3% picked]

Why: TBA

Problem 42:

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) remains unchanged, but the orbit is no longer circular. [2.9% picked]

(B) decreases. [2.1% picked]

(D) increases. [88.5% picked]

Answer: (D) increases. [88.5% picked]

Why: TBA

Problem 43:

At high altitude, a commercial jetliner must compress the outside air before circulating it in the pressurized cabin. The compressed air must first be sent through an air conditioner, however, because compressing air

(A) increases its humidity. [1.6% picked]

(B) increases its temperature. [95.5% picked]

(D) decreases its humidity. [0.0% picked]

Answer: (B) increases its temperature. [95.5% picked]

Why: TBA

Problem 44:

You are an astronaut floating in empty space far from Earth or anything else. You throw a wrench forward as hard as you can. While that wrench is moving forward at great speed, the net force on that wrench is

(A) the force of the wrench's velocity. [0.4% picked]

(B) zero. [91.4% picked]

(D) the force of the wrench's mass. [4.5% picked]

Answer: (B) zero. [91.4% picked]

Why: TBA

Problem 45:

It's a stormy day, and you're struggling to hold your umbrella steady. The wind is blowing toward the east and your motionless umbrella is pulling your hands upward and eastward. In which direction is the umbrella pushing on the air that passes it?

(A) Upward and westward. [2.9% picked]

(B) Downward and eastward. [4.1% picked]

(D) Downward and westward. [71.2% picked]

Answer: (D) Downward and westward. [71.2% picked]

Why: TBA

Problem 46:

Firefighters are battling a fire on the 12th floor of an apartment building. When they stand on the ground, their fire hose can only shoot the steady stream of water up to the 10th floor. So they carry the end of the same fire hose up a 2-story-tall ladder and shoot water upward. The other end of the hose remains attached to the same water source as before. Now the steady stream of water rises to the

(A) 12th floor. [5.3% picked]

(B) 8th floor. [1.2% picked]

(D) 11th floor. [0.8% picked]

Answer: (C) 10th floor. [92.6% picked]

Why: TBA

Problem 47:

You're playing tennis on a hot, humid day in August and you are covered with perspiration. Unfortunately, that perspiration doesn't cool you off much because

(A) the air density is extremely low and its pressure is too low to permit water to evaporate. [2.1% picked]

(B) the air is almost saturated with water vapor, so there is almost no evaporation. [91.4% picked]

(D) the air density is extremely low and its pressure is too high to permit water to evaporate. [5.8% picked]

Answer: (B) the air is almost saturated with water vapor, so there is almost no evaporation. [91.4% picked]

Why: TBA

Problem 48:

Flowing pancake syrup is less likely to become turbulent than flowing water because

(A) water's larger pressure favors turbulent flow. [0.0% picked]

(B) water's larger viscosity favors turbulent flow. [0.8% picked]

(D) pancake syrup's larger density favors laminar flow. [2.5% picked]

Answer: (C) pancake syrup's larger viscosity favors laminar flow. [96.7% picked]

Why: TBA

Problem 49:

You are swinging a can of paint around you in a big horizontal circle at a constant speed. You are at the center of its circular path. The net force on the paint can points directly

(A) away from you. [2.1% picked]

(B) toward you. [96.3% picked]

(D) along the paint can's velocity (along its direction of travel). [1.2% picked]

Answer: (B) toward you. [96.3% picked]

Why: TBA

Problem 50:

While decorating your new guitar, you accidently coat one of the guitar strings with spray paint. That paint increases the string's mass. How will that change affect the string?

(A) The string's frequency of vibration (pitch) will increase. [1.6% picked]

(B) The string's frequency of vibration (pitch) will not change, but its amplitude of motion will increase. [1.6% picked]

(D) The string's frequency of vibration (pitch) will not change, but its amplitude of motion will decrease. [1.6% picked]

Answer: (C) The string's frequency of vibration (pitch) will decrease. [94.7% picked]

Why: TBA

Problem 51:

If the earth had no atmosphere, the temperature at the earth's surface would be

(A) almost that of empty space—a few degrees above absolute zero. [8.6% picked]

(B) significantly cooler than it is now. [81.5% picked]

(D) essentially the same as it is now. [0.8% picked]

Answer: (B) significantly cooler than it is now. [81.5% picked]

Why: TBA

Problem 52:

You are at a tropical resort and your friends have talked you into diving off the top of a cliff into the ocean below. It's a 4-second fall to the ocean. When you have been falling for exactly 2 seconds, you change your mind and instantly grab onto the tree projecting outward from the cliff. Miraculously, you stop safely and hold onto the tree. Your decision to climb up or down now depends on your height on the cliff. Where are you and the tree located on that cliff? [Note: neglect any effects due to the air.]

(A) You are midway between the top of the cliff and the ocean. [1.2% picked]

(B) You are much closer to the top of the cliff than to the ocean below. [88.5% picked]

(D) You are slightly above the midway point between the top of the cliff and the ocean. [4.1% picked]

Answer: (B) You are much closer to the top of the cliff than to the ocean below. [88.5% picked]

Why: TBA

Problem 53:

A typical heat pump (an air conditioner run backwards so that it heats the indoor air) consumes 2000 watts of electric power. How much thermal power does it provide to the indoor air?

(A) About 200 watts. [7.0% picked]

(B) About 2000 watts. [43.2% picked]

(D) Significantly more than 2000 watts. [33.3% picked]

Answer: (D) Significantly more than 2000 watts. [33.3% picked]

Why: TBA

Problem 54:

You are watching an airshow at the local airport and an acrobatic airplane is flying upside down to impress the crowd. That airplane is traveling forward at a constant horizontal velocity, just 10 feet above the runway! In which direction is the overall force that the airplane exerts on the air?

(A) The airplane is pushing the air directly downward. [91.8% picked]

(B) The airplane is pushing the air downward and forward. [2.9% picked]

(D) The airplane is pushing the air downward and backward. [3.3% picked]

Answer: (A) The airplane is pushing the air directly downward. [91.8% picked]

Why: TBA

Problem 55:

Increasing the amount of carbon dioxide (a greenhouse gas) in the earth's atmosphere

(A) increases the rate at which the earth radiates heat into space. [7.4% picked]

(B) has no effect on the earth's thermal radiation or the effective surface that emits that thermal radiation. [0.8% picked]

(D) increases the altitude of the earth's effective radiating surface. [73.7% picked]

Answer: (D) increases the altitude of the earth's effective radiating surface. [73.7% picked]

Why: TBA

Problem 56:

When a liquid flows through a collection of stationary obstacles, its flow can be smooth and laminar, or it can be swirling and turbulent. The liquid's flow is more likely to become turbulent if you increase its

(A) pressure. [3.7% picked]

(B) speed. [95.5% picked]

(D) transparency. [0.0% picked]

Answer: (B) speed. [95.5% picked]

Why: TBA

Problem 57:

You are riding a tourist train through the hills of northern Italy. The train is currently climbing uphill at constant velocity along a train track that points directly toward a high pass between hills. The force that the train is exerting on you points

(A) horizontally forward, toward the base of the hills. [0.8% picked]

(B) uphill, in the direction of the train track. [7.0% picked]

(D) down and forward, at right angles to the train track. [0.8% picked]

Answer: (C) directly upward. [91.4% picked]

Why: TBA

Problem 58:

Your coffee has cooled off and you decide to heat it the hard way: you stir the coffee vigorously for 10 minutes. The coffee does become slightly hotter. What causes this rise in temperature?

(A) You are making the coffee move as though it were boiling, so that it becomes hotter. [0.4% picked]

(B) You are encouraging the conduction of heat from the cup to the coffee. [1.6% picked]

(D) You do work on the coffee as you stir it and viscous effects turn that work into thermal energy. [96.3% picked]

Answer: (D) You do work on the coffee as you stir it and viscous effects turn that work into thermal energy. [96.3% picked]

Why: TBA

Problem 59:

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. So they carry the nozzle end of the same fire hose to the top of a 4-story-tall ladder and again point the nozzle upward. How does the speed of the water as it emerges from the nozzle near the top of the ladder compare to its speed when it emerged from the nozzle near the ground?

(A) The water emerging from the nozzle near the top of the ladder travels faster. [2.9% picked]

(B) The water emerges from the nozzle at the same speed in both cases. [14.4% picked]

(D) No water emerges from the nozzle when the nozzle is near the top of the ladder. [0.4% picked]

Answer: (C) The water emerging from the nozzle near the top of the ladder travels more slowly. [82.3% picked]

Why: TBA

Problem 60:

You are playing with a squirt gun and the narrow stream of water from your toy hits your friend's cheek. At the center of the stream of water, right where it hits his cheek, the water is coming to a complete stop. The pressure in the water at the center of the stream as it touches his cheek is

(A) exactly zero. [4.5% picked]

(B) higher than atmospheric pressure. [86.4% picked]

(D) equal to atmospheric pressure. [6.6% picked]

Answer: (B) higher than atmospheric pressure. [86.4% picked]

Why: TBA