Final Examination Solutions

Physics 1050 - Fall 2017 - Final Exam - Posted Version with Solutions

Problem 1:
You're the first person to visit Mars and you've just met a group of Martian schoolchildren. They are playing street hockey with parts from the Mars Polar Lander and, after years of watching Star Trek reruns, they are fluent in English. One of them asks you how your weight and mass have changed since you left Earth. You take a moment to measure both and reply correctly that
(A) your weight is essentially unchanged but your mass is less than on Earth. [3.1% picked]
(B) your mass is essentially unchanged but your weight is less than on Earth. [94.2% picked]
(C) your weight and mass have both changed significantly. [0.4% picked]
(D) neither your weight nor your mass have changed much. [2.2% picked]
Answer: (B) your mass is essentially unchanged but your weight is less than on Earth. [94.2% picked]
Why: TBA
Problem 2:
A friend has purchased an old pendulum clock at a yard sale. Its pendulum is discolored, so your friend applies a thick layer of brass-colored spray paint. That paint increases the pendulum's mass and its weight. How does painting the pendulum affect the pendulum's period? [Ignore effects due to the air and other insignificant influences.]
(A) The pendulum's period increases (more time). [32.9% picked]
(B) The pendulum's period is unchanged. [56.4% picked]
(C) The pendulum becomes unstable and does not swing back and forth anymore. It there has no period. [0.4% picked]
(D) The pendulum's period decreases (less time.) [10.2% picked]
Answer: (B) The pendulum's period is unchanged. [56.4% picked]
Why: TBA
Problem 3:
Increasing the tension in a guitar string
(A) raises the string's frequency (pitch) by softening the string's restoring force. [0.0% picked]
(B) raises the string's frequency (pitch) by stiffening the string's restoring force. [94.7% picked]
(C) lowers the string's frequency (pitch) by softening the string's restoring force. [0.4% picked]
(D) lowers the string's frequency (pitch) by stiffening the string's restoring force. [4.9% picked]
Answer: (B) raises the string's frequency (pitch) by stiffening the string's restoring force. [94.7% picked]
Why: TBA
Problem 4:
You are enjoying an elegant candlelit dinner for two but the waiter has been a little careless with the ice. Your glass of ice water contains 25% water and 75% ice, while your friend's glass contains 75% water and 25% ice. The temperature of your glass of ice water
(A) is the same as that of your friend. [82.2% picked]
(B) depends on when it was filled. The glass that was filled first will now be warmer than the glass that was filled second. [4.0% picked]
(C) is higher than that of your friend. [1.3% picked]
(D) is lower than that of your friend. [12.4% picked]
Answer: (A) is the same as that of your friend. [82.2% picked]
Why: TBA
Problem 5:
Why does a paper cup of hot coffee cool so much more slowly if you cover it with a thin plastic lid?
(A) The plastic lid prevents heat from flowing out through the sides of the plastic cup. [7.1% picked]
(B) Plastic radiates heat much more slowly than coffee radiates heat. [9.8% picked]
(C) The lid traps water vapor so that the coffee evaporates much more slowly. [56.4% picked]
(D) Heat conducts through plastic much more slowly than it conducts through air. [26.7% picked]
Answer: (C) The lid traps water vapor so that the coffee evaporates much more slowly. [56.4% picked]
Why: TBA
Problem 6:
As you knead bread dough, squeezing it and folding it repeatedly, the dough becomes hotter. What causes this rise in temperature? [Neglect any heat that flows between your hands and the dough due to differences in their temperatures.]
(A) You are forcing convection to occur so that heat flows more rapidly through the dough. [1.8% picked]
(B) You are allowing radiation to carry heat from one part of the dough to another part. [0.9% picked]
(C) You are doing work on the dough and viscous effects turn that work into thermal energy in the dough. [96.0% picked]
(D) You are encouraging the conduction of heat from one side of the side of the dough to the other. [1.3% picked]
Answer: (C) You are doing work on the dough and viscous effects turn that work into thermal energy in the dough. [96.0% picked]
Why: TBA
Problem 7:
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 it
(A) stops the downward motion of your foot faster and thus absorbs more of your momentum. [2.2% picked]
(B) stops the downward motion of your foot faster and thus absorbs less of your momentum. [8.4% picked]
(C) pushes up on your foot just as hard as your foot pushes down on it, unlike the soft dry sand. [3.1% picked]
(D) barely moves downward as you push downward on it, so you do almost zero work on it. [86.2% picked]
Answer: (D) barely moves downward as you push downward on it, so you do almost zero work on it. [86.2% picked]
Why: TBA
Problem 8:
Water flows gently from a kitchen faucet. You block most of the faucet's opening with your thumb and now the water sprays out at high speed because you
(A) are compressing the water and increasing its density. [5.8% picked]
(B) are keeping air from getting into the faucet and reducing the pressure. [3.6% picked]
(C) have slowed the water flow through the faucet and plumbing. Water therefore wastes less of its total ordered energy doing work against viscous forces. [81.3% picked]
(D) are doing work on the water with your thumb and greatly increasing its total ordered energy. [9.3% picked]
Answer: (C) have slowed the water flow through the faucet and plumbing. Water therefore wastes less of its total ordered energy doing work against viscous forces. [81.3% picked]
Why: TBA
Problem 9:
How is a steam engine able to obtain work (ordered energy) from hot steam?
(A) The hot steam has a large amount of momentum and that momentum can be converted into work. [2.2% picked]
(B) The steam's thermal energy can be converted into work because both are energy and energy is conserved. [7.6% picked]
(C) As heat flows from the hot steam to the colder environment, the steam engine is able to divert a fraction of that heat and convert it into work. [85.8% picked]
(D) The hot steam has a large amount of entropy and that entropy can be converted into work. [4.4% picked]
Answer: (C) As heat flows from the hot steam to the colder environment, the steam engine is able to divert a fraction of that heat and convert it into work. [85.8% picked]
Why: TBA
Problem 10:
As the temperature difference between indoor air and outdoor air decreases, the electric energy an air conditioner consumes as it moves one joule of heat also decreases. Why?
(A) Less ordered energy must become thermal energy to make up for the entropy lost when heat moves against its natural direction of flow. [58.7% picked]
(B) Less thermal energy must become ordered energy to make up for the entropy produced when heat moves against its natural direction of flow. [7.1% picked]
(C) Less thermal energy must become ordered energy to make up for the entropy lost when heat moves against its natural direction of flow. [14.7% picked]
(D) Less ordered energy must become thermal energy to make up for the entropy produced when heat moves against its natural direction of flow. [19.6% picked]
Answer: (A) Less ordered energy must become thermal energy to make up for the entropy lost when heat moves against its natural direction of flow. [58.7% picked]
Why: TBA
Problem 11:
A birdfeeder hangs from a tree-branch on a long string and it can swing back and forth like a pendulum. As its birdseed is consumed, the birdfeeder's mass decreases and its center of mass moves downward. How do these two changes affect the period of the birdfeeder's swinging motion?
(A) Its decrease in mass causes its period to decrease. The lowering of its center of mass causes its period to increase. [17.3% picked]
(B) Its decrease in mass does not change its period. The lowering of its center of mass does not change its period. [19.1% picked]
(C) Its decrease in mass does not change its period. The lowering of its center of mass causes its period to increase. [47.6% picked]
(D) Its decrease in mass causes its period to decrease. The lowering of its center of mass does not change its period. [16.0% picked]
Answer: (C) Its decrease in mass does not change its period. The lowering of its center of mass causes its period to increase. [47.6% picked]
Why: TBA
Problem 12:
You are a patent examiner for the government and the invention you are examining claims to convert heat continuously into electric power. According to the inventor, when you put this device in the middle of a room and turn it on, it gradually cools the room air while providing a steady and endless supply of electric power. You wisely reject this patent application because, in order to work as described, it would have to have to
(A) violate the law of entropy. [76.0% picked]
(B) turn work into heat, or more precisely, into thermal energy. [2.2% picked]
(C) violate the law of conservation of energy. [20.9% picked]
(D) turn stored energy into power. [0.9% picked]
Answer: (A) violate the law of entropy. [76.0% picked]
Why: TBA
Problem 13:
A common type of water pump produces pressurized water by spinning a cylindrical volume of water rapidly about its center. Water enters the pump where the water pressure is smallest and leaves the pump where the water pressure is greatest. Where on the cylinder of water does the water enter and leave the pump?
(A) Water enters and leaves at the cylinder's outer edge, but the water completes one full rotation between entering and leaving. [2.2% picked]
(B) Water enters at the cylinder's outer edge and leaves at the cylinder's center. [20.4% picked]
(C) Water enters and leaves at the cylinder's outer edge, but on opposite sides of the cylinder. [4.0% picked]
(D) Water enters at the cylinder's center and leaves at the cylinder's outer edge. [73.3% picked]
Answer: (D) Water enters at the cylinder's center and leaves at the cylinder's outer edge. [73.3% picked]
Why: TBA
Problem 14:
At the cruising altitude of a commercial jet plane, the air's density is too low to support life. The plane must therefore compress the outside air before sending it into the pressurized cabin. After compressing the air, the plane must send that air through an air conditioner because the act of compressing that air
(A) increases the air's temperature. [94.2% picked]
(B) reduces the air's energy. [1.3% picked]
(C) turns it into a liquid and the air conditioner helps it evaporate back into a gas. [4.0% picked]
(D) increases the air's momentum. [0.4% picked]
Answer: (A) increases the air's temperature. [94.2% picked]
Why: TBA
Problem 15:
To win a stuffed animal at the state fair, you simply need to drop a glass marble onto a stationary glass plate and have the marble come to rest on that plate. Unfortunately, when the marble hits the plate, it bounces upward because the marble
(A) retains essentially all of its energy and momentum. [5.3% picked]
(B) transfers a large amount of momentum and energy to the plate. [0.9% picked]
(C) retains essentially all of its momentum but transfers a large amount of energy to the plate. [4.9% picked]
(D) retains essentially all of its energy but transfers a large amount of momentum to the plate. [88.9% picked]
Answer: (D) retains essentially all of its energy but transfers a large amount of momentum to the plate. [88.9% picked]
Why: TBA
Problem 16:
You fill a rubber balloon with water and use your hand to throw the balloon forward horizontally. As the balloon accelerates forward rapidly, where in the balloon is water pressure greatest?
(A) At the front of the balloon (farthest from your hand). [49.3% picked]
(B) At the bottom of the balloon (nearest the ground). [2.7% picked]
(C) At the back of the balloon (nearest your hand). [46.2% picked]
(D) At the top of the balloon (farthest from the ground). [1.8% picked]
Answer: (C) At the back of the balloon (nearest your hand). [46.2% picked]
Why: TBA
Problem 17:
Someone props open the door to your jazz club on a cold winter evening and the air temperature in the club decreases dramatically. The air pressure remains unchanged. How does this decrease in air temperature affect the sound of the Saxaphone (a wind instrument)?
(A) Its loudness (amplitude of pressure fluctuation) increases. [2.2% picked]
(B) Its frequency (pitch) decreases. [73.8% picked]
(C) Its frequency (pitch) increases. [20.9% picked]
(D) Its loudness (amplitude of pressure fluctuation) decreases. [3.1% picked]
Answer: (B) Its frequency (pitch) decreases. [73.8% picked]
Why: TBA
Problem 18:
You seal a leftover sprig of parsley in a large zippered plastic bag and put them in the refrigerator. There was a substantial volume of air trapped in the bag, but the air's volume decreases as the refrigerator cools the bag. The decrease in the bag's temperature caused
(A) an increase in the density of the air inside the bag. [76.8% picked]
(B) a decrease in the density of the air inside the bag. [5.4% picked]
(C) an increase in the pressure of the air inside the bag. [3.6% picked]
(D) a decrease in the pressure of the air inside the bag. [14.3% picked]
Answer: (A) an increase in the density of the air inside the bag. [76.8% picked]
Why: TBA
Problem 19:
Liquid water can coexist with ice at 0 °C. If you dissolve sugar in the water, it
(A) can coexist with ice at a temperature greater than 0 °C. [8.0% picked]
(B) cannot coexist with ice at any temperature. [16.4% picked]
(C) can coexist with ice at 0 °C. [12.4% picked]
(D) can coexist with ice at a temperature less than 0 °C. [63.1% picked]
Answer: (D) can coexist with ice at a temperature less than 0 °C. [63.1% picked]
Why: TBA
Problem 20:
As heat flows from a hot woodstove to a cold room, the entropy of the woodstove
(A) decreases, the entropy of the room increases, and their combined entropy remains unchanged. [8.4% picked]
(B) decreases, the entropy of the room increases, and their combined entropy increases. [87.1% picked]
(C) increases, the entropy of the room decreases, and their combined entropy remains unchanged. [0.0% picked]
(D) increases, the entropy of the room decreases, and their combined entropy increases. [4.4% picked]
Answer: (B) decreases, the entropy of the room increases, and their combined entropy increases. [87.1% picked]
Why: TBA
Problem 21:
You are riding a roller coaster and your speed is increasing as you plunge downhill on the first hill. Your acceleration is directed
(A) downhill, your feeling of acceleration is directed uphill, and your apparent weight is smaller in amount than your actual weight. [59.1% picked]
(B) downhill, your feeling of acceleration is directed downhill, and your apparent weight is directed vertically downward. [1.8% picked]
(C) vertically downward, your feeling of acceleration is directed vertically upward, and your apparent weight is vertically upward. [4.9% picked]
(D) vertically downward, your feeling of acceleration is directed vertically upward, and your apparent weight is smaller in amount than your actual weight. [34.2% picked]
Answer: (A) downhill, your feeling of acceleration is directed uphill, and your apparent weight is smaller in amount than your actual weight. [59.1% picked]
Why: TBA
Problem 22:
A gold flute produces a tone as the flautist blows across a hole near the end of the flute. What is vibrating to produce this tone?
(A) The gold body of the flute is vibrating as a single arc in which its middle moves opposite its two ends. [0.9% picked]
(B) A column of air inside the flute is vibrating along its length. [93.8% picked]
(C) The gold body of the flute is vibrating along its length. [1.8% picked]
(D) The gold body of the flute is vibrating as a single arc in which its middle moves back and forth while its ends remain motionless. [3.6% picked]
Answer: (B) A column of air inside the flute is vibrating along its length. [93.8% picked]
Why: TBA
Problem 23:
During your ride in a hot air balloon, the pilot turns on the flame to heat up the air inside the balloon's envelope (the balloon's fabric skin). That envelope is open at the bottom, where the flame heats it. As the temperature of the air inside the unsealed envelope increases,
(A) the upward buoyant force on the balloon increases and it begins to rise. [12.9% picked]
(B) air molecules flow into the envelope's opening and the pressure of the air inside the envelope increases. [6.2% picked]
(C) air molecules flow into the envelope's opening and the density of the air inside the envelope increases. [4.4% picked]
(D) air molecules flow out of the envelope's opening and the weight of the air inside the envelope decreases. [76.4% picked]
Answer: (D) air molecules flow out of the envelope's opening and the weight of the air inside the envelope decreases. [76.4% picked]
Why: TBA
Problem 24:
During its compression stroke, a car-engine's piston squeezes the fuel and air mixture into a smaller volume within a cylinder. As the piston moves into the cylinder, the piston does work on the gas and the
(A) pressure and density of gas increase, but the temperature of the gas remains unchanged. [2.2% picked]
(B) density and temperature of the gas increase, but the pressure of the gas remains unchanged. [1.3% picked]
(C) pressure, density, and temperature of the gas all increase. [87.6% picked]
(D) pressure and temperature of the gas increase, but the density of the gas remains unchanged. [8.9% picked]
Answer: (C) pressure, density, and temperature of the gas all increase. [87.6% picked]
Why: TBA
Problem 25:
After John Glenn completed 3 orbits of the Earth in May 1961, it was time to return his spaceship to the ground. In which direction did rocket engines push his spaceship to cause it to descend into the atmosphere for reentry?
(A) The engines pushed the spaceship forward (in the direction of its velocity). [5.8% picked]
(B) The engines pushed the spaceship upward (away from the center of the Earth). [7.6% picked]
(C) The engines pushed the spaceship backward (opposite its velocity). [57.3% picked]
(D) The engines pushed the spaceship downward (toward the center of the Earth). [29.3% picked]
Answer: (C) The engines pushed the spaceship backward (opposite its velocity). [57.3% picked]
Why: TBA
Problem 26:
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. [1.3% picked]
(B) 6th floor. [0.0% picked]
(C) 10th floor. [96.9% picked]
(D) 8th floor. [1.8% picked]
Answer: (C) 10th floor. [96.9% picked]
Why: TBA
Problem 27:
As a soccer ball moves swiftly through the air, it experiences pressure drag because the air pressure
(A) below the ball is higher than the air pressure above the ball. [0.9% picked]
(B) in front of the ball is higher than the air pressure behind it. [93.3% picked]
(C) in front of the ball is lower than the air pressure behind it. [5.3% picked]
(D) around the ball is uniform everywhere. [0.4% picked]
Answer: (B) in front of the ball is higher than the air pressure behind it. [93.3% picked]
Why: TBA
Problem 28:
An electric space heater is perfectly efficient: it consumes 1500 watts of electric power and provides 1500 watts of thermal power to the indoor air. In comparison, a heat pump (an air conditioner run backwards so that it heats the indoor air) that consumes 1500 watts of electric power provides
(A) about 1500 watts. [2.7% picked]
(B) significantly more than 1500 watts of thermal power. [93.8% picked]
(C) about 100 watts. [0.9% picked]
(D) about 1000 watts. [2.7% picked]
Answer: (B) significantly more than 1500 watts of thermal power. [93.8% picked]
Why: TBA
Problem 29:
When a log is floating on water, much of the log is above the water and is actually surrounded by air. If that surrounding air were to disappear suddenly, the log would
(A) sink to the bottom of the water. [16.4% picked]
(B) move upward slightly and float somewhat higher (less deep) in the water. [40.4% picked]
(C) float at the same height as before the air left. [4.0% picked]
(D) move downward slightly and float somewhat lower (deeper) in the water. [39.1% picked]
Answer: (D) move downward slightly and float somewhat lower (deeper) in the water. [39.1% picked]
Why: TBA
Problem 30:
You purchase an antique balance-ring clock. When you place the clock upright on your dresser, it keeps perfect time. A friend tips the clock gently onto its back, so that its face and hands are visible from above instead of from the side. How does this change in orientation affect the clock?
(A) The clock continues to keep perfect time. [51.6% picked]
(B) The clock runs fast (its hands move through the minutes and hours too quickly). [2.2% picked]
(C) The clock stops operating because its balance ring cannot rotate back and forth in the new orientation. [33.8% picked]
(D) The clock runs slow (its hands move through the minutes and hours too slowly). [12.4% picked]
Answer: (A) The clock continues to keep perfect time. [51.6% picked]
Why: TBA
Problem 31:
A space tourism company offers to carry people to orbit in a luxurious spaceship. To perform that task, the ship's rocket engines must lift the ship above Earth's atmosphere and
(A) give the ship such a large upward vertical speed that the falling ship cannot descend to the ground. [2.2% picked]
(B) push upward on the ship with a force that balances the ship's downward weight so that the ship can coast at constant velocity. [4.9% picked]
(C) give the ship such a large downward acceleration that the ship's apparent weight is zero. [0.4% picked]
(D) give the ship such a large horizontal speed that the falling ship arcs endlessly around the Earth rather than descending to the ground. [92.4% picked]
Answer: (D) give the ship such a large horizontal speed that the falling ship arcs endlessly around the Earth rather than descending to the ground. [92.4% picked]
Why: TBA
Problem 32:
The elevator in a fancy hotel has table in it and there is a vase of flowers on that table. As the elevator rises at constant velocity toward the penthouse suite, the table is
(A) doing zero work on the vase of flowers because the vase and flowers are moving at constant velocity. [27.6% picked]
(B) doing (positive) work on the vase of flowers. [67.6% picked]
(C) doing zero work on the vase of flowers because the table is rigid. [2.7% picked]
(D) doing negative work on the vase of flowers. [2.2% picked]
Answer: (B) doing (positive) work on the vase of flowers. [67.6% picked]
Why: TBA
Problem 33:
If you blow across the top of a half-full bottle of water, you can produce a clear tone. If you drink some of the water and try this again, the frequency of the sound produced will
(A) be the same as before your drink. [2.2% picked]
(B) be smaller or larger than before your drink, depending on how much water you drank. [0.4% picked]
(C) be smaller than before your drink. [61.8% picked]
(D) be larger than before your drink. [35.6% picked]
Answer: (C) be smaller than before your drink. [61.8% picked]
Why: TBA
Problem 34:
You are pushing a shopping cart up a ramp. Both you and the cart are moving at constant velocity. The net force on the shopping cart is
(A) zero and the net force on you is zero. [91.6% picked]
(B) directed downhill and the net force on you is directed uphill. [1.8% picked]
(C) directed uphill and the net force on you is directed uphill. [1.8% picked]
(D) directed uphill and the net force on you is directed downhill. [4.9% picked]
Answer: (A) zero and the net force on you is zero. [91.6% picked]
Why: TBA
Problem 35:
As spacecraft travels from Earth to Mars, it is exposed to full sunlight continuously for about six months. Since nothing is touching that spacecraft, why doesn't the spacecraft continue to get hotter and hotter?
(A) Because it is isolated from the sun by empty space, the sun's heat can't reach it and it doesn't experience any changes in temperature. [8.0% picked]
(B) The spacecraft's temperature rises until it is able to radiate heat away into space as fast as that heat arrives from the sun. [81.8% picked]
(C) It uses an air conditioner to eliminate the excess thermal energy as rapidly as it arrives. [4.9% picked]
(D) It has solar panels that convert the sun's thermal radiation completely into electricity and avoids any need to eliminate heat. [5.3% picked]
Answer: (B) The spacecraft's temperature rises until it is able to radiate heat away into space as fast as that heat arrives from the sun. [81.8% picked]
Why: TBA
Problem 36:
You are on the outdoor deck of a restaurant, watching athletes compete in winter sports. The weather is cold, but the red-hot heating element positioned about 10 feet above you keeps you surprisingly warm. Heat from that element reaches you
(A) primarily via convection and conduction, with almost no heat flowing to you via radiation. [0.4% picked]
(B) equally well via conduction, convection, and radiation. [0.4% picked]
(C) primarily via convection and radiation, with almost no heat flowing to you via conduction. [4.4% picked]
(D) primarily via radiation, with almost no heat flowing to you via conduction or convection. [94.7% picked]
Answer: (D) primarily via radiation, with almost no heat flowing to you via conduction or convection. [94.7% picked]
Why: TBA
Problem 37:
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 experience a larger inward acceleration than your friend does. [40.0% picked]
(B) You and your friend experience equal inward accelerations, but your friend's speed is larger than your speed. [16.4% picked]
(C) Your friend experiences a larger inward acceleration than you do. [15.6% picked]
(D) You and your friend experience equal inward accelerations, but your speed is larger than your friend's speed. [28.0% picked]
Answer: (A) You experience a larger inward acceleration than your friend does. [40.0% picked]
Why: TBA
Problem 38:
As a window air conditioner cools the air in your room on a hot summer day, which of the following statements about energy and entropy is correct?
(A) The energy in the room air is decreasing, but the entropy in the room air is increasing. [6.7% picked]
(B) The room air's energy is becoming entropy while the outdoor air's entropy is becoming energy. [2.7% picked]
(C) The total energy in the room air and the outdoor air is constant, but the total entropy in the room air and the outdoor air is increasing. [34.7% picked]
(D) The energy and entropy in the room air are decreasing and the energy and entropy in the outdoor air are increasing. [56.0% picked]
Answer: (D) The energy and entropy in the room air are decreasing and the energy and entropy in the outdoor air are increasing. [56.0% picked]
Why: TBA
Problem 39:
You put a metal pot of water on the stove at sea level and begin heating the water. When the water's temperature reaches 100 °C (212 °F), the water begins to boil. As heat continues to flow into the boiling water,
(A) its temperature begins rising more rapidly as the amount of water remaining in the pot decreases. [4.0% picked]
(B) its temperature remains almost constant because that heat is being used to convert liquid water into gaseous steam. [75.6% picked]
(C) water molecules begin leaving the water's surface for the first time and gaseous water starts rising above the pot. [19.1% picked]
(D) its temperature decreases slightly because cold air is flowing into it to form the bubbles. [1.3% picked]
Answer: (B) its temperature remains almost constant because that heat is being used to convert liquid water into gaseous steam. [75.6% picked]
Why: TBA
Problem 40:
After drinking all the water in your plastic water bottle, you seal the bottle closed and place it in the hot sun. A while later, you find that the bottle's sides are bent slightly outward so that it has slightly more volume than before. Since you sealed the bottle, the air pressure inside the bottle has
(A) decreased and the air density inside the bottle has decreased. [3.1% picked]
(B) increased and the air density inside the bottle has decreased. [85.8% picked]
(C) increased and the air density inside the bottle has increased. [6.2% picked]
(D) decreased and the air density inside the bottle has increased. [4.9% picked]
Answer: (B) increased and the air density inside the bottle has decreased. [85.8% picked]
Why: TBA
Problem 41:
A weather balloon is used to study the atmosphere as it float upward. The balloons starts as an empty rubber bag, but adding helium to it gradually inflates the balloon until it is many feet in diameter. At first, the balloon doesn't float. As you keep adding helium to it and it grows larger, however, it eventually begins floating because
(A) the average density of the helium-filled balloon decreases as it fills and its density eventually becomes less than that of the air it displaces. [73.8% picked]
(B) 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. [4.4% picked]
(C) the upward buoyant force on a balloon full of helium is larger than the buoyant force on an identical balloon full of air. [19.6% picked]
(D) the helium-filled balloon's weight decreases as you put more lightweight helium particles inside it and it eventually becomes weightless. [2.2% picked]
Answer: (A) the average density of the helium-filled balloon decreases as it fills and its density eventually becomes less than that of the air it displaces. [73.8% picked]
Why: TBA
Problem 42:
An artist constructs a minimalist violin that has no wooden body at all. Instead, the violin's four strings are mounted on a thin streel rod. In fact, that is the entire violin! The strings are under tension and vibrate properly, but you can barely hear their sound. Why is this minimalist violin so quiet?
(A) Because air flows easily around the narrow strings as they vibrate, they produce little sound. [60.7% picked]
(B) Because the steel rod is thin, it bends as the strings vibrate and absorbs their sound. [19.2% picked]
(C) Because the steel is magnetic, it attracts and traps the sound of the strings before that sound can escape into the room. [0.9% picked]
(D) Because steel is so much more dense than wood, it reflects the strings' sound back to the strings. [19.2% picked]
Answer: (A) Because air flows easily around the narrow strings as they vibrate, they produce little sound. [60.7% picked]
Why: TBA
Problem 43:
You have a large block of slippery ice in the back of your pickup truck. As the truck turns toward the left at high speed, the block of ice slides up against the right side of the truck bed. What force, if any, is pushing the ice toward the right?
(A) A support force from the truck bed is pushing the ice toward the right. [0.9% picked]
(B) The force of the ice's momentum is pushing it toward the right. [4.0% picked]
(C) A friction force from the truck bed is pushing the ice toward the right. [1.3% picked]
(D) There is no force pushing the ice toward the right. [93.8% picked]
Answer: (D) There is no force pushing the ice toward the right. [93.8% picked]
Why: TBA
Problem 44:
Water is flowing smoothly through horizontal plumbing. As the water move through a bend in that pipe, compare the water on the inside of the bend to water on the outside of the bend.
(A) Water on the inside has lower pressure and lower speed than water on the outside. [10.2% picked]
(B) Water on the inside has lower pressure and higher speed than water on the outside. [82.2% picked]
(C) Water on the inside has higher pressure and lower speed than water on the outside. [6.2% picked]
(D) Water on the inside has higher pressure and higher speed than water on the outside. [1.3% picked]
Answer: (B) Water on the inside has lower pressure and higher speed than water on the outside. [82.2% picked]
Why: TBA
Problem 45:
It's a warm summer day and you're having lunch outdoors on a patio. A window air conditioning unit hums quietly nearby as it cools an office inside the building. Your friend notices that the unit's outdoor part is emitting a considerable amount of heat and comments on how strange that is. You explain correctly that the air conditioner's outside part is emitting the heat
(A) it removes from the inside air and the heat it produces from the electricity it consumes. [88.4% picked]
(B) it produces from the electricity it consumes. [4.0% picked]
(C) that is left over when it converts thermal energy from the inside air into electricity. [6.7% picked]
(D) it produces during its defrost cycle; when it warms up its evaporator to remove ice that forms because of the humidity. [0.9% picked]
Answer: (A) it removes from the inside air and the heat it produces from the electricity it consumes. [88.4% picked]
Why: TBA
Problem 46:
When smokejumpers parachute into forest fires, they have with them emergency tents made of shiny aluminum foil. If they're caught in the blaze, they lie flat on the dirt in a low spot with the aluminum tent propped just above them. If the fire passes quickly enough, they have a good chance of surviving it. The tent is made of aluminum because
(A) aluminum reflects most thermal radiation from the fire. [79.1% picked]
(B) aluminum does not conduct heat well. [9.3% picked]
(C) aluminum blocks convective heat transfer from the fire. [10.7% picked]
(D) aluminum does not burn. [0.9% picked]
Answer: (A) aluminum reflects most thermal radiation from the fire. [79.1% picked]
Why: TBA
Problem 47:
You are designing clothing and equipment that will help a downhill ski racer to win her races. Why should you minimize the size of the turbulent wake the skier produces in the air behind her?
(A) The skier gives mass to the wake in order to make it turbulent, so having less air in the wake means retaining as much mass as possible. [0.9% picked]
(B) The skier exerts a backward force on the wake in order to make it turbulent and that backward force slows the skier's forward velocity. The smaller the wake, the smaller the force. [8.0% picked]
(C) The wake shifts the skier's center of gravity backward, making it harder for her to accelerate forward. The smaller the wake, the smaller the shift and the faster the skier can move. [6.7% picked]
(D) The skier gives forward momentum to the wake, so having less air in the wake means giving away less forward momentum. [84.4% picked]
Answer: (D) The skier gives forward momentum to the wake, so having less air in the wake means giving away less forward momentum. [84.4% picked]
Why: TBA
Problem 48:
The air that flows over the wing of an airplane in flight bends downward, toward the top surface of the wing. Compared to its speed and pressure before it encountered the wing, the air flowing just above the wing's top surface has a
(A) smaller speed and a smaller pressure. [5.8% picked]
(B) larger speed and a larger pressure. [3.1% picked]
(C) larger speed and a smaller pressure. [80.4% picked]
(D) smaller speed and a larger pressure. [10.7% picked]
Answer: (C) larger speed and a smaller pressure. [80.4% picked]
Why: TBA
Problem 49:
Mike and Johnny are in trouble for scuffling in the school cafeteria. While Mike admits that he pushed Johnny, who immediately fell over backward, Mike claims that Johnny pushed back and is thus just as guilty. From the perspective of physics,
(A) Johnny pushed back on Mike, but with more force than Mike exerted on him. [0.0% picked]
(B) Johnny pushed back on Mike, but with less force than Mike exerted on him. [17.8% picked]
(C) Johnny didn't push back on Mike. [10.7% picked]
(D) Johnny did push back on Mike, with exactly the same amount of force. [71.6% picked]
Answer: (D) Johnny did push back on Mike, with exactly the same amount of force. [71.6% picked]
Why: TBA
Problem 50:
When the A-string of a violin vibrates in its fundamental mode without anyone touching it, it has a frequency (or pitch) of 440 cycles per second. When that same string is vibrating at 880 cycles per second without anyone touching it, it is vibrating in its
(A) fundamental mode, but traveling half as far during each vibrational cycle. [2.2% picked]
(B) second harmonic mode, as 2 half-length strings. [88.9% picked]
(C) fundamental mode, but traveling twice as far during each vibrational cycle. [1.8% picked]
(D) fundamental mode, but completing the same motion twice as quickly. [7.1% picked]
Answer: (B) second harmonic mode, as 2 half-length strings. [88.9% picked]
Why: TBA
Problem 51:
Two steel balls, one of which weighs twice as much as the other, roll off a horizontal table with the same speeds. In this situation,
(A) the heavier ball hits the floor at about half the horizontal distance from the base of the table than does the lighter. [4.0% picked]
(B) the lighter ball hits the floor at about half the horizontal distance from the base of the table than does the heavier. [0.9% picked]
(C) the heavier ball hits considerably closer to the base of the table than the lighter, but not necessarily half the horizontal distance. [0.9% picked]
(D) both balls hits the floor at approximately the same horizontal distance from the base of the table. [94.2% picked]
Answer: (D) both balls hits the floor at approximately the same horizontal distance from the base of the table. [94.2% picked]
Why: TBA
Problem 52:
A child is riding on a playground merry-go-round (a horizontal platform that rotates about a fixed center). As the merry-go-round rotates at a steady rate, the net force on the child points directly
(A) along the child's velocity (along the child's direction of travel). [7.6% picked]
(B) downward. [4.4% picked]
(C) away from the center of the merry-go-round. [2.7% picked]
(D) toward the center of the merry-go-round. [85.3% picked]
Answer: (D) toward the center of the merry-go-round. [85.3% picked]
Why: TBA
Problem 53:
You can put your hand in a 400 °F (200 °C) oven briefly without getting burned. However, even a second or two of contact with 212 °F (100 °C) steam can be quite painful. The steam delivers heat much faster to your hand because steam
(A) is actually a liquid with a high thermal conductivity. It is composed of countless tiny droplets suspended in the air. [12.9% picked]
(B) is a gas with a much higher thermal conductivity than air. [8.4% picked]
(C) condenses into water and deposits its heat of vaporization into your hand. [76.4% picked]
(D) undergoes much more rapid convection than air. [2.2% picked]
Answer: (C) condenses into water and deposits its heat of vaporization into your hand. [76.4% picked]
Why: TBA
Problem 54:
A child is using a crayon (a stick of colored wax) to draw a picture on a sheet of paper. The paper rests on a horizontal tabletop and the child's crayon is moving across the paper at a steady speed in a straight line. Which of the following statements about the forces acting on the crayon is correct?
(A) If the child were to exert twice as much force on the crayon, the crayon would slide across the paper at twice its original speed. [1.3% picked]
(B) The amount of the force the child is exerting on the crayon must be more than the amount of the force that friction is exerting on the crayon. [8.9% picked]
(C) The amount of the force the child is exerting on the crayon must be more than the amount of the crayon's weight. [3.6% picked]
(D) The amount of the force the child is exerting on the crayon must be equal to the amount of force that friction is exerting on the crayon. [86.2% picked]
Answer: (D) The amount of the force the child is exerting on the crayon must be equal to the amount of force that friction is exerting on the crayon. [86.2% picked]
Why: TBA
Problem 55:
When a wax candle burns in air, the water and carbon dioxide molecules that form as the result of combustion reactions have
(A) more chemical momentum than the original wax and air molecules [0.0% picked]
(B) less chemical potential energy than the original wax and air molecules [90.7% picked]
(C) more chemical potential energy than the original wax and air molecules [5.8% picked]
(D) less chemical momentum than the original wax and air molecules [3.6% picked]
Answer: (B) less chemical potential energy than the original wax and air molecules [90.7% picked]
Why: TBA
Problem 56:
When you blow gently across the top of a particular soda bottle, it emits a tone. The column of air in the bottle is vibrating up and down in its fundamental mode. If you replace the air in the bottle with helium and then blow gently across the top of the bottle, it will emit
(A) a tone at the same pitch as before. [3.6% picked]
(B) a tone at a higher pitch than before. [84.9% picked]
(C) no sound at all. [4.0% picked]
(D) a tone at a lower pitch than before. [7.6% picked]
Answer: (B) a tone at a higher pitch than before. [84.9% picked]
Why: TBA
Problem 57:
A sculptor is working with three non-flammable materials: a silvery metal, a black ceramic, and a white ceramic. When the sculptor heats these three materials to 1800 °C, they all begin to glow with visible thermal radiation. Which material glows most brightly?
(A) The black ceramic. [95.6% picked]
(B) They are all glowing with equal brightness. [0.9% picked]
(C) The white ceramic. [1.8% picked]
(D) The silvery metal. [1.8% picked]
Answer: (A) The black ceramic. [95.6% picked]
Why: TBA
Problem 58:
You have two cups in front of you, one filled with water and the other filled with honey. Honey has a much greater viscosity than water. Honey has a slightly greater density than water. If you stir both liquids at equal speeds using identical spoons, which liquid is more likely to become turbulent and why?
(A) The water is more likely to become turbulent because it has the much smaller viscosity. [90.2% picked]
(B) The water is more likely to become turbulent because it has the slightly smaller density. [4.9% picked]
(C) The honey is more likely to become turbulent because it has the much larger viscosity. [4.0% picked]
(D) The honey is more likely to become turbulent because it has the slightly larger density. [0.9% picked]
Answer: (A) The water is more likely to become turbulent because it has the much smaller viscosity. [90.2% picked]
Why: TBA
Problem 59:
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 where you plucking it. [5.8% picked]
(B) The string's fundamental vibrational frequency depends on its amplitude of vibration and that amplitude depends on where you pluck the string. [11.1% picked]
(C) Where you pluck the string divides the string into two parts that vibrate separately, so where you pluck affects those two partial strings. [5.8% 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. [77.3% 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. [77.3% picked]
Why: TBA
Problem 60:
Most clocks use harmonic oscillators as timekeepers because of what important characteristic of a harmonic oscillator?
(A) A harmonic oscillator's period of oscillation does not depend on its amplitude (extent of motion). [86.2% picked]
(B) A harmonic oscillator's amplitude (extent of motion) is constant, regardless of its total energy. [4.0% picked]
(C) A harmonic oscillator's momentum is constant throughout its oscillation. [4.0% picked]
(D) A harmonic oscillator's total energy is constant, regardless of its amplitude (extent of motion). [5.8% picked]
Answer: (A) A harmonic oscillator's period of oscillation does not depend on its amplitude (extent of motion). [86.2% picked]
Why: TBA