Conceptual Questions

ssm Solution is in the Student Solutions Manual

| 1 | A pile of empty aluminum cans has a volume of 1.0 m3. The density of aluminum is 2700 kg/m3. Explain why the mass of the pile is not .

| 2 | A method for resealing a partially full bottle of wine under a vacuum uses a specially designed rubber stopper to close the bottle. A simple pump is attached to the stopper, and to remove air from the bottle, the plunger of the pump is pulled up and then released. After about 15 pull-and-release cycles the wine is under a partial vacuum. On the 15th pull-and-release cycle, it is harder to pull up the plunger than on the first cycle. Why?

| 3 | A person could not balance her entire weight on the pointed end of a single nail, because it would penetrate her skin. However, she can lie safely on a “bed of nails.” A “bed of nails” consists of many nails driven through a sheet of wood so that the pointed ends form a flat array. Why is the “bed of nails” trick safe?

| 4 | As you climb a mountain, your ears “pop” because of the changes in atmospheric pressure. In which direction does your eardrum move (a) as you climb up and (b) as you climb down? Give your reasoning.

| 5 | A bottle of juice is sealed under partial vacuum, with a lid on which a red dot or “button” is painted. Around the button the following phrase is printed: “Button pops up when seal is broken.” Explain why the button remains pushed in when the seal is intact.

| 6 | A sealed container of water is full, except for a tube that is attached to it, as the drawing at right shows. Water is poured into this tube, and before the tube is full the container bursts. Explain why.

Question 6

| 7 | A large closed tank is completely filled with water. A valve is then opened at the bottom of the tank and water begins to flow out. When the water stops flowing, will the tank be completely empty, or will there still be a noticeable amount of water in it? Explain your answer.

| 8 | Could you use a straw to sip a drink on the moon? Explain.

| 9 | ssm Why does the cork fly out with a loud “pop” when a bottle of champagne is opened? To answer this question on an exam a student says, “Because the gas pressure in the bottle is approximately .” Explain whether this answer refers to absolute or gauge pressure.

| 10 | A scuba diver is below the surface of the water when a storm approaches, dropping the air pressure above the water. Would a sufficiently sensitive pressure gauge attached to his wrist register this drop in air pressure? Give your reasoning, assuming that the diver’s wrist does not move as the storm approaches.

| 11 | A steel beam is suspended completely under water by a cable that is attached to one end of the beam, so it hangs vertically. Another identical beam is also suspended completely under water, but by a cable that is attached to the center of the beam so it hangs horizontally. Which beam, if either, experiences the greater buoyant force? Provide a reason for your answer. Neglect any change in water density with depth.

| 12 | A glass beaker, filled to the brim with water, is resting on a scale. A block is placed in the water, causing some of it to spill over. The water that spills is wiped away, and the beaker is still filled to the brim. How do the initial and final readings on the scale compare if the block is made from (a) wood and (b) iron? In both cases, explain your answer, using data from Table 11-1 and Archimedes’ principle.

| 13 | On a distant planet the acceleration due to gravity is less than it is on earth. Would you float more easily in water on this planet than on earth? Account for your answer.

| 14 | Put an ice cube in a glass, and fill the glass to the brim with water. When the ice cube melts and the temperature of the water returns to its initial value, would the water level drop, remain the same, or rise (causing water to spill out)? Explain what you would observe in terms of Archimedes’ principle.

| 15 | As a person dives toward the bottom of a swimming pool, the pressure increases noticeably. Does the buoyant force also increase? Justify your answer. Neglect any change in water density with depth.

| 16 | ssm A pot is partially filled with water, in which a plastic cup is floating. The floating cup contains two small blocks, one of wood and one of lead. The wooden block is then removed from the cup and put into the water, where it floats. The level of the water in the pot is observed not to change when this is done. Next, the wooden block is put back into the cup and the lead block is put into the water, where it sinks to the bottom. Now the level of water in the pot does change. It falls. Explain these observations.

| 17 | The drawing shows a side view of part of a system of gates whose purpose is to protect the city of Venice, Italy, against flooding due to very high tides. When not in use the hollow, triangular-shaped gate is filled with seawater and rests on the ocean floor. The gate is anchored to the ocean floor by a hinge mechanism, about which it can rotate. When a dangerously high tide is expected, water is pumped out of the gate, and the structure rotates into a position where it can protect against flooding. Explain why the gate rotates into position.

Question 17

| 18 | In steady flow, the velocity of a fluid particle at any point is constant in time. On the other hand, a fluid accelerates when it moves into a region of smaller cross-sectional area. (a) Explain what causes the acceleration. (b) Explain why the condition of steady flow does not rule out such an acceleration.

| 19 | Concept Simulation 11.1 at reviews the central idea in this question. The cross-sectional area of a stream of water becomes smaller as the water falls from a faucet. Account for this phenomenon in terms of the equation of continuity. What would you expect to happen to the cross-sectional area when the water is shot upward, as it is in a fountain?

| 20 | Have you ever had a large truck pass you from the opposite direction on a narrow two-lane road? You probably noticed that your car was pulled toward the truck as it passed. What can you conclude about the speed of the air between your car and the truck compared to that on the opposite side of the car? Provide a reason for your answer.

| 21 | Can Bernoulli’s equation describe the flow of water that is cascading down a rock-strewn spillway? Explain.

| 22 | Hold two sheets of paper by adjacent corners, so that they hang downward. They should be parallel and slightly separated, so that you can see the floor through the gap between them. Blow air strongly down through the gap. Do the sheets move further apart, or do they come closer together? Discuss what you observe in terms of Bernoulli’s equation.

| 23 | Which way would you have to spin a baseball so that it curves upward on its way to the plate? In describing the spin, state how you are viewing the ball. Justify your answer.

| 24 | You are traveling on a train with your window open. As the train approaches its rather high operating speed, your ears “pop.” Your eardrums respond to a decrease or increase in the air pressure by “popping” outward or inward, respectively. Do your ears “pop” outward or inward on the train? Give your reasoning in terms of Bernoulli’s equation.

| 25 | A passenger is smoking in the backseat of a moving car. To remove the smoke, the driver opens his window just a bit. Explain why the smoke is drawn to and out of the driver’s window.

| 26 | ssm The airport in Phoenix, Arizona, has occasionally been closed to large planes because of weather conditions that do not entail storms or low visibility. Instead, the conditions combine to create an unusually low air density. Using Bernoulli’s equation, explain why such an air density would make it difficult for a large, heavy plane to take off, especially if the runway were not exceptionally long.

| 27 | To change the oil in a car, you remove a plug beneath the engine and let the old oil run out. Your car has been sitting in the garage on a cold day. Before changing the oil, it is advisable to run the engine for a while. Why?

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