Conceptual Physics Fundamentals

Paul G. Hewitt

Chapter 6

Fluid Mechanics - all with Video Answers

Educators

Chapter Questions

Problem 1

Stand on a bathroom scale and read your weight. When you lift one foot up so you're standing on one foot, does the reading change? Does a scale read force or pressure?

Prabhu Ramji

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Problem 2

The photo shows physics teacher Marshall Ellenstein walking barefoot on broken glass bottles in his class. What physics concept is Marshall demonstrating, and why is he careful that the broken pieces are small and numerous?

Prabhu Ramji

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Problem 3

In a deep dive, a whale is appreciably compressed by the pressure of the surrounding water. What happens to the whale's density?

Prabhu Ramji

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Problem 4

The density of a rock doesn't change when it is submerged in water. Does your density change when you are

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

Numerade Educator

Problem 5

Why are persons who are confined to bed less likely to develop bedsores on their bodies if they lie on a waterbed rather than on an ordinary mattress?

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

Numerade Educator

Problem 6

If water faucets upstairs and downstairs are turned fully on, will more water per second flow out the downstairs faucet? Or will the flows from the faucets be the same?

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

Numerade Educator

Problem 7

Which do you suppose exerts more pressure on the ground - an elephant or a woman standing on spike heels? (Which will be more likely to make dents in a linoleum floor?) Can you approximate a rough calculation for each?

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Problem 8

Suppose you wish to lay a level foundation for a home on hilly and bushy terrain. How can you use a garden hose filled with water to determine equal elevations for distant points?

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Problem 9

When you are bathing on a stony beach, why do the stones hurt your feet less when you get in deep water?

Prabhu Ramji

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Problem 10

If liquid pressure were the same at all depths, would there be a buoyant force on an object submerged in the liquid? Explain.

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

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Problem 11

The Himalayas are slightly less dense than the mantle material upon which they "float." Do you suppose that, like floating icebergs, these mountains are deeper than they are high?

Ben Nicholson

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Problem 12

How much force is needed to push a nearly weightless but rigid 1 -L carton beneath a surface of water?

Prabhu Ramji

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Problem 13

Why is it inaccurate to say that heavy objects sink and that light objects float? Give exaggerated examples to support your answer.

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

Numerade Educator

Problem 14

Compared to an empty ship, would a ship loaded with a cargo of Styrofoam sink deeper into water or rise in water? Defend your answer.

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

Numerade Educator

Problem 15

A barge filled with scrap iron is in a canal lock. If the iron is thrown overboard, does the water level at the side of the lock rise, fall, or remain unchanged? Explain.

Prabhu Ramji

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Problem 16

Would the water level in a canal lock go up or down if a battleship in the lock were to $\sin k ?$

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Problem 17

A balloon is weighted so that it is barely able to float in water. If it is pushed beneath the surface, will it return to the surface, stay at the depth to which it is pushed, or sink? Explain. (Hint: Does the balloon's density change?)

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Problem 18

A ship sailing from the ocean into a freshwater harbor sinks slightly deeper into the water. Does the buoyant force on it change? If so, does it increase or decrease?

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

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Problem 19

Suppose you are given the choice between two life preservers that are identical in size, the first a light one filled with Styrofoam and the second a very heavy one filled with lead pellets. If you submerge these life preservers in the water, upon which will the buoyant force be greater? Upon which will the buoyant force be ineffective? Why are your answers different?

Prabhu Ramji

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Problem 20

The relative densities of water, ice, and alcohol are $1.0,$ $0.9,$ and $0.8,$ respectively. Do ice cubes float higher or lower in a mixed alcoholic drink? What can you say about a cocktail in which the ice cubes lie submerged at the bottom of the glass?

Ben Nicholson

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Problem 21

When an ice cube in a glass of water melts, does the water level in the glass rise, fall, or remain unchanged? Does your answer change if the ice cube contains many air bubbles? Does your answer change if the ice cube contains many grains of heavy sand?

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

Numerade Educator

Problem 22

A half-filled bucket of water is on a spring scale. Will the reading of the scale increase or remain the same if a fish is placed in the bucket? (Will your answer be different if the bucket is initially filled to the brim?

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Problem 23

We say that the shape of a liquid is that of its container. But with no container and no gravity, what is the natural shape of a blob of water? Why?

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

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Problem 24

If you release a Ping-Pong ball beneath the surface of water, it will rise to the surface. Would it do the same if it were submerged in a big blob of water floating weightless in an orbiting spacecraft?

Ben Nicholson

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Problem 25

It is said that a gas fills all the space available to it. Why, then, doesn't the atmosphere go off into space?

Eduard Sanchez

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Problem 26

Count the tires on a large tractor trailer that is unloading food at your local supermarket, and you may be surprised to count 18 tires. Why so many tires? (Hint: See Active Exploration $5 .)$

Keshav Singh

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Problem 27

How does the density of air in a deep mine compare with the air density at Earth's surface?

Keshav Singh

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Problem 28

Two teams of eight horses each were unable to pull the Magdeburg hemispheres apart (Figure 21 ). Why? Suppose two teams of nine horses each could pull them apart. Then would one team of nine horses succeed if the other team were replaced with a strong tree? Defend your answer.

Keshav Singh

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Problem 29

Before boarding an airplane, you buy a bag of chips (or any item packaged in an airtight bag) and, while in flight, you notice that the bag is puffed up. Explain why this occurs.

Shital Rijal

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Problem 30

Why do you suppose that airplane windows are smaller than bus windows?

Abhishek Jana

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Problem 31

A half cup or so of water is poured into a 5 - $L$ can, which is placed on a source of heat until most of the water has boiled away. Then the top of the can is screwed on tightly and the can is removed from the source of heat and allowed to cool. What happens to the can and why?

Eduard Sanchez

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Problem 32

We can understand how pressure in water depends on depth by considering a stack of bricks. The pressure below the bottom brick is determined by the weight of the entire stack. Halfway up the stack, the pressure is half because the weight of the bricks above is half. To explain atmospheric pressure, we should consider compressible bricks, like foam rubber. Why is this so?

Shital Rijal

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Problem 33

The "pump" in a vacuum cleaner is merely a high-speed fan. Would a vacuum cleaner pick up dust from a rug on the Moon? Explain.

Abhishek Jana

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Problem 34

If you could somehow replace the mercury in a mercury barometer with a denser liquid, would the height of the liquid column be greater or less than with mercury? Why?

Shital Rijal

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Problem 35

Would it be slightly more difficult to draw soda through a straw at sea level or on top of a very high mountain? Explain.

Abhishek Jana

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Problem 36

Your friend says that the buoyant force of the atmosphere on an elephant is significantly greater than the buoyant force of the atmosphere on a small helium-filled balloon. What do you say?

Shital Rijal

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Problem 37

Why is it so difficult to breathe when snorkeling at a depth of $1 \mathrm{~m}$, and practically impossible at a $2-\mathrm{m}$ depth? Why can't a diver simply breathe through a hose that extends to the surface?

Shital Rijal

Numerade Educator

Problem 38

When you replace helium in a balloon with hydrogen, which is less dense, does the buoyant force on the balloon change if the balloon remains the same size? Explain.

Abhishek Jana

Numerade Educator

Problem 39

A steel tank filled with helium gas doesn't rise in air, but a balloon containing the same helium easily does? Why?

Keshav Singh

Numerade Educator

Problem 40

Two identical balloons of the same volume are pumped up with air to more than atmospheric pressure and suspended on the ends of a stick that is horizontally balanced. One of the balloons is then punctured. Is there a change in the stick's balance? If so, which way does it tip?

Keshav Singh

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Problem 41

The force of the atmosphere at sea level against the outside of a 10 -square-meter store window is about a million $N$. Why does this not shatter the window? Why might the window shatter in a strong wind blowing past the window?

Keshav Singh

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Problem 42

In the hydraulic arrangement shown, the larger piston has an area that is 50 times that of the smaller piston. The strong man hopes to exert enough force on the large piston to raise the $10 \mathrm{~kg}$ that rest on the small piston. Do you think he will be successful? Defend your answer

Ben Nicholson

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Problem 43

When a steadily flowing gas flows from a larger-diameter pipe to a smaller-diameter pipe, what happens to (a) its speed, (b) its pressure, and (c) the spacing between its streamlines?

Abhishek Jana

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Problem 44

How is an airplane able to fly upside down?

Abhishek Jana

Numerade Educator

Problem 45

When a jet plane is cruising at a high altitude, the flight attendants have more of a "hill" to climb as they walk forward along the aisle than when the plane is cruising at a lower altitude. Why does the pilot have to fly with a greater "angle of attack" at a high altitude than at a lower one?

Michael Sorola

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Problem 46

What physics principle underlies the following three observations? When passing an oncoming truck on the highway, your car tends to sway toward the truck. The canvas roof of a convertible automobile bulges upward when the car is traveling at high speeds. The windows of older passenger trains sometimes break when a high-speed train passes by on the next track.

Shital Rijal

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Problem 47

On a windy day, waves in a lake or the ocean are higher than their average height. How does Bernoulli's principle contribute to the increased height?

Eduard Sanchez

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Problem 48

Wharves are made with pilings that permit the free passage of water. Why would a solid-walled wharf be disadvantageous to ships attempting to pull alongside?

Abhishek Jana

Numerade Educator