Conceptual Physics Fundamentals

Paul G. Hewitt

Chapter 4

Equilibrium and Linear Motion - all with Video Answers

Educators

Chapter Questions

Problem 1

A bowling ball rolling along a lane gradually slows as it rolls. How would Aristotle interpret this observation? How would Galileo interpret it?

Donald Albin

Donald Albin

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Problem 2

What Aristotelian idea did Galileo discredit in his fabled Leaning Tower of Pisa experiment? With his inclined plane experiments?

Keshav Singh

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Problem 3

When a ball rolls down an inclined plane, it gains speed because of gravity. When rolling up, it loses speed because of gravity. Why doesn'\operatorname{tg} r a v i t y ~ p l a y ~ a ~ r o l e ~ w h e n ~ i t ~ r o l l s ~ o n ~
a horizontal surface?

Kara Merfeld

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Problem 4

What physical quantity is a measure of how much inertia an object has?

Esmaeil Mahdavi

Esmaeil Mahdavi

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Problem 5

Which has more mass, a 2 -kg fluffy pillow or a 3 -kg small piece of iron? More volume? Why are your answers different?

Sri Datta Vikas Buchemmavari

Sri Datta Vikas Buchemmavari

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Problem 6

Does a dieting person more accurately lose mass or lose weight?

Eduard Sanchez

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Problem 7

A favorite class demonstration by the author is lying on his back with a blacksmith's anvil placed on his chest. When an assistant whacks the anvil with a strong sledgehammer blow, Hewitt is not hurt. How is the physics here similar to that illustrated in Figure $8 ?$

Keshav Singh

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Problem 8

What is your own mass in kilograms? Your weight in newtons?

Eduard Sanchez

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Problem 9

Gravitational force on the Moon is only $1 / 6$ that of the gravitational force on Earth. What would be the weight of a 10 -kg object on the Moon and on Earth? What would its mass be on the Moon and on Earth?

Jerrah Biggerstaff

Jerrah Biggerstaff

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Problem 10

Consider a pair of forces, one with a magnitude of $25 \mathrm{~N}$ and the other with a magnitude of $15 \mathrm{~N}$. What maximum net force is possible for these two forces? What minimum net force is possible?

Prabhu Ramji

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Problem 11

The sketch shows a painter's scaffold in mechanical equilibrium. The person in the middle weighs $250 \mathrm{~N}$, and the tensions in each rope are $200 \mathrm{~N}$. What is the weight of the scaffold?

Vishal Gupta

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Problem 12

A different scaffold that weighs $300 \mathrm{~N}$ supports two painters, one weighing $250 \mathrm{~N}$ and the other weighing $300 \mathrm{~N}$. The reading in the left scale is $400 \mathrm{~N}$. What should the reading in the right scale be?

Abhishek Jana

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Problem 13

Nellie Newton hangs at rest from the ends of the rope as shown. How does the reading on the scale compare to her weight?

cm

Charles Magnusen

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Problem 14

Harry the painter swings year after year from his bosun's chair. His weight is $500 \mathrm{~N}$ and the rope, unknown to him, has a breaking point of $300 \mathrm{~N}$. Why doesn't the rope break when he is supported as shown below? One day, Harry was painting near a flagpole, and, for a change, he tied the free end of the rope to the flagpole instead of to his chair, as shown to the right. Why did Harry end up taking his vacation early?

Prabhu Ramji

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Problem 15

A hockey puck slides across the ice at a constant velocity. Is it in mechanical equilibrium? Why or why not?

cm

Charles Magnusen

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Problem 16

If you push horizontally on a crate and it slides across the floor, slightly gaining speed, how does the friction acting on the crate compare with your push?

Hubert Agamasu

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Problem 17

When you place a heavy book on a table, the table pushes up on the book. Why doesn't this upward push cause the book to rise from the table?

Prabhu Ramji

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Problem 18

An empty jug of weight $W$ rests on a table. What is the support force exerted on the jug by the table? What is the support force when water of weight $w$ is poured into the jug?

Abhishek Jana

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Problem 19

In order to slide a heavy cabinet across the floor at constant speed, you exert a horizontal force of $600 \mathrm{~N}$. Is the force of friction between the cabinet and the floor greater than, less than, or equal to 600 N? Defend your answer.

Abhishek Jana

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Problem 20

Consider a crate at rest on a factory floor. As a pair of workers starts to lift it, does the support force on the crate provided by the floor increase, decrease, or remain unchanged? What happens to the support force on the workers' feet?

Prabhu Ramji

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Problem 21

Correct your friend who says, "The dragster rounded the curve at a constant velocity of $100 \mathrm{~km} / \mathrm{h}$."

Shital Rijal

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Problem 22

What is the impact speed when a car moving at $100 \mathrm{~km} / \mathrm{h}$ bumps into the rear of another car traveling in the same direction at $98 \mathrm{~km} / \mathrm{h} ?$

Kara Merfeld

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Problem 23

Harry Hotshot can paddle a canoe in still water at $8 \mathrm{~km} / \mathrm{h}$. How successful will he be at canoeing upstream in a river that flows at $8 \mathrm{~km} / \mathrm{h} ?$

Keshav Singh

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Problem 24

A destination 120 miles away is posted on a highway sign, and the speed limit is 60 miles/hour. If you drive at the posted speed, will you reach the destination in 2 hours? Or will you reach it in more than 2 hours?

Supratim Pal

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Problem 25

Suppose that a freely falling object were somehow equipped with a speedometer. By how much would its speed reading increase with each second of fall?

Kara Merfeld

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Problem 26

Suppose that the freely falling object in the preceding exercise were also equipped with an odometer. Would the readings of distance fallen each second indicate equal or unequal distances of fall for successive seconds? Explain.

Shital Rijal

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Problem 27

When a ballplayer throws a ball straight up, by how much does the speed of the ball decrease each second while moving upward? In the absence of air resistance, by how much does its speed increase each second while it is moving downward? How much time is required while moving upward? How much time is required while moving downward?

Kara Merfeld

Numerade Educator

Problem 28

Someone standing at the edge of a cliff (as in Figure 24$)$ throws a ball straight up at a certain speed and another ball straight down with the same initial speed. If air resistance is negligible, which ball has the greater speed when it strikes the ground below?

Keshav Singh

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Problem 29

What is the acceleration of a car that moves at a steady velocity of $100 \mathrm{~km} / \mathrm{h}$ for 100 seconds? Explain your answer, and state why this question is an exercise in careful reading as well as in physics.

Kajal Raghubanshi

Kajal Raghubanshi

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Problem 30

For a freely falling object dropped from rest, what is its acceleration at the end of the fifth second of fall? At the end of the tenth second? Defend your answers (and distinguish between velocity and acceleration).

Kara Merfeld

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Problem 31

Two balls, $A$ and $B$, are released simultaneously from rest at the left end of the equal-length tracks $A$ and $B$, as shown. Which ball will reach the end of its track first?

Kara Merfeld

Numerade Educator

Problem 32

Refer to the tracks above.
a. Does ball $\mathrm{B}$ roll faster along the lower part of track $\mathrm{B}$ than ball A rolls along track A?
b. Is the speed gained by ball $\mathrm{B}$ going down the extra dip the same as the speed it loses going up near the right-hand end - and doesn't this mean that the speed of balls $\mathrm{A}$ and $\mathrm{B}$ will be the same at the ends of both tracks?
c. On track $\mathrm{B}$, won't the average speed dipping down and up be greater than the average speed of ball A during the same time?
d. So, overall, does ball $A$ or ball $B$ have the greater average speed? (Do you wish to change your answer to the previous exercise?)

Donald Albin

Numerade Educator