Explorations: An Introduction to Astronomy (Arny), 7th Edition

Chapter 3: Gravity and Motion

Thought Questions

1
A cinder block can be weightless in space. Would you want to kick it with your bare foot? Even if it is weightless, does it have inertia?
2
In some amusement park rides, you are spun in a cylinder and are pressed against the wall as a result of the spin. People sometimes describe that effect as being due to "centrifugal force." What is really holding you against the wall of the spinning cylinder? Drawing a sketch and using Newton's first law may help you answer the question.
3
Is there a force of gravity between the orbiting International Space Station and the Earth? If so, is it large enough that the ISS is affected by it? Why do the astronauts in the ISS float freely?
4
Use Newton's second law of motion to explain why smaller cars tend to get better mileage than larger ones.
5
Is there a force of gravity between you and this textbook? What is the dependence of the force of gravity on distance? If the book is much closer to you than the center of the Earth, why does it accelerate to the ground instead of toward you when you drop it?
6
Using F = ma and F = GMm/d 2, deduce the units of Newton's gravitational constant G if masses are measured in kilograms, times in seconds, and distances in meters.
7
How many times greater is the Earth's gravitational force on the Moon than the Moon's gravitational force on the Earth? Think about Newton's third law of motion before answering this.
8
When you walk, does the ground push on you, or do you push on the ground? Explain clearly how you are accelerated forward. Why is it harder to walk across the beach than down a road?
9
How many times greater is the Sun's gravitational force on the Earth than the Earth's gravitational force on the Sun? (Consider Newton's third law and the law of gravity.)
10
Consider a binary star system in which one star has ten times the mass of the other. Which star's orbit would you expect to have a larger circumference? Why?
11
Explain how a larger planet could have a smaller surface gravity than a smaller planet, or why it could not.
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