1. Chapter 9
Gravity

2. Gravity was first discovered by
a. Aristotle.
b. Galileo.
c. Isaac Newton.
d. early humans.
Answer: D

3. Gravity was first discovered by
a. Aristotle.
b. Galileo.
c. Isaac Newton.
d. early humans.
Explanation: Newton did not discover gravity. What Newton discovered was that gravity is universal, existing far beyond Earth. Gravity itself is discovered by all of us as babies when we first fall, a discovery that goes back eons.
Answer: D

4. Legend tells us that Newton realized that the idea of an apple in free fall applies to
a. all apples.
b. the Moon.
c. Both of these.
d. Neither of these.
Answer: C

5. Legend tells us that Newton realized that the idea of an apple in free fall applies to
a. all apples.
b. the Moon.
c. Both of these.
d. Neither of these.
Explanation: It was the fall of an apple that triggered the notion of the Moon falling.
Answer: C

6. The Moon falls toward Earth in the sense that it falls
a. with an acceleration of 10 m/s2, as do apples on Earth.
b. beneath the straight-line path it would follow without gravity.
c. Both of these.
d. Neither of these.
Answer: B

7. The Moon falls toward Earth in the sense that it falls
a. with an acceleration of 10 m/s2, as do apples on Earth.
b. beneath the straight-line path it would follow without gravity.
c. Both of these.
d. Neither of these.
Answer: B

8. A planet remains in orbit while falling around the Sun due to
a. tangential velocity.
b. zero tangential velocity.
c. acceleration of about 10 m/s2.
d. centrifugal force.
Answer: A

9. A planet remains in orbit while falling around the Sun due to
a. tangential velocity.
b. zero tangential velocity.
c. acceleration of about 10 m/s2.
d. centrifugal force.
Answer: A

10. The gravitational constant G in Newton’s law of gravity
a. produces the correct units of force in Newton’s equation.
b. indicates the strength of gravity.
c. changes the proportion form of the law of gravity to an exact equation.
d. All of these.
Answer: D

11. The gravitational constant G in Newton’s law of gravity
a. produces the correct units of force in Newton’s equation.
b. indicates the strength of gravity.
c. changes the proportion form of the law of gravity to an exact equation.
d. All of these.
Answer: D

12. The force of gravity between two planets depends on their
a. planetary compositions.
b. planetary atmospheres.
c. rotational motions.
d. None of these.
Answer: D

13. The force of gravity between two planets depends on their
a. planetary compositions.
b. planetary atmospheres.
c. rotational motions.
d. None of these.
Explanation: Letting the equation for gravitational force guide our thinking, we can see none of the choices is valid.
Answer: D

14. If the Sun were half as massive, its pull on Mars would be
a. unchanged.
b. twice.
c. half.
d. 4 times as much.
Answer: C

15. If the Sun were half as massive, its pull on Mars would be
a. unchanged.
b. twice.
c. half.
d. 4 times as much.
Explanation: Letting the equation for gravitational force guide our thinking, half the mass with no other changes means half the force.
Answer: C

16. An apple halfway up a tree falls due to gravity
An apple halfway up a tree falls due to gravity. An apple at the top of the tree is pulled by a gravitational force that is
a. twice as much.
b. half as much.
c. one-fourth as much.
d. about the same.
Answer: D

17. An apple halfway up a tree falls due to gravity
An apple halfway up a tree falls due to gravity. An apple at the top of the tree is pulled by a gravitational force that is
a. twice as much.
b. half as much.
c. one-fourth as much.
d. about the same.
Explanation: Oops—unless you want to be real picky, the distance of the apple from the Earth’s center is about the same either way, so the force of gravity on the apple would be the same. For Earth gravity, the inverse-square law relates to the distance between an object and the center of Earth.
Answer: D

18. Our sensation of weight
a. is due to our sensation of mass.
b. is due to the force of gravity.
c. may or may not be due to the force of gravity.
d. remains constant as long as mass is constant.
Answer: C

19. Our sensation of weight
a. is due to our sensation of mass.
b. is due to the force of gravity.
c. may or may not be due to the force of gravity.
d. remains constant as long as mass is constant.
Explanation: Recall from the previous chapter that one feels a sense of weight in a rotating habitat regardless of the force of gravity there.
Answer: C

20. When an astronaut in orbit is weightless, he or she is
a. beyond the pull of Earth’s gravity.
b. still in the grips of Earth’s gravity.
c. in the grips of interstellar gravity.
d. None of these.
Answer: B

21. When an astronaut in orbit is weightless, he or she is
a. beyond the pull of Earth’s gravity.
b. still in the grips of Earth’s gravity.
c. in the grips of interstellar gravity.
d. None of these.
Explanation: If the astronaut were not in the grip of Earth’s gravity, would circling the Earth occur?
Answer: B

22. Inhabitants of the International Space Station do not have a
a. force of gravity on their bodies.
b. sufficient mass.
c. support force.
d. condition of free fall.
Answer: C

23. Inhabitants of the International Space Station do not have a
a. force of gravity on their bodies.
b. sufficient mass.
c. support force.
d. condition of free fall.
Answer: C

24. The body most responsible for ocean tides on Earth is
a. Earth itself.
b. the Sun.
c. the Moon.
d. Jupiter and other massive planets.
Answer: C

25. The body most responsible for ocean tides on Earth is
a. Earth itself.
b. the Sun.
c. the Moon.
d. Jupiter and other massive planets.
Answer: C

26. Most educated people know what body produces ocean tides, but which pulls more strongly on Earth and its oceans?
a. The Moon.
b. The Sun.
c. Both about equally.
d. Both exactly equally.
Answer: B

27. Most educated people know what body produces ocean tides, but which pulls more strongly on Earth and its oceans?
a. The Moon.
b. The Sun.
c. Both about equally.
d. Both exactly equally.
Answer: B

28. The highest ocean tides occur when Earth and the Moon are
a. lined up with the Sun.
b. at right angles to the Sun.
c. at any angle to the Sun.
d. lined up during spring.
Answer: A

29. The highest ocean tides occur when Earth and the Moon are
a. lined up with the Sun.
b. at right angles to the Sun.
c. at any angle to the Sun.
d. lined up during spring.
Answer: A

30. Tides in Earth’s atmosphere
a. are nonexistent.
b. exist only during the time of a new or full Moon.
c. result from the same physics responsible for ocean tides.
d. have no effect on Earth’s creatures.
Answer: C

31. Tides in Earth’s atmosphere
a. are nonexistent.
b. exist only during the time of a new or full Moon.
c. result from the same physics responsible for ocean tides.
d. have no effect on Earth’s creatures.
Answer: C

32. Compared to the gravitational field of Earth at its surface, Earth’s gravitational field at a distance 3 times as far from Earth’s center is about
a. one-third as much.
b. one-half as much.
c. one-ninth as much.
d. zero.
Answer: C

33. Compared to the gravitational field of Earth at its surface, Earth’s gravitational field at a distance 3 times as far from Earth’s center is about
a. one-third as much.
b. one-half as much.
c. one-ninth as much.
d. zero.
Explanation: By way of the inverse-square law, 3 times as far means one-ninth the gravitational field.
Answer: C

34. Compared to the gravitational field of Earth at its surface, Earth’s gravitational field at its center is
a. one-third as much.
b. one-half as much.
c. one-ninth as much.
d. zero.
Answer: D

35. Compared to the gravitational field of Earth at its surface, Earth’s gravitational field at its center is
a. one-third as much.
b. one-half as much.
c. one-ninth as much.
d. zero.
Explanation: At Earth’s center, gravitation cancels to zero.
Answer: D

36. Imagine you're standing on the surface of a shrinking planet
Imagine you're standing on the surface of a shrinking planet. If it shrinks to one-tenth its original diameter with no change in mass, on the shrunken surface you'd weigh
a. 1/100 as much.
b times as much.
c times as much.
d times as much.
Answer: C

37. Imagine you're standing on the surface of a shrinking planet
Imagine you're standing on the surface of a shrinking planet. If it shrinks to one-tenth its original diameter with no change in mass, on the shrunken surface you'd weigh
a. 1/100 as much.
b times as much.
c times as much.
d times as much.
Answer: C

38. A black hole is a. simply a collapsed star.
b. a two-dimensional surface in space.
c. barely visible with high-powered telescopes.
d. a new form of gravity.
Answer: A

39. A black hole is a. simply a collapsed star.
b. a two-dimensional surface in space.
c. barely visible with high-powered telescopes.
d. a new form of gravity.
Answer: A

40. A spacecraft on its way from Earth to the Moon is pulled equally by Earth and the Moon when it is
a. closer to Earth’s surface.
b. closer to the Moon’s surface.
c. halfway from Earth to the Moon.
d. At no point, since Earth always pulls more strongly.
Answer: B

41. A spacecraft on its way from Earth to the Moon is pulled equally by Earth and the Moon when it is
a. closer to Earth’s surface.
b. closer to the Moon’s surface.
c. halfway from Earth to the Moon.
d. At no point, since Earth always pulls more strongly.
Answer: B