# Question Set 5 Review Questions: Newton's Law of Gravitation Answers

## Multiple Choice Questions

1. A woman standing on the surface of the Earth has a mass of 70 kilograms and a weight of 686 N. If instead she is floating freely inside a nonrotating space habitat far from the Earth, her mass would be

a. Less than 70 kg
b. 70 kg
c. More than 70 kg.

2. and her weight would be

a. Less than 686 N
b. 686 N
c. More than 686 N.

Reasoning: Mass is independent of locations. Thus her mass does not change. However, weight depends on the force of the gravity of the Earth and inside the space station, there is a smaller force of gravity acting on the woman.

3. A 400-N person stands on the surface of the Earth. If he were somehow able to stand on a ladder so that he was twice as far from the center of the Earth, he would weight

a. 0 N
b. 100 N
c. 200 N
d. 400 N

Reasoning: As shown in the text (pages157-158), and in the computer lessons for the review of gravity, the force of gravity falls with distace squared. Thus doubling the distance to the center of the earth causes the force of gravity to become 1/4 compared to the surface of the earth. The 400-N person now weighs 1/4 as much or 100 N.

4. This would be because

a. He would be above the atmosphere
b. His mass would be the same wherever he was
c. Gravitational force obeys an inverse square law.

5. The Earth and the moon are attracted to each other by gravitational force. The larger Earth attracts the smaller moon with a force whose magnitude is

a. Smaller than that of the force which the moon exerts on the Earth
b. Greater than that of the force which the moon exerts on the Earth
c. The same as that of the force which the moon exerts on the Earth.

6. This is in accordance with

a. The law of inertia
b. Kepler's Law's
c. The conservation of energy
d. The fact that both moon and Earth orbit about a common point, the center of mass
e. The law of action and reaction.

Reasoning: Forces are the same, however, the results are different. The moon with a smaller mass than the earth will have a larger acceleration due to this forcethan the earth does. (Note: this acceleration is NOT the acceleration of gravity on the moon. This is referring to the acceleration of the moon as it revolves around the earth.)

7. High tides are predominately produced by the

a. Strong gravitational pull of the sun
b. Centrifugal force created by the Earth-moon orbit
c. Differences in gravitational pulls of the moon on opposite sides of the Earth.

Reasoning:See page 161 of the text.

8. Because

a. Of rotation of the Earth and moon about the sun
b. The moon pulls with a significantly greater force on the closer side of the Earth than on the side farther away.
c. The sun pulls the Earth with a force 200 times stronger than the gravitational pull of the moon.

9. The body chiefly responsible for ocean tides is the

a. Moon
b. Sun

10. chiefly because

a. The moon is closer to the Earth than the sun
b. Gravitational pull by the moon is less than that of the sun
c. The difference between the moon's pull on the near and far sides of the Earth is appreciable.

11. According to Kelper's laws, the paths of planets about the sun are

a. Parabolas
b. Circles
c. Straight lines
d. Ellipses
e. None of these.

12. According to Newton, the greater the masses of interacting objects, the

a. Less the gravitational force between them
b. Greater the gravitational force between them
c. Greater the force between them by the square of the masses.

Reasoning: See the computer lesson on the review of gravity.

13. According to Newton, doubling the distance between two interacting objects

a. Divides by 2 the gravitational force between them
b. Multiplies by 2 the gravitational force between them
c. Divides by 4 the gravitational force between them
d. Multiples by 4 the gravitational force between them.

Reasoning: See the computer lesson on the review of gravity.

14. What is the force of gravity on a 500-newton woman standing on the Earth's surface?

a. 50 N
b. 250 N
c. 500 N
d. 509.8 N
e. None of these

Reasoning: A 500-N woman is acted upon by a force of 500 N from the earth. This is precisely the weight of the person.

15. Inside a freely-falling elevator, there would be no

a. Gravitational force on you
b. Apparent weight for you
c. Both of these
d. None of these.

Reasoning: See page 160 of the text.

16. An asteroid exerts a 360-N gravitational force on a nearby spacecraft. This force is directed

a. Toward the asteroid
b. Away from the asteroid
c. Toward the sun.

Reasoning: The gravitational force is an attractive force. Thus the astroid pulls the spacecraft toward itself.

17. If the radius of the Earth somehow decreased with no change in mass, your weight would

a. Increase
b. Not change
c. Decrease.

Reasoning: The decrease in radius will produce a much larger acceleration of gravity at the surface. This means your weight will increase.

18. A very massive object A and a less massive object B move toward each other under the influence of gravitation. Which force, if either, is greater?

a. The force on A
b. The force on B
c. Both forces are the same.

Reasoning: Newton's third law.

19. The reason the moon does not crash into the Earth is that the

a. Earth's gravitational field is weak at the moon
b. Gravitational pull of other planets keeps the moon up
c. Moon has a sufficient tangential speed
d. Moon has less mass than the Earth
e. None of these.

20. How far must one travel to get away from the Earth's gravitational field?

a. To a region above the Earth's atmosphere
b. To a region well beyond the moon
c. To a region beyond the solar system
d. Forget it; you can't travel far enough.

Reasoning: Gravitation is a long range force. Its range is infinite.

21. Which pulls on the oceans of the Earth with the greater force?

a. The moon
b. The sun
c. Both pull the same.

Reasoning: Sun applies a stronger force on the earth than the moon does. However, this does not mean that the sun creates bigger tidal effects.

22.The weight of an apple near the surface of the earth is 1 N. What is the weight of the earth in the gravitational field of the apple?

a) 1 N
b) 6 x 1024 N
c) 0 N
d) millions of millions of tons

Reasoning: Newton's third law.

23.The planet Jupiter is about 300 times more massive than the earth is. However, an object on the Jupiter weighs only about three times as much as the earth. The explanation for this is that

a) the Jupiter is 100 times smaller in radius than the earth is
c) the Jupiter being gaseous does not apply a full force on objects near its surface
d) the high temperature at the surface makes the force less effective

24. If you were in a freely falling elevator and you dropped a pencil, you’d see the pencil hovering. Is the pencil falling?

a) yes: it is falling at the same rate as the elevator
b) yes: it is falling, but it has much smaller acceleration because of its small mass
c) no: it is basically standing still exactly at the location it was released
d) no: it is actually going upward compared to the elevator

25. The value of g at the surface of the earth is 9.8m/s2. What is the value of g at a distance of twice the radius of the earth (from the center of the earth)?

a) 9.8 m/s2
b) 4.9 m/s2
c) 2.45 m/s2
d) 0 m/s2

Reasoning: See question 3. Also this is shown in ndetails in the computer review lesson for gravity.