Chapter 5: Circular Motion, the Planets, and Gravity
1. Earth’s gravity attracts a person with a force of 120 lbs. The force with which the Earth is
attracted towards the person is
A. Zero.
B. Small but not zero.
C. billions and billions of tons.
D. 120 lbs.
Answer: D
2. Which planets exhibit retrograde motion, that is, periodically appear to reverse their direction
of motion across the sky?
A. Mars, Mercury, and Venus
B. Mars, Jupiter, and Venus
C. Mars, Jupiter, and Saturn
D. Venus, Saturn, and Jupiter
E. All of the above.
Answer: C
3. Astronauts orbiting the Earth in the space shuttle experienced weightlessness because
A. they, and all things with them, were in free fall.
B. the Earth’s gravity does not extend that far.
C. gravity is diluted because the space shuttle is spinning.
D. the shuttle shielded them from the Earth’s gravity.
Answer: A
4. A rock that weighs 100 lb on Earth is taken to the Moon. As strange as it may seem, it is just
as difficult to lift the 100 lb rock on the Moon as on the Earth. This is because
A. the mass of the rock is independent of gravity.
B. the weight of the rock is independent of gravity.
C. Nonsense! The rock is more easily lifted on the Moon.
Answer: C
5. For reasons known only to them, a group of extraterrestrials offers you your choice of three
gold ingots. One weighs 10 lb on Earth, the second weighs 10 lb on Jupiter, and the third weighs
10 lb on the Moon. To get the most gold, you should choose the ingot that weighs 10 lb on
A. Earth.
B. the Moon.
C. Jupiter.
D. No difference: 10 lbs is 10 lbs, wherever you go.
Answer: B
6. An object weighs 20 Newtons on Earth. What is its approximate mass ?
A. 2 kg.
B. 20 g.
C. 10 kg.
D. 196 kg.
E. 640 kg.
Answer: A
7. An object undergoes circular motion. Which pair of quantities MUST change?
A. the object’s speed and acceleration.
B. the object’s velocity and acceleration.
C. the object’s mass and speed.
D. all of the above.
Answer: B
8. Which of the following is NOT a vector?
A. acceleration
B. weight
C. mass
D. velocity
E. All of the above.
Answer: C
9. A banked curve in a roadway is designed for a speed of 35 mph. During an ice storm cars
should be able to safely negotiate this curve at 35 mph, because at this speed
A. the necessary centripetal force is supplied entirely by gravity.
B. at all speeds the acceleration vector of the car points down, helping maintain stability.
C. the centrifugal force of the car exactly balances the centripetal force.
D. the necessary centripetal force is supplied entirely by the normal force from the road.
Answer: D
10. The mass of an apple on the Earth is 0.2 kg. On the Moon, the mass of the same apple
would be
A. zero.
B. greater than zero but less than 0.2 kg.
C. 0.2 kg.
D. greater than 0.2 kg.
Answer: C
11. The weight of an oxygen tank of the Moon is 10 pounds. On the Earth, the same oxygen
tank would have a weight of
A. less then 10 pounds.
B. 10 pounds.
C. more than 10 pounds.
Answer: C
12. Six identical physics books, each with mass 2.0 kg, are taken to the Moon. On the Moon,
their combined mass is
A. zero.
B. 2.0 kg.
C. 6.0 kg.
D. 12 kg.
Answer: D
13. Two cars are moving around a circular track at the same constant speed. If car 1 is at the
inner edge of the track and car 2 is at the outer edge, then
A. the acceleration of car 1 is less than the acceleration of car two.
B. the accelerations of both cars are equal and greater than zero.
C. the acceleration of car 1 is greater than the acceleration of car 2.
D. the accelerations of both cars are zero.
Answer: C
14. A cyclist races around a circular track at the constant speed of 20 m/s. The radius of the
track is 50 m. The acceleration of the cyclist is
A. zero.
2
B. 8 m/s , toward the center of the track.
2
C. 10 m/s , downward.
2
D. 20 m/s , in the direction of travel.
Answer: B
15. A man weighs 600 N while on the surface of Earth. If he is transported to the planet Mythos,
which has the same mass as Earth but a radius that is twice as large as Earth’s, his weight would
be
A. 1200 N.
B. 600 N.
C. 300 N.
D. 150 N.
E. 100 N.
Answer: D
16. The orbit of Earth about the Sun is not a circle; Earth is closer to the Sun in December than
it is in June. Thus, the speed of Earth as it moves about the Sun is
A. greater in December than in June.
B. greater in June than it is in December.
C. steadily increasing all year long.
D. steadily decreasing all year long.
E. the same all year long.
Answer: A
17. An asteroid moving around the Sun happens to experience only negligible forces from other
objects in the solar system. The path of this asteroid will be
A. an ellipse.
B. a parabola.
C. a sinusoid.
D. a straight line.
Answer: A
18. A car travels around a curve with constant speed. The correct statement from the following
is:
A. The velocity of the car is constant.
B. The car has an acceleration directed inward toward the center of the curve.
C. The car has an acceleration directed outward from the center of the curve.
D. The car has zero acceleration.
E. The car has an acceleration directed in the instantaneous direction of the velocity vector.
Answer: B
19. A ball is whirled on the end of a string in a horizontal circle at constant speed. Suddenly, the
string breaks. Immediately after the string breaks, the ball will
A move horizontally outward from the center of the circle.
B. move horizontally inward toward the center of the circle.
C. drop straight down.
D. have a horizontal velocity that is tangential to the circle.
Answer: D
20. Car A travels with speed v around curve number one, which has a radius r. Car B travels
with speed 2v around curve number two, which has a radius 2r. The acceleration will be
A. zero for both cars.
B. the same for both cars.
C. greater for car A.
D. greater for car B.
Answer: D
21. If a ball at the end of a string is whirled in a vertical circle at constant speed, the tension will
be
A. the same throughout the motion.
B. greatest at the lowest point in the motion.
C. greatest at the highest point in the motion.
D. greatest at a point where the string is instantaneously parallel to the ground.
Answer: B
22. The first scientist to determine that the orbits of the planets were ellipses was
A. Copernicus.
B. Ptolemy.
C. Kepler.
D. Galileo.
E. Newton.
Answer: C
23. The heliocentric model of the solar system gained preference over the early Greek epicycle
model because
A. the heliocentric model gave a more accurate description of observed planetary motions.
B. only the heliocentric model could explain retrograde motion.
C. the heliocentric model was simpler.
Answer: C
24. According to Newton’s Law of Gravitation, if the distance between two bodies is doubled
the attractive force between them becomes
A. twice as large.
B. half as large.
C. four times as large.
D. one quarter as large.
E. unchanged.
Answer: D
25. The sun and Moon both have an effect on the tides. Which one has the larger effect, and
why?
A. The sun, because it exerts a larger force on the ocean.
B. The Moon, because it exerts a larger force on the ocean.
C. Both the sun and Moon equally affect the tides.
D. The Moon, because its force differs more between the surface and center of the Earth.
Answer: D
26. Suppose a planet has a mass of 100 times that of the Earth and a radius that is 10 times that
of the Earth. The acceleration of gravity on the surface of the planet, expressed in units of the
Earth's acceleration of gravity, g, is
A. g.
B. 10 g.
C. 100 g.
D. g/10.
E. g/100.
Answer: A
27. Suppose an artificial satellite has been put into circular orbit about the Earth, at a distance
from the center of the Earth equal to 1/4 the distance from the Earth’s center to the Moon's
center. In terms of the Moon’s period Tm, what will be the period of the satellite?
A. 16Tm.
B. 8Tm.
C. Tm.
D. Tm/8.
E. Tm/16.
Answer: D
28. On a two-lane highway (not divided), a car headed north experiences a centripetal
acceleration directed toward the east. Simultaneously, a truck passes the car, headed south in the
other lane. The direction of the centripetal acceleration on the truck is
A. west.
B. east.
C. north.
D. south.
Answer: B
29. What are the units for the constant G used in Newton’s law of universal gravitation?
A. (N m²) / kg².
B. kg m /s².
C. kg²/m².
D. N/m².
E. None of these.
Answer: A
30. If you are ever fortunate enough to experience a total eclipse of the sun, you can be sure that
it will happen when the Moon is
A. in the “new Moon” phase.
B. full.
C. in its “first quarter” phase.
D. in its “last quarter” phase.
E. No way to tell...it’s a matter of chance.
Answer: A
31. In Vienna, there is a Ferris wheel designed so that the passengers ride in a standing position.
If one of the passengers were standing on a bathroom scale, the scale would read lowest at
A. the highest point in the ride.
B. the lowest point in the ride.
C. the point in the ride where they were ascending most rapidly.
D. the point in the ride where they were descending most rapidly.
Answer: A
32. During a new Moon, when the Sun and the Moon are on the same side of the Earth, the
people who live next to the ocean will see about how many high tides per day?
A. One
B. Two
C. Three
D. Four
Answer: B
33. Two artificial satellites are in circular orbits about the Earth. Which of the two will be
moving faster in its orbit?
A. The higher one.
B. The lower one.
C. The one with the smaller mass.
D. The one with the larger mass.
Answer: B
34. A full Moon is just now rising. What time of day is it?
A. Dusk.
B. Dawn.
C. Midnight.
D. Noon.
E. No way to tell.
Answer: A
35. A satellite is in an elliptical orbit with distance from the center of the Earth varying from a
maximum of 2D to a minimum of D. If its speed is v when its distance from the center of the
Earth is 2D, what is its speed when its distance is D?
A. v
B. It is stationary.
C. ½ v.
D. 2v.
Answer: D.
36. In his model of the motions of the planets, Copernicus
A. assumed that the Earth is the center of the Solar system.
B. continued to use epicycles to explain planetary motion.
C. found that the planets move in paths shaped like ellipses.
D. was able to make predictions that were much more accurate than Ptolemy’s model gave.
Answer: B.
37. Strictly speaking, Newton’s law of universal gravitation, F = Gm1m2/r², is valid only if the
masses are either point masses or __________________ .
Answer: perfect (uniform) spheres.
38. The ____________ of a body decreases as it is moved away from the surface of the Earth.
Answer: weight
39. A body moving in a circle path at constant speed exhibits acceleration because its
____________ is changing.
Answer: velocity
40. A car can move at constant speed on a level curve on a highway as long as the force of
___________between the pavement and tires is sufficient to provide the necessary centripetal
force.
Answer: friction
41. A car could move at constant speed on an icy curve which is banked for _______________
(all, one, no) speed(s) of the car.
Answer: one
42. A person weighing 500 N rides on a Ferris wheel sitting on a bathroom scale. At the
highest point the scale reads zero while the person continues to move in a circular path. The
centripetal force on the person at this point is ___________ N.
Answer: 500
43. To explain the retrograde motion of planets, Ptolemy introduced the concept of
_______________.
Answer: epicycles
44. If a person sits on a bathroom scale while riding on a Ferris wheel, the reading on the scale
will be highest while passing through _______________ point (indicate a point in the path).
Answer: the lowest
45. Two satellites are projected into circular orbits about the Earth. The one closer to the Earth
has a period of 90 minutes. The one further away will have a period which is _____________
(longer, shorter or same as) compared to 90 minutes.
Answer: longer