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QUESTIONS FOR THOUGHT FOR FURTHER ANALYSIS

1. An insect inside a bus flies from the back toward the front at 1. What are the significant similarities and diff erences between
2 m/s. The bus is moving in a straight line at 20 m/s. What is the speed and velocity?

speed of the insect? 2. What are the significant similarities and diff erences between

2. Disregarding air friction, describe all the forces acting on a bullet velocity and acceleration?

shot from a rifle into the air. 3. Compare your beliefs and your own reasoning about motion

3. Can gravity act in a vacuum? Explain. before and after learning Newton’s three laws of motion.
4. Is it possible for a small car to have the same momentum as a 4. Newton’s law of gravitation explains that every object in the universe is
large truck? Explain. attracted to every other object in the universe. Describe a conversation
5. Without friction, what net force is needed to maintain a 1,000 kg between yourself and another person who does not believe this law,
car in uniform motion for 30 minutes? as you persuade her or him that the law is indeed correct.
6. How can there ever be an unbalanced force on an object if every 5. Why is it that your weight can change by moving from one place
action has an equal and opposite reaction? to another, but your mass stays the same?

7. Why should you bend your knees as you hit the ground aft er 6. Assess the reasoning that Newton’s first law of motion tells us
jumping from a roof? that centrifugal force does not exist.
8. Is it possible for your weight to change while your mass remains
constant? Explain.
INVITATION TO INQUIRY
9. What maintains the speed of Earth as it moves in its orbit around
the Sun?
The Domino Effect
10. Suppose you are standing on the ice of a frozen lake and there
is no friction whatsoever. How can you get off the ice? (Hint: The domino effect is a cumulative effect produced when one event initi-

Friction is necessary to crawl or walk, so that will not get you off ates a succession of similar events. In the actual case of dominoes, a row is
the ice.) made by standing dominoes on their ends so they stand face to face in a

11. A rocket blasts off from a platform on a space station. An line. When the domino on the end is tipped over, it will fall into its neigh-

identical rocket blasts off from free space. Considering bor, which falls into the next one, and so on until the whole row has fallen.
everything else to be equal, will the two rockets have the same How should the dominoes be spaced so the row falls with maxi-
acceleration? Explain. mum speed? Should one domino strike the next one as high as possi-
12. An astronaut leaves a spaceship that is moving through free ble, in the center, or as low as possible? If you accept this invitation, you
space to adjust an antenna. Will the spaceship move off and leave must determine how to space the dominoes and measure the speed.

the astronaut behind? Explain.

PARALLEL EXERCISES

The exercises in groups A and B cover the same concepts. Solutions to group A exercises are located in appendix E.
Note: Neglect all frictional forces in all exercises.
Group A Group B
1. What is the average speed in km/h of a car that travels 160 km 1. What was the average speed in km/h of a car that travels 400.0 km
for 2 h? in 4.5 h?
2. What is the average speed in km/h for a car that travels 50.0 km 2. What was the average speed in km/h of a boat that moves
in 40.0 min? 15.0 km across a lake in 45 min?
3. What is the weight of a 5.2 kg object? 3. How much would a 80.0 kg person weigh (a) on Mars, where
2
the acceleration of gravity is 3.93 m/s , and (b) on Earth’s Moon,
2
where the acceleration of gravity is 1.63 m/s ?
4. What net force is needed to give a 40.0 kg grocery cart an 4. What force is needed to give a 6,000 kg truck an acceleration of
2
2
acceleration of 2.4 m/s ? 2.2 m/s over a level road?
5. What is the resulting acceleration when an unbalanced force of 5. What is the resulting acceleration when a 300 N force acts on an
100.0 N is applied to a 5.00 kg object? object with a mass of 3,000 kg?
6. What is the average speed, in km/h, for a car that travels 22 km 6. A boat moves 15.0 km across a lake in 30.0 min. What was the
in exactly 15 min? average speed of the boat in kilometers per hour?
8
7. Suppose a radio signal travels from Earth and through space at a 7. If the Sun is a distance of 1.5 × 10 km from Earth, how long does
8
8
speed of 3.0 × 10 m/s. How far into space did the signal travel it take sunlight to reach Earth if light moves at 3.0 × 10 m/s?
during the first 20.0 minutes?

8. How far away was a lightning strike if thunder is heard 5.00 seconds 8. How many meters away is a cliff if an echo is heard 0.500 s aft er
after the flash is seen? Assume that sound traveled at 350.0 m/s the original sound? Assume that sound traveled at 343 m/s on

during the storm. that day.
2-35 CHAPTER 2 Motion 59