1. Pretest 1. What is relative motion?
Chapter 12
1. What is relative motion?
2. What is the difference between distance and displacement?
3. How is average speed calculated?
4. On a distance-time graph, what does the slope represent?

2. Pretest (continued) 5. What is velocity?
Chapter 12
5. What is velocity?
6. How is acceleration related to velocity?
7. A backpack falls out of an open window. The backpack starts from rest and hits the ground 1.0 second later with a velocity of 9.8 m/s. What is the average acceleration of the backpack?
a. 9.8 m/s b. 9.8 m c. 9.8 m/s2 d. all of the above
8. How are mass and weight different?

3. Interest Grabber Objects in Free Fall
Section 12.1
Objects in Free Fall
What factors affect a falling object? Perform the following simple activity to begin learning about the forces that act on falling objects.
1. Stand beside your desk. Hold a sheet of notebook paper level at eye level. Release the sheet of paper and watch it fall. Describe the motion of the paper.
2. Hold a sheet of notebook paper that has been crumpled into a tight ball at eye level. Release the crumpled paper and watch it fall. Describe the motion of the paper.
3. How do the motions of the flat sheet of paper and the crumbled ball of paper compare? What forces do you think are acting on each sheet of paper?

4. Relating Text and Visuals
Reading Strategy
Section 12.1
Relating Text and Visuals
a. Yes
b. No motion
c. Yes
d. No motion
e. Yes
f. No motion
g. Yes
h. No motion
i. No
j. Potted plant accelerates.

5. Combining Forces Acting on an Object
Figure 4

6. Combining Forces Acting on an Object
Figure 4

7. Combining Forces Acting on an Object
Figure 4

8. Combining Forces Acting on an Object
Figure 4

9. Combining Forces Acting on an Object
Figure 4

10. Combining Forces Acting on an Object
Figure 4

11. Interest Grabber Describing Motion
Section 12.2
Describing Motion
Imagine you are carrying a bowl of soup on a food tray as you walk toward your favorite table. Suddenly, someone walks in front of you. You stop abruptly to avoid a collision.
1. Describe the motion of the soup in the bowl immediately after your abrupt stop.
2. Explain why you think the soup in the bowl behaved the way it did.

12. Reading Strategy Building Vocabulary
Section 12.2
Building Vocabulary
a. Inertia is the tendency of an object to resist a change in its motion
b. Mass
c. Mass is the amount of matter an object contains as measured by its inertia.
d. Weight
e. Weight is the force of gravity acting on an object.

13. Newton’s Second Law
Section 12.2

14. Newton’s Second Law
Section 12.2

15. Newton’s Second Law
Section 12.2

16. Newton’s Second Law
Section 12.2

17. Effects of a Force on Acceleration
Figure 13

18. Effects of a Force on Acceleration
Figure 13

19. Effects of a Force on Acceleration
Figure 13

20. Effects of a Force on Acceleration
Figure 13

21. Effects of a Force on Acceleration
Figure 13

22. Effects of a Force on Acceleration
Figure 13

23. Interest Grabber Changing Motion
Section 12.3
Changing Motion
Did you know that billiard balls can be used to study the transfer of energy between objects?
1. Describe the motion of the balls in Figure A.
2. Figure B shows the motion of the billiard balls after impact. Explain why the motion of the balls changes in Figure B.

24. Reading Strategy Summarizing a. kg•m/s
Section 12.3
Summarizing
a. kg•m/s
b. mass (or velocity) c. velocity (or mass)

25. Conservation of Momentum
Figure 17A and 17B

26. Conservation of Momentum
Figure 17A and 17B

27. Conservation of Momentum
Figure 17A and 17B

28. Conservation of Momentum
Figure 17A and 17B

29. Conservation of Momentum
Figure 17C

30. Conservation of Momentum
Figure 17C

31. Interest Grabber Comparing Forces
Section 12.4
Comparing Forces
No matter where you are in the universe, certain types of forces are present. You are already familiar with two of these forces—electric force and magnetic force.
1. Describe the behavior of two bar magnets that are positioned so that their north and south poles are nearly touching.
2. Describe a common behavior of clothes when they are removed from a clothes dryer.
3. How are these two forces the same? How are they different?

32. Comparing and Contrasting
Reading Strategy
Section 12.4
Comparing and Contrasting
a. Neutrons and proton
b. Very short (decreases rapidly beyond the diameter of a few protons)
c. Very strong (100 times stronger than electrical repulsion force)
d. All particles
e. Short
f. Weaker than the strong force

33. Gravitational Forces Acting on Masses at Different Distances
Figure 21

34. Gravitational Forces Acting on Masses at Different Distances
Figure 21

35. Gravitational Forces Acting on Masses at Different Distances
Figure 21

36. Gravitational Forces Acting on Masses at Different Distances
Figure 21

37. Forces Acting on the Moon
Figure 22

38. Pretest Answers 1. What is relative motion?
Chapter 12
1. What is relative motion?
2. What is the difference between distance and displacement?
3. How is average speed calculated?
4. On a distance-time graph, what does the slope represent?
Relative motion is movement in relation to a frame of reference.
Distance is the length of a path between two points. Displacement is the direction from the starting point and the length of a straight line from the starting point to the ending point.
Total distance is divided by total time.
The slope represents the speed.
Click the mouse button to display the answers.

39. Pretest Answers (continued)
Chapter 12
5. What is velocity?
6. How is acceleration related to velocity?
7. A backpack falls out of an open window. The backpack starts from rest and hits the ground 1.0 second later with a velocity of 9.8 m/s. What is the average acceleration of the backpack?
a. 9.8 m/s b. 9.8 m c. 9.8 m/s2 d. all of the above
8. How are mass and weight different?
Velocity is speed with direction.
Acceleration is change in velocity, that is, any change in speed, direction, or both.
Mass is a measure of inertia; weight is the measure of the force of gravity acting on an object.
Click the mouse button to display the answers.

40. Interest Grabber Answers
Section 12.1
1. Stand beside your desk. Hold a sheet of notebook paper level at eye level. Release the sheet of paper and watch it fall. Describe the motion of the paper.
The paper flutters slowly to the ground.
2. Hold a sheet of notebook paper that has been crumpled into a tight ball at eye level. Release the crumpled paper and watch it fall. Describe the motion of the paper.
The crumpled sheet of paper falls straight to the ground.
3. How do the motions of the flat sheet of paper and the crumbled ball of paper compare? What forces do you think are acting on each sheet of paper?
The flat sheet of paper fluttered slowly to the ground whereas the crumpled ball of paper fell more quickly to the ground and followed a straight-line path. Do not assess students on correctly identifying the two opposing forces on the paper; accept any reasonable response. The two opposing forces are gravity and air resistance.

41. Interest Grabber Answers
Section 12.2
1. Describe the motion of the soup in the bowl immediately after your abrupt stop.
The soup in the bowl continues forward and spills over the bowl’s front edge onto the food tray.
2. Explain why you think the soup in the bowl behaved the way it did.
The soup’s motion is explained by momentum. The momentum of the soup keeps it moving forward after the forward motion of the food tray has been halted.

42. Interest Grabber Answers
Section 12.3
1. Describe the motion of the balls in Figure A.
The racked balls in Figure A are motionless. The cue ball moves in a straight line and at high speed toward the racked balls.
2. Figure B shows the motion of the billiard balls after impact. Explain why the motion of the balls changes in Figure B.
The high-speed cue ball strikes the motionless balls and transfers momentum and kinetic energy to them, causing them to move.

43. Interest Grabber Answers
Section 12.4
1. Describe the behavior of two bar magnets that are positioned so that their north and south poles are nearly touching.
The opposite poles of the bar magnets attract each other. If the magnets are close enough, they will move together.
2. Describe a common behavior of clothes when they are removed from a clothes dryer.
Clothes removed from a clothes dryer often stick together.
3. How are these two forces the same? How are they different?
Both forces are associated with charged particles. Both forces attract. The magnets are pulled together by magnetic forces, whereas the clothes cling together because of electric forces.

44. Go Online Data sharing Self-grading assessment
Chapter 12
Data sharing
Self-grading assessment
For links on forces, go to and enter the Web Code as follows: ccn-2121.
For links on mass, go to and enter the Web Code as follows: ccn-2122.
For links on Newton’s laws, go to and enter the Web Code as follows: ccn-2123.
For links on gravity, go to and enter the Web Code as follows: ccn-2124.