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. Chapter 12 Forces & Motion
Unit 3 Chapter 12

6. 12.1 Forces A force is a push or a pull that acts on an object.
A force can cause a resting object to move, or it can accelerate a moving object by changing the object's speed or direction.
Force is measured by the stretch of a spring in newtons, abv. as N.
1 newton is equal to 1 kilogram-meter per second squared (1 N = 1 kg·m/s2).

7. Representing Forces
You can use an arrow (vector) to represent the direction and strength of a force.
The direction of the arrow represents the direction of the force.
The length of the arrow represents the strength, or magnitude, of the force.
You can combine force vectors by vector addition to show the resultant vector or the net force of how forces combine.

8. Combining Forces
The net force is the overall force acting on an object after all the forces are combined.
When the forces on an object are balanced, the net force is zero and there is no change in the object's motion.
When an unbalanced force acts on an object, the object accelerates in the direction of the largest combined force.

9. Combining Forces (Vectors)
Forces acting in opposite directions can also combine to produce an unbalanced force.
The net force equals the size of the larger force minus the size of the smaller force.
Two unequal forces act in opposite directions but combine to produce an unbalanced net force.

10. Combining Forces Acting on an Object
Figure 4

11. Combining Forces Acting on an Object
Figure 4

12. Combining Forces Acting on an Object
Figure 4

13. Combining Forces Acting on an Object
Figure 4

14. Combining Forces Acting on an Object
Figure 4

15. Combining Forces Acting on an Object
Figure 4

16. Friction
Friction is a force that opposes the motion of objects that touch, as they move past each other.
There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

17. Friction - 2
Static friction is the friction force that acts on objects that are not moving.
Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface .
Rolling friction is the friction force that acts on rolling objects like wheels.
Fluid friction opposes the motion of an object through a fluid, the higher the viscosity, the more resistance to flow.
Air resistance comes under fluid friction.

18. Gravity
Gravity is a force of attraction that acts between any two masses.
Earth's gravitational force exerts a force of attraction on every other object that is near Earth.
Gravity can act over large distances.
Earth's gravity acts downward toward the center of Earth.

19. Falling Objects
On Earth, both gravity and air resistance affect the motion of a falling object.
Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

20. Projectile Motion
When you throw a ball forward, you'll notice that it actually follows a curved path.
This curved path (a parabola) is an example of projectile motion.
The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.

21. 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.

22. 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.

23. 12.2 Newton’s Laws
Newton built on the work of scientists such as Galileo.
Newton summarized his study of force and motion in several laws of motion.
Previous errors held back progress in the study of motion for most of recorded. history, but the plague had forced Newton to leave college and study on his own.

24. Newton’s 1st Law
According to Newton's first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.
In other words, an object in motion stays in motion, & an object at rest stays at rest, unless acted upon by an outside force.
Inertia is the tendency of an object to resist a change in its motion.

25. Newton’s 2nd Law
An unbalanced force causes an object's velocity to change.
According to Newton's second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object's mass.
Acceleration = Net force/Mass or a = F/m
Mass is a measure of the inertia of an object and depends on the amount of matter the object contains.

26. Newton’s Second Law
Section 12.2

27. Newton’s Second Law
Section 12.2

28. Newton’s Second Law
Section 12.2

29. Newton’s Second Law
Section 12.2

30. Effects of a Force on Acceleration
Figure 13

31. Effects of a Force on Acceleration
Figure 13

32. Effects of a Force on Acceleration
Figure 13

33. Effects of a Force on Acceleration
Figure 13

34. Effects of a Force on Acceleration
Figure 13

35. Effects of a Force on Acceleration
Figure 13

36. Combining Laws 1 & 2
Weight is the force of gravity acting on an object.
Formula for weight is w = mg (g = 9.8 m/s2) or
Weight = mass X acceleration of gravity.
Formula for force is F = ma or
Force = mass X acceleration.
Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.

37. 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.

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

39. 12.3 Newton’s 3rd Law Forces always exist in pairs.
According to Newton's third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
These two forces are called action and reaction forces.

40. Momentum Momentum is the energy of motion that an object has.
The momentum for any object at rest is zero.
An object has a large momentum if the product of its mass and velocity is large.
A bullet fired at high velocity has a high momentum even though it is of low mass.

41. Momentum - 2
Momentum is measured in units of kilogram-meters per second kg•m/s.
Momentum increases as an objects speed increases.
You can calculate momentum by multiplying an object's mass (in kilograms) and its velocity (in meters per second).
Momentum = mass x velocity. M = mv

42. Momentum Is Conserved
Collisions obey the law of conservation of momentum if the system is closed.
A closed system means other objects and forces cannot enter or leave a system.
According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change.
In a closed system, the loss of momentum of one object equals the gain in momentum of another object. I.e. momentum is conserved

43. Conservation of Momentum
Figure 17A and 17B

44. Conservation of Momentum
Figure 17A and 17B

45. Conservation of Momentum
Figure 17A and 17B

46. Conservation of Momentum
Figure 17A and 17B

47. Conservation of Momentum
Figure 17C

48. Conservation of Momentum
Figure 17C

49. 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?

50. 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

51. 12.4 Universal Forces
The four universal forces are the electromagnetic, strong nuclear, weak nuclear, and gravitational .
All the universal forces act over a distance between particles of matter (particles don’t need to touch each other).
Each of these forces is affected by the distance between the particles of matter.

52. Electromagnetic Force
Electromagnetic force is associated with charged particles.
Electric force and magnetic force are the only forces that can both attract and repel.
Electric & magnetic force are two different aspects of the electromagnetic force.
Electric forces act between charged objects & particles like electrons & protons.
Opposites attract (+ & - , N&S) & like ones repel (N&N, S&S, + & +, - & -).

53. Nuclear Forces
Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together.
The strong nuclear force overcomes the electric force of repulsion that acts among the protons in the nucleus.
The weak nuclear force is involved in certain types of radioactive processes.

54. Gravitational Force
Gravitational force is an attractive force that acts between any two masses.
Newton's law of universal gravitation states that every object in the universe attracts every other object.
The gravitational force between two objects is proportional to their masses and decreases rapidly as the distance between the masses increases.

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

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

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

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

59. Earth, Moon, & Tides
Earth’s gravitational attraction keeps the moon in a nearly circular orbit.
Centripetal force is a center-directed force that continuously changes the direction of an object to make it move in a circle.
The gravitational pull of the moon causes two bulges in the Earth’s oceans.
The Earth rotates once each day beneath these bulges causing two high tides and two low tides.

60. Forces Acting on the Moon
Figure 22

61. 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.

62. 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.

63. 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.

64. 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.

65. 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.

66. 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.

67. 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.