1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Motion and Force Objectives Explain the effects of unbalanced forces on the motion of objects. Compare and contrast static and kinetic friction. Describe how friction may be either harmful or helpful. Identify ways in which friction can be reduced or increased. Chapter 10

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The concept of force explains many occurrences in our everyday lives. From your own experience, state what will happen in the following situations: 1.A marble is placed at the top of a smooth ramp. What happens to the marble? What force causes this? 2.A marble is rolling around in the back of a small toy wagon as the wagon is pulled along the sidewalk. When the wagon is stopped suddenly by a rock under one of the wheels, the marble rolls towards the front of the wagon. Why does the marble keep going when the wagon stops? (Hint: Consider what it takes to change the velocity of the wagon and the marble.) 3.If you dropped a flat uncrumpled sheet of notebook paper and a similar piece of notebook paper that was crushed into a ball from the same height, which would reach the floor first? Why are the forces on these two pieces of paper different? Section 3 Motion and Force Chapter 10 Bellringer

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Balanced and Unbalanced Forces Force is an action exerted on a body in order to change the body’s state of rest or motion. Force has magnitude and direction. The net force is the combination of all forces acting on an object. Objects subjected to balanced forces either do not move or move at constant velocity. An unbalanced force must be present to cause any change in an object’s state of motion or rest. Section 3 Motion and Force Chapter 10

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Force Section 3 Motion and Force Chapter 10

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Force of Friction Friction is a force that opposes motion between two surfaces that are in contact. Friction opposes the applied force. Section 3 Motion and Force Chapter 10

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Force of Friction, continued Section 3 Motion and Force Chapter 10

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Force of Friction, continued Static friction resists the initiation of sliding motion between two surfaces that are in contact and at rest. Kinetic friction opposes the movement of two surfaces that are in contact and are sliding over each other. Static friction is greater than kinetic friction. There are many different types of kinetic friction, such as sliding friction and rolling friction. Fluid friction, such as air resistance, also opposes motion. Section 3 Motion and Force Chapter 10

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Types of Friction Section 3 Motion and Force Chapter 10

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Frictional Forces and Acceleration Section 3 Motion and Force Chapter 10

10. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Friction and Motion Friction can be helpful or harmful. Friction is necessary to roll a vehicle or hold an object. However, friction can also cause excessive heating or wear of moving parts. Harmful friction can be reduced. Helpful friction can be increased. Section 3 Motion and Force Chapter 10

11. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Ways to Reduce or Increase Friction Section 3 Motion and Force Chapter 10