1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapter 5. Force and Motion Chapter Goal: To establish a connection between force and motion.

2. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Student Learning Objectives To recognize what does and does not constitute a force. To identify the specific forces acting on an object. To draw an accurate free-body diagram of an object. To begin the process of understanding the connection between force and motion. To begin learning how to explain an observation on the basis of physical principles.

3. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A force is an interaction between two objects A force is a push or a pull on an object. If I push a book across a table, the book pushes me back (inanimate objects can exert force!) A force is a vector. It has both a magnitude and a direction. The force (interaction) has the same magnitude for both me and the book. However the direction of the force on me is opposite to the direction of the force on the book.

4. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A force is an interaction between two objects A force requires an agent. Another object does the pushing or pulling. This is another way of confirming that a force is an interaction between objects. It takes two to tango! A force is either a contact force or a long- range force. Gravity is the only long-range force we will study this semester. All other forces only exist when the two objects are in contact

5. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Tactics: Drawing force vectors

6. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Combining Forces

7. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Two of three forces exerted on an object are shown. The net force points to the left. What is the missing third force?

8. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Two of three forces exerted on an object are shown. The net force points to the left. What is the missing third force?

9. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces Gravity is a long-range attractive force between two objects. In this class, our emphasis is on the interaction between the Earth and objects on or near its surface (the weight force).

10. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces

11. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces

12. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces

13. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces

14. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces

15. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A Short Catalog of Forces

16. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball? A. Gravity, acting downward B. The normal force, acting upward C. The force of the kick, acting in the direction of motion D. Friction, acting opposite the direction of motion E. All of the above

17. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball? A. Gravity, acting downward B. The normal force, acting upward C. The force of the kick, acting in the direction of motion D. Friction, acting opposite the direction of motion E. All of the above Ans: A, B, and D

18. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Tactics: Identifying forces

19. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Tactics: Identifying forces

20. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 5.2 Forces on a skier QUESTION:

21. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 5.2 Forces on a skier

22. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Workbook Exercises 3-8

23. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. What do forces do? Can you apply a net force to an object and make it move at a constant speed in a straight line? –Stapler –Cart on low-friction track

24. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. An object pulled with a constant force moves with a constant acceleration a = cF where c, the proportionality constant, is the slope of the graph.

25. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. What is the proportionality constant? As the “size” of the object increases, the slope decreases. The size of the object is it’s mass, a measure of the amount of matter it contains. Now we can write the equation as: a = F/m

26. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Proportional or Ratio Reasoning If u = cv, then u 1 = cv 1 and u 2 = cv 2 : We can deduce information about u or v, without having to know the value of c.

27. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Non-linear Proportionality If w changes is tripled, by how much does u change?

28. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Non-linear Proportionality If w is tripled, by how much does u change? Use ratio reasoning to find u2: where w 2 = 3w 1 u 2 = (1/3) 2 u 1 = u 1 /9

29. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Workbook problems 10, 11

30. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Workbook problems 10 10: B, C, 3/5

31. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Workbook problems 11 11: –20 m/s 2 –4 m/s 2 –10 m/s 2 –40 m/s 2

32. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.

33. Newton’s first law is also known as the law of inertia. If an object is at rest, it will stay at rest. If it is moving, it will continue moving with the same speed in the same direction.

34. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Tactics: Drawing a free-body diagram

35. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Tactics: Drawing a free-body diagram