1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. PHY 2048C General Physics I with lab Spring 2011 CRNs 11154, 11161 & 11165 Dr. Derrick Boucher Assoc. Prof. of Physics Session 5-6, Chapters 5-6

2. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapters 5 & 6 Practice Problems Chap 5: 19, 25, 27, 29, 33, 45, 47, 51 Chap 6: 5, 9, 19, 21, 29, 31, 33, 37, 45 Unless otherwise indicated, all practice material is from the “Exercises and Problems” section at the end of the chapter. (Not “Questions.”)

3. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. This is a conceptual chapter. No math! Ah…but chapter 6 will introduce the math AFTER we master these concepts. Chapter 5. Force and Motion

4. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. What is a force? A force is a push or a pull on an object. A force is a vector. It has both a magnitude and a direction. A force requires an agent. Something does the pushing or pulling. A force is either a contact force or a long- range force. Gravity is the only long- range force we will deal with until much later in the book.

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. A Short Catalog of Forces The box also pulls the Earth up toward it! Force the Earth exerts on the box.

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

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

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

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. EXAMPLE 5.1 Forces on a bungee jumper This is a “Free Body” diagram. It shows only the forces, as vectors.

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

15. Newton’s first law is also known as the law of inertia. If an object is at rest, it has a tendency to stay at rest. If it is moving, it has a tendency to continue moving with the same velocity.

16. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. EXAMPLE 5.4 An elevator accelerates upward

17. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapter 5. Summary Slides

18. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. General Principles

19. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Aristotle vs. Galileo (and Newton) Aristotle (c. 300 BCE) His ideas predominated for ~2000 years Stated that a CONSTANT FORCE was needed to keep an object in motion Galileo (c. 1600) Challenged Aristotle’s ideas Stated that constant motion is natural UNLESS forces intervene to slow it down (e.g. friction) or speed it up 

20. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Aristotle vs. Galileo (and Newton) “It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong” Richard Feynman, American physicist (1918-1988)

21. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. General Principles

22. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Important Concepts

23. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapter 4 Clicker QUIZ Get your clickers ready…

24. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A ball is thrown upward at a 45° angle. In the absence of air resistance, the ball follows a A.tangential curve. B.sine curve. C.parabolic curve. D.linear curve.

25. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. A hunter points his rifle directly at a coconut that he wishes to shoot off a tree. It so happens that the coconut falls from the tree at the exact instant the hunter pulls the trigger. Consequently, A.the bullet passes above the coconut. B.the bullet hits the coconut. C.the bullet passes beneath the coconut. D.This wasn’t discussed in Chapter 4.

26. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. The quantity with the symbol ω is called A.the circular weight. B.the circular velocity. C.the angular velocity. D.the centripetal acceleration.

27. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. For uniform circular motion, the acceleration A.points toward the center of the circle. B.points away from the circle. C.is tangent to the circle. D.is zero.

28. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapter 6. Dynamics I: Motion Along a Line This chapter focuses on objects that move in a straight line, such as runners, bicycles, cars, planes, and rockets. Gravitational, tension, thrust, friction, and drag forces will be essential to our understanding. Chapter Goal: To learn how to solve problems about motion in a straight line.

29. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Topics: Mass, Weight, and Gravity Equilibrium Using Newton’s Second Law Friction Drag More Examples of Newton’s Second Law Chapter 6. Dynamics I: Motion Along a Line

30. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Mass Mass is a scalar quantity that describes an object’s inertia. Loosely speaking, it also describes the amount of matter in an object. Mass is an intrinsic property of an object. It tells us something about the object, regardless of where the object is, what it’s doing, or whatever forces may be acting on it.

31. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Gravity It was Newton who first recognized that gravity is an attractive, long-range force between any two objects. Somewhat more loosely, gravity is a force that acts on mass. When two objects with masses m 1 and m 2 are separated by distance r, each object pulls on the other with a force given by Newton’s law of gravity, as follows:

32. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Gravity We can write the gravitational force even more simply as where the quantity g is defined to be R is the radius of the Earth. This approximation is only valid when the distance from the Earth’s center to the object is nearly R. (Within 1% if altitude is less than 32 km  20 miles.)

33. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Weight If the scale is at rest relative to the earth, then the object being weighed is in static equilibrium. The upward spring force exactly balances the downward gravitational force, so that F sp = F G = mg. Because we defined weight as the reading F sp of a spring scale, the weight of a stationary object is If g = 9.8 m/s 2, then m must be in kg so that w is in N

34. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Equilibrium An object on which the net force is zero is said to be in equilibrium. The object might be at rest in static equilibrium, or it might be moving along a straight line with constant velocity in dynamic equilibrium. Both are identical from a Newtonian perspective because the net force and the acceleration are zero.

35. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Problem-Solving Strategy: Equilibrium Problems

36. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chap 6 EXAMPLE Problem #30, page 179

37. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Using Newton’s Second Law The essence of Newtonian mechanics can be expressed in two steps. The forces on an object determine its acceleration a = F net /m, and The object’s trajectory can be determined by using the equations of kinematics.

38. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Problem-Solving Strategy: Dynamics problems

39. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chap 6 EXAMPLE Problem #12, page 178

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

41. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Simulations of Atomic-scale friction Friction is an atomic- scale phenomenon Poorly understood until very recently It can be understood via computer modeling (need to calculate atomic forces…not easy)

42. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Static Friction The box is in static equilibrium, so the static friction must exactly balance the pushing force:

43. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Static Friction There’s clearly a limit to how big f s can get. If you push hard enough, the object slips and starts to move. In other words, the static friction force has a maximum possible size f s max. An object remains at rest as long as f s < f s max. The object slips when f s = f s max. A static friction force f s > f s max is not physically possible. where the proportionality constant μ s is called the coefficient of static friction.

44. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Kinetic Friction The kinetic friction force is proportional to the magnitude of the normal force. where the proportionality constant μ k is called the coefficient of kinetic friction.

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

46. Chap 6 EXAMPLE Problem #18, page 178

47. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chap 6 EXAMPLE Problem #42, page 180

48. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Chapter 6. Summary Slides

49. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. General Strategy

50. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. General Strategy

51. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Important Concepts

52. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley. Important Concepts

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

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