1. © 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Lecture Outlines Chapter 5 Physics, 3 rd Edition James S. Walker

2. Chapter 5 Newton’s Laws of Motion

3. Units of Chapter 5 Force and Mass Newton’s First Law of Motion Newton’s Second Law of Motion Newton’s Third Law of Motion The Vector Nature of Forces: Forces in Two Dimensions Weight Normal Forces

4. Chapter 5 (p.107) The description of motion: kinematics The study of causes of motion: dynamics Newton’s laws must be modified for objects moving at speeds near that of light (relativitetsteori) and for objects comparable in size to atoms (kvantfysik). In the world of everyday experience, however, Newton’s laws still reign supreme.

5. 5-1 Force and Mass Force: push or pull Force is a vector – it has magnitude and direction

6. 5-1 Force and Mass Mass is the measure of how hard it is to change an object’s velocity. Mass can also be thought of as a measure of the quantity of matter in an object.

7. 5-2 Newton’s First Law of Motion If you stop pushing an object, does it stop moving? Only if there is friction! In the absence of any net external force, an object will keep moving at a constant speed in a straight line, or remain at rest. This is also known as the Law of Inertia (“Tröghetslagen”).

8. Photo 5-1 An air track provides a nearly frictionless environment for experiments involving linear motion.

9. Figure 5-1 The air track

10. An object at rest remains at rest as long as no net force acts on it. An object moving with constant velocity continues to move with the same speed and in the same direction as long as no net force acts on it. If the net force on an object is zero, its velocity is constant. 5-2 Newton’s First Law of Motion

11. In order to change the velocity of an object – magnitude or direction – a net force is required. An inertial reference frame (“inertialsystem”) is one in which the first law is true. The surface of the earth is to a very good approximation an inertial reference system. (Hastighet vid ekvatorn? Acceleration?) Accelerating reference frames are not inertial. 5-2 Newton’s First Law of Motion

12. 5-3 Newton’s Second Law of Motion Two equal weights exert twice the force of one; this can be used for calibration of a spring:

13. 5-3 Newton’s Second Law of Motion Now that we have a calibrated spring, we can do more experiments. Acceleration is proportional to force:

14. 5-3 Newton’s Second Law of Motion Acceleration is inversely proportional to mass:

15. 5-3 Newton’s Second Law of Motion Combining these two observations gives (En naturlag som kommer från jämförelse med experiment) Or, more familiarly,

16. 5-3 Newton’s Second Law of Motion An object may have several forces acting on it; the acceleration is due to the net force: (5-1)

17. 5-3 Newton’s Second Law of Motion

18. Conceptual Checkpoint 5-1(p.112) Tightening a Hammer/Your height

19. Figure 5-5 Constructing and using a free-body diagram

20. 5-3 Newton’s Second Law of Motion Free-body diagrams: A free-body diagram shows every force acting on an object. Sketch the forces Isolate the object of interest Choose a convenient coordinate system Resolve the forces into components Apply Newton’s second law to each coordinate direction

21. 5-3 Newton’s Second Law of Motion Example of a free-body diagram:

22. Example 5-1 (p.115) Three Forces (80,5 N each, m boat = 752 kg)

23. Figure 5-7 An astronaut using a jet pack to push a satellite (m=655 kg) that moves 0,675 m in 5,00 seconds

24. Photo 5-3 A technician inspects the landing gear of an airliner in a test of Foamcrete, a solid paving material that is just soft enough to collapse under the weight of an airliner. A plane that has run off the runway will slow safely to a stop as its wheels plow through the crumbling Foamcrete.

25. Active Example 5-1 (p.116/117) The Force Exerted by Foamcrete (m=175 000 kg)

26. Example 5-2 Pitch Man: Estimate the Force on the Ball (ignoring gravity, OK?) (Δx = 2,0 m, m = 0,15 kg, v = 90 mi/h = 40,225 m/s)

27. 5-4 Newton’s Third Law of Motion Forces always come in pairs, acting on different objects: If object 1 exerts a force F on object 2, then object 2 exerts a force –F on object 1. These forces are called action-reaction pairs.

28. Example 5-3 (p.119) Tippy Canoe, m 1 = 150 kg, m 2 = 250 kg, F 2 = 46 N a 1 = ? a 2 =?, What is the separation after 1,2 s?

29. 5-4 Newton’s Third Law of Motion Although the forces are the same, the accelerations will not be unless the objects have the same mass. Contact forces: The force exerted by one box on the other is different depending on which one you push.

30. Example 5-4a When Push Comes to Shove

31. Example 5-4b (p.112, fundera och diskutera nästa gång) When Push Comes to Shove

32. 5-5 The Vector Nature of Forces: Forces in Two Dimensions (m=940kg, F 1 =26N, F 2 =41N) The easiest way to handle forces in two dimensions is to treat each dimension separately, as we did for kinematics.

33. Example 5-5 (p.122) Jack and Jill (m = 1,3 kg, F 1 = 7,0 N, F 2 = 11 N, θ =? för att accelerationen endast skall vara i y-led)

34. Active Example 5-2 Find the Speed of the Sled (m= 4,60 kg, F= 6,20 N, θ= 35,0°, t = 1,15s)

35. 5-6 Weight The weight of an object on the Earth’s surface is the gravitational force exerted on it by the Earth.

36. Figure 5-10 Weight and Mass

37. Example 5-6 (p.125) (m= 97 kg, starting from rest, acceleration = const, t = 1,2 s, F=?) Where’s the Fire?

38. 5-6 Weight Apparent weight: Your perception of your weight is based on the contact forces between your body and your surroundings. If your surroundings are accelerating, your apparent weight may be more or less than your actual weight.

39. Example 5-7 (p.127) How Much Does the Salmon (m=5,0 kg) Weigh? a) a = 0 b) a = 3,2 m/s 2 uppåt c) a = = 3,2 m/s 2 nedåt

40. 5-7 Normal Forces The normal force is the force exerted by a surface on an object.

41. 5-7 Normal Forces The normal force may be equal to, greater than, or less than the weight.

42. 5-7 Normal Forces The normal force is always perpendicular to the surface.

43. Figure 5-15 Components of the weight on an inclined surface

44. Example 5-9 (p.131) Toboggan to the Bottom (a) Vad blir a x ? (b) N = ?

45. Summary of Chapter 5 Force: a push or pull Mass: measures the difficulty in accelerating an object Newton’s first law: if the net force on an object is zero, its velocity is constant Inertial frame of reference: one in which the first law holds Newton’s second law: Free-body diagram: a sketch showing all the forces on an object

46. Summary of Chapter 5 Newton’s third law: If object 1 exerts a force F on object 2, then object 2 exerts a force –F on object 1. Contact forces: an action-reaction pair of forces produced by two objects in physical contact Forces are vectors Newton’s laws can be applied to each component of the forces independently Weight: gravitational force exerted by the Earth on an object

47. Summary of Chapter 5 On the surface of the Earth, W = mg Apparent weight: force felt from contact with a floor or scale Normal force: force exerted perpendicular to a surface by that surface Normal force may be equal to, lesser than, or greater than the object’s weight