2. List of Demo Materials Large BB Pendulum bob on string (no metal support) Blocky mass Book Ball (tennis, bouncy, golf, …) Dynamics cart Hi-tech cart List of Demo Materials Large BB Pendulum bob on string (no metal support) Blocky mass Book Ball (tennis, bouncy, golf, …) Dynamics cart Hi-tech cart

3. “Newton’s Laws” These laws explain Why things do what they do. FORCES

4. Motivation Why is the cup of coffee in the car more likely to spill when you: Turn, Stop, or Start? Why are people with filled backpacks less agile? Why are bicycles easy to balance when they are rolling?

5. Drawing Forces An introduction to Newton’s First Law

6. An object is dropped. Why does it fall? Draw a picture of the ball, and draw the _ _ _ _ _ that acts on the ball.

8. Weight This ball accelerates.

9. The object is hanging from a string. Why doesn’t it fall? Draw a picture of the ball, and draw the forces that act on the ball.

10. What forces act on the ball?

11. String (tension) Are there any other forces that act on the ball?

12. String (tension) Weight

13. String (tension)  F=0 String - Weight = 0 If the sum of the forces is zero, then the object does not accelerate. If the object does not accelerate, then the sum of the forces is zero.

14. An object is on the table. Draw the forces that act on the object.

15. What forces act on the object while it is on the table?

16. TABLE WEIGHT  F=0 Table - Weight = 0 If the sum of the forces is zero, then the object does not accelerate. If the object does not accelerate, then the sum of the forces is zero.

17. Pull a cart so it speeds up: Friction Pull Motion

18. Pull a cart so it slows down: Friction Pull Motion

19. Pull a cart so it goes at one speed: Friction Pull Tough idea: If the forces are equal, then the acceleration is zero, even if the object is moving! Motion

20. Newton’s Difficult First Law (2-case version) Easy Case: No Motion (_ _ _ _ _ _ _ _ _ _ _ _ is zero, and the sum of the forces is zero). [Book on table.] Tough Case: Constant Motion (_ _ _ _ _ _ _ _ _ _ _ _ is zero, and the sum of the forces is zero). [Cart at constant speed.]

21. Newton’s Difficult First Law (2-case version) Easy Case: No Motion (Acceleration is zero, and the sum of the forces is zero). [Book on table.] Tough Case: Constant Motion (_ _ _ _ _ _ _ _ _ _ _ _ is zero, and the sum of the forces is zero). [Cart at constant speed.]

22. Newton’s Difficult First Law (2-case version) Easy Case: No Motion (Acceleration is zero, and the sum of the forces is zero). [Book on table.] Tough Case: Constant Motion (Acceleration is zero, and the sum of the forces is zero). [Cart at constant speed.]

23. A car is on ‘cruise control.’ The speed is a constant 60 mph. Draw the forces:

24. Friction Road Force (due to engine) A car is on ‘cruise control.’ The speed is a constant 60 mph. Road Weight

25. How could you use Newton’s first law to get the ketchup out of the bottle?

26. Transition from the First to the Second Law …

27. A moving cart in an inverted position. [Maximum Friction] Aristotle: “Objects stop.” [Notice that the acceleration ≠ 0.] Motion

28. A moving cart in a traditional position. [Moderate Friction] Can you hear the squeaking and the grinding? There is still friction. a≠0 Motion

29. A $70 cart. [Small Friction] The friction is less. The cart barely slows down. Galileo: “Objects slow down, but only if there is a force to make it happen. That force is often friction.” Objects do not stop by themselves!

30. Galileo built a rounder ball on a smoother track: start end start roughest smoothest end The ball never gets back to its original height, but Galileo saw the pattern. Friction stops the ball (it doesn’t stop on its own).

31. … it would _ _ _ _ _ _ _ _ _! start ? If you could build a perfect marble on a perfect track …

32. … it would go forever! “ Inertia ” Objects in motion stay in motion, and objects at rest stay at rest (unless acted on by a force). start ? If you could build a perfect marble on a perfect track …

33. Uptight version of Law 1 If the total force acting on an object is zero then the object will not accelerate. Conversely, if the object is not accelerating, then the total force acting on the object is zero.

34. An Application of the First Law: The BB and the Cart The system is moving. Then the cart hits a barrier. What will happen? motion

35. An Application of the First Law: The BB and the Cart The system is at rest. Then the cart gets a shove. What will happen? What if a reference object is used to help us track the BB?

36. The core idea of the 2nd law:

37. Two trucks. One empty, one Full. Empty m Full M Compare their accelerations.

38. A =  F ÷ m Different accelerations can arise from different masses (and the same force). a =  F ÷ M

39. The Vector Nature of Forces The Direction of a Force Matters a Lot

40. Two 5 Newton forces act on the book. What happens? Top View Book 5 N 2 kg

41. Two 5 Newton forces act on the book. What happens? Top View Book 5 N 2 kg

42. Two 5 Newton forces act on the book. What happens? a = ? Book 5 N 2 kg

43. Two 5 Newton forces act on the book. What happens? To deal with this you need to learn about vectors. Book 5 N 2 kg There can be many forces that act on one object, but the object has only one acceleration.

44. Uptight version of Law 2 An object will accelerate if the total force acting on it is not zero. The direction of the acceleration is the same as the direction of the total force. The amount of acceleration depends on the mass of the object (M) and the total force acting on the object:

45. What is the core difference between Laws 1 and 2?

46. Quiz. Which Law fits better, the First or the Second? Velocity is constant or changing? a = 0 or a ≠ 0 ?  F = 0 or  F ≠ 0 ? First Law or Second Law? You pull a wagon at one velocity. A wagon rolls to a stop on a sidewalk.

47. Quiz. Which Law fits better, the First or the Second? Velocity is constant or changing? a = 0 or a ≠ 0 ?  F = 0 or  F ≠ 0 ? First Law or Second Law? You pull a wagon at constant velocity. Constanta = 0  F = 0 First A wagon rolls to a stop on a sidewalk. Changing a ≠ 0  F ≠ 0 Second

48. What is a “LAW” in science? A pattern in what we see, that covers many situations. Examples: –People want more stuff –a=∑F÷M Not an example: A tossed ball spends as much time going up as going down. [It’s not an important pattern.]

49. Competition between your teacher and a football player Each competitor gets to punch a tissue as hard as possible. Who can exert the most force?

50. Why would it hurt if you hit the wall?

51. Intro to Newton’s Third Law (The weird one that does not really belong.) This law is a bizarre pattern about forces, but has nothing to say about motion. There is no acceleration here, no velocity, nothing about what is actually going to happen. The nickname of this law is the “Action - Reaction” law. That is a misleading name and you should never say it.

52. Newton’s Third Law, in small bites: Forces always occur in pairs. [Push on your desk, your desk pushes on you.] The forces in the pair act on different objects. [One force is on your desk, and the other is on you.] The forces are in opposite directions. [You push down on the desk, your desk pushes up on you.] The forces in the pair are equal. [Weird, but when we measure, we see it’s true.]

53. Newton’s Third Law If Object A exerts a force on Object B, then Object B will exert an equal force on Object A, in the opposite direction.

54. 3 Easy Examples Demo: One student asks another student for a helping hand out of a chair. A collision between two football players. Carry a box.

55. An Interesting Example Hold a piece of cardboard at an angle, and spin around. Does this put a force on the air? Does this put a force on the cardboard?

56. An Interesting Example Hold a piece of cardboard at an angle, and spin around. Does this put a force on the air? Does this put a force on the cardboard?

57. An Interesting Example Hold a piece of cardboard at an angle, and spin around. Does this put a force on the air? Does this put a force on the cardboard? Why does it have the small propellor on the back?

58. Tough Examples The Truck and the Bug. During the collision, compare the accelerations and compare the forces. [Hint: see the two trucks from before. We see that different accelerations do not always mean that there are different forces.] A Rock falling. [The earth barely accelerates because of its large mass.]

59. It’s time for the ‘Stations.’

60. Post - Stations Discussion 5 6 7 9

61. Post - Stations Mini Lecture Apply Newton’s Second Law to a falling ball, and see why we conclude that W = Mg. Fill in the steps: Weight 9.8 m/s 2 = Weight / M g = W / M W = Mg Weight is the force the earth puts on an object. Mass is the amount of matter an object has (protons & neutrons).

62. Post - Stations Mini Lecture Apply Newton’s Second Law to an object hanging from a string ( or spring scale ), and see why we conclude that T = W = Mg. Fill in the steps: Tension Weight  F = 0 T - W = 0 T = W T = Mg

63. Post - Stations Mini Lecture Apply Newton’s Second Law to a book on a table, and see why we conclude that the force of the table (N) is: N = W = Mg. Fill in the steps: Weight Table (Normal Force)  F = 0 N - W = 0 N = W N = Mg