2. CHAPTER SIX: LAWS OF MOTION  6.1 Newton’s First Law  6.2 Newton’s Second Law  6.3 Newton’s Third Law and Momentum

3. Ch. 6.1 Force changes motion  A force is a push or pull, or any action that is able to change motion.

4. Galileo’s Inertia  Inertia is the tendency of things to resist changes in motion  Example: Removing the tablecloth from a table: Too little force, too little time to overcome "inertia" of tableware.

5.  Newton’s first law says that objects continue the motion they already have unless they are acted on by a net force.  If the net force is zero, an object at rest will stay at rest.  If an object is acted upon by unbalanced forces, its motion will change. Newton’s First Law of motion Often Called the Law of Inertia

6. Newton’s First Law of motion  Motion tends to continue unchanged.  Example: The elephant at rest tends to remain at rest.

7. MASS AND INERTIA  Inertia is also called mass  Mass – measure of the quantity of matter in an object  Mass is measured in kilograms  One kilogram is the amount of mass in a 2.2 pound weight  1 Kg = 9.8 Newton's on Earth

8. NET FORCE  Newton’s first law is often written in terms of the net force.  “An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.” According to these vectors, in what direction is the net force?

9. FORCE CHANGES MOTION  Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.

10. LAW OF INERTIA  Inertia is the property of an object that resists changes in motion.  Objects with more mass have more inertia and are more resistant to changes in their motion. Which ball has more inertia?

12. SOLVING PROBLEMS A car drives along the highway at constant velocity. Find the car’s weight and the friction force if the engine produces a force of 2,000 newtons between the tires and the road and the normal force on the car is 12,000 N.

13. SOLVING PROBLEMS 1. Looking for:  weight of car in newtons, force due to friction 2. Given:  Force N = 12,000N (up);  Force E = 2,000N (forward) 3. Relationships:  Newton’s 1 st Law: net force = zero at constant velocity; so Force N = Force W and Force E = Force F

14. F E = 200 N SOLVING PROBLEMS 4. Solution  Draw a free body diagram.  There is no net force upward, so the weight of the car is an equal downward force of − 12,000 N.  The forward engine force balances the friction force so the friction force is − 2,000 N. F F = -200 N F W = -12,000N F N = 12,000N

15. THREE EXAMPLES OF INERTIA

16. Ch. 6.2 Newton’s Second Law of Motion  The force of a moving object is directly proportional to the object’s mass and acceleration.

17. ACCELERATION Equals Force / Mass Inversely Proportional to Mass

18. EFFECTS OF A FORCE ON ACCELERATION

19. Newton’s second law  Newton’s first law tells us that motion cannot change without a net force.  According to Newton’s second law, the amount of acceleration depends on both the force and the mass.

20. Newton’s second law There are three main ideas related to Newton’s Second Law: 1. Acceleration is the result of unbalanced forces. 2. A larger force makes a proportionally larger acceleration. 3. Acceleration is inversely proportional to mass.  Which means that when mass increases, acceleration decreases.

21. Newton’s second law  Unbalanced forces cause changes in speed, direction, or both.

22. ACCELERATION AND DIRECTION  Another important factor of the second law is that the acceleration is always in the same direction as the net force.

24. ACCELERATION AND FORCE  The second law says that acceleration is proportional to force.  If force is increased or decreased, acceleration will be increased or decreased by the same factor.

25. ACCELERATION AND MASS  The greater the mass, the smaller the acceleration for a given force.  This means acceleration is inversely proportional to mass.

26. ACCELERATION, FORCE AND MASS  The acceleration caused by a force is proportional to force and inversely proportional to mass. F a m

27.  The stronger the force on an object, the greater its acceleration.  Force is directly proportional to acceleration.  If twice the force is applied, the acceleration is twice as great.

28.  The greater the mass, the smaller the acceleration for a given force.  Mass is inversely related to force.  An object with twice the mass will have half the acceleration if the same force is applied.

29. APPLYING THE SECOND LAW  Keep the following important ideas in mind: 1. The net force is what causes acceleration. 2. If there is no acceleration, the net force must be zero. 3. If there is acceleration, there must also be a net force. 4. The force unit of newtons is based on kilograms, meters, and seconds.

30. SOLVING PROBLEMS A car has a mass of 1,000 kilograms. If a net force of 2,000 N is exerted on the car, what is its acceleration? 1. Looking for: car’s acceleration 2. Given: mass = 1,000 kg; net force = 2,000 N 3. Relationships: a = F / m 4. Solution: 2,000 N ÷ 1,000 kg = 2 N/kg = 2 m/s 2

31. Newton’s Third Law of Motion  Third Law – Whenever an object exerts a force on a second object, the second object exerts an equal and opposite force on the first.  For every action there is an equal and opposite reaction, or All forces occur in pairs.

32.  Newton pushes on the elephant and the elephant pushes back! The Third Law: Action/Reaction

33.  Newton’s Third Law (action-reaction) applies when a force is placed on any object, such as a basketball.  There can never be a single force, alone, without its action-reaction partner. The Third Law: Action/Reaction

34. 6.3 ACTION AND REACTION  When sorting out action and reaction forces it is helpful to examine or draw diagrams. Here the action force is on the ________________, and the reaction force is on the _______________.

35. ACTION-REACTION  Note: each of the two forces in the pair acts on a different object.  Hammer pushes on stake. Stake pushes on hammer.  The hammer acts, the stake re-acts.

36. ACTION-REACTION FORCES DO NOT CANCEL  Action reaction forces do not act on the same object.  Only when action reaction forces act on the same object will they result in a net force of zero.

38. ACTION-REACTION PAIR EXAMPLES

39. RECOILING GUN  Expanding gas pushes bullet out of gun barrel. Why does the gun recoil?

40. ACTION-REACTION  If action-reaction forces are equal but opposite, why don't they cancel?

41. SOLVING PROBLEMS  A woman with a weight of 500 newtons is sitting on a chair.  Describe one action-reaction pair of forces in this situation.

42. SOLVING PROBLEMS 1. Looking for:  …pair of action-reaction forces 2. Given  …girl’s force W = -500 N (down) 3. Relationships:  Action-reaction forces are equal and opposite and act on different objects. 4. Solution  Draw a free body diagram  The downward force of 500 N exerted by the woman on the chair is an action.  Therefore, the chair acting on the woman provides an upward force of 500 N and is the reaction. Fw = -500 N Fc = 500 N

43. Write Newton’s three Laws of motion! Bell Work.

44. 6.3 COLLISIONS  Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other.  The effect of the force is not always the same.

45. MOMENTUM  How hard is it to stop an object.  the mass matters  the speed matters  p = mv  Yes, that’s “p ” for momentum!

46. 6.3 MOMENTUM  Momentum is the mass of a object times its velocity.  The units for momentum are kilogram-meter per second (kg·m/s).  All moving objects have momentum.

47. MOMENTUM  If the boulder and the boy have the same momentum, will the boulder crush the boy?  Hint: Which would have the larger speed?  No

48. 6.3 MOMENTUM  The law of conservation of momentum states that as long as the interacting objects are not influenced by outside forces (like friction) the total amount of momentum is constant or does not change.

49. 6.3 MOMENTUM  The result of a skateboarder throwing a 1-kg ball at a speed of -20 m/sec is that he and the skateboard with a total mass of 40 kg move backward at a speed of +0.5 m/sec (if you ignore friction). We use positive and negative numbers to show opposite directions.

50. 6.3 COLLISIONS  When a large truck hits a small car, the forces are equal.  The small car experiences a much greater change in velocity much more rapidly than the big truck. Which vehicle ends up with more damage?

51. MOMENTUM IS CONSERVED DURING A COLLISION  What does this mean?  Total momentum for the system stays the same before and after the collision.  Momentum lost by one object is gained by another. Conservation of Momentum

52. HARD TO STOP, SMALL MASS m v m v

53. HARD TO STOP, SMALL VELOCITY m v

54. mV

55. CONSERVATION OF MOMENTUM  Momentum Before = 0  Momentum After = 0  After firing, the opposite momentum cancel.

56. CONSERVATION OF MOMENTUM  Momenta are equal but opposite.  M v = m V  4 kg v = 0.010 kg x 300 m/s v = 0.75 m / s

57. CONSERVATION OF MOMENTUM after before during

58. CONSERVATION OF MOMENTUM

62. SOLVING PROBLEMS  If an astronaut in space were to release a 2-kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed?  The astronaut’s mass is 100 kilograms.

63. SOLVING PROBLEMS 1. Looking for:  … the velocity of the astronaut (backward) 2. Given  …velocity 1 = 10 m/s; mass 1 = 2 kg; ...mass 2 = 100 kg; 3. Relationships:  m 1 v 1 = m 2 v 2 4. Solution  Draw a free body diagram.