Newton’s Laws

1. Newton’s Laws Of Motion Copy the slides with a pencil

2. Inertia An object at rest will stay at rest… an object in motion will stay in motion at a constant velocity… UNLESS ACTED ON BY AN UNBALANCED FORCE!

3. Here’s an object at rest

4. It’s not moving

6. Look! It’s still at rest!

8. Why?

9. INERTIA!

10. This object will stay at rest until a force comes in and acts on it.

11. A “force” is a push or pull in a direction and is measured in “Newtons.” What happens if you give this block a push?

12. It will keep moving… FORCE

13. FORCE UNTIL… The object is acted on by force!

14. Now the object is at rest.

15. Do you need to keep pushing for the object to keep moving? It will keep moving because of its inertia NO! FORCE

16. Let’s examine that again…

17. An object at rest stays at rest because of its inertia.

18. FORCE Forces cause a change in motion. An object in motion will stay in motion at a constant velocity.

19. FORCE UNLESS… The object is acted on by force!

21. In outer space, objects keep moving in a straight line at the same speed FOREVER. Unless they hit something or get too close to a planet or other source of gravity (outside force).

22. A ball will go in a circle because of the force of the string. Once the string breaks it will go in a straight line.

23. Equilibrium: When the forces are balanced. When the forces are balanced there is no change in the object’s motion.

24. Is this object in equilibrium? 10 N up from table 10 N down from gravity

25. Is this object in equilibrium? YES! 10 N up from table The up and down forces are balanced. 10 N down from gravity Net force = 0

26. Is this object in equilibrium? 26 N up from the rope 26 N up from the rope 52 N down from gravity

27. Is this object in equilibrium? YES! 26 N + 26 N up from the rope The up and down forces are balanced. 52 N down from gravity Net force = 0

28. Is this object in equilibrium? 800 N up from the rope 800 N up from the rope 2000 N down from gravity

29. Is this object in equilibrium? NO! 800 N + 800 N up from the rope The up and down forces are NOT balanced. 2000 N down from gravity Net force = 400 N down

30. Is this object in equilibrium? NO! The car’s motion changes. Net force = 400 N down

31. Is this object in equilibrium? 50 N backwards from friction 40 N forwards from motor

32. Is this object in equilibrium? NO! 50 N backwards from friction The forces are NOT balanced. 40 N forwards from motor Net force = 10 N backward

33. Is this object in equilibrium? NO! The car’s motion changes. (It slows down!) Net force = 10 N backward

34. F = m x a Newton's Second Law of Motion Net Force = (Mass) * (Acceleration)

35. F = m x a Same force (swinging a club) applied to objects of different mass:

36. F = m x a Objects of different mass are accelerated equally: F = ma F = ma

37. F = m x a Objects that are the same mass are accelerated differently: Big force makes it accelerate faster F = ma F = ma Small force makes it accelerate slower

38. Equal, Opposite Reaction Newton's Third Law of Motion

39. Newton’s Third Law Every action force has an equal, opposite reaction force.

40. Action = Gun barrel pushing on bullet with 75N of force Reaction = _________________ Action = Person pushes wall with 100 N of force Reaction = _________________

41. You can only pull as hard as the other end can pull back at you…

42. They hit each other with the same force! The little truck will accelerate more because it has less mass.

43. Summary • Inertia (an object will change its motion only when acted on by an unbalanced force) • Net Force = Mass * Acceleration • Every action force has an equal, opposite reaction force