Newton’s Three Laws of Motion – Law # 1

1. Newton’s ThreeLaws of Motion – Law #1

2. Newton’s First Law: This Law has two parts ~ 1) An object at rest will stay at restunless it is acted upon by an unbalanced force. 2) An object in motion will stay in motionwith the same speed and in the same direction unless acted upon by an unbalanced force.

3. The first law is also known as the Law of Inertia. • Inertia: two definitions:1) the tendency of an object to keepdoing what it’s alreadydoing2) An object’s resistance to change its speed and/or direction (its velocity)

4. Anything with mass has inertia. • .All matter (solids, liquids, and gases) has mass therefore all matter (solids, liquids, and gases) has inertia.

5. The more mass an object has, the more inertia it has. • More massive objects are harder to stop and start moving. • Example: Bowling ball vs. Ping Pong ball* A bowling ball has more mass, therefore more inertia, and is harder to move. * A ping pong ball has less mass, therefore less inertia, and is easier to move.

6. The more velocity an object has, the more inertia it has. • .Faster moving objects are harder and take longer to stop. • Example: Running vs. Walking * If you are running down the hall and someone steps in front of you, you are more likely to run into them. * If you are walking down the hall and someone steps in front of you, you are less likely to run into them.

7. Inertia Video: Enter the number 1 in the box below the screen. Inertia begins at 1:24ends at 4:50 http://www.animatedscience.co.uk/flv/

8. According to the First Law of Motion: • Once an object is moving, it will keep moving until an unbalanced force causes it to stop. • If an object is at rest, it will stay at rest until an unbalanced force causes it to move.

9. If an object is moving in a northward direction, it will continue moving northward until an unbalanced force causes it to change direction. • Remember, an object’s motion will only change when an unbalanced force acts upon the object.

10. Balanced vs. Unbalanced Forces Unbalanced: • The forces on the object are NOT equal in size • The net force is NOT zero. • The forces may be in the same or opposite direction(s). • The object DOES experience a CHANGE of motion; changes direction and/or speed.

11. Remember number lines? • Going to the right is ___________________. • Going to the left is _____________________.  positive negative  Figure the net force the same way: Give arrows going to the right a positive number and arrows going to the left a negative number.

12. Example #1: 20 N 40 N • Figure the net force: _____ + _____= _____ • The net force is _____________ • The net force is _____ zero, therefore it is _________________. • The box will be moving to the ______with a force of _____ ___.  20  -40  -20  -20 N NOT  unbalanced left  20  N

13. Example #2: 60 N 10 N • Figure the net force: ___ + ___ + ___= _____ • The net force is _____________ • The net force is _____ zero, therefore it is _________________. • The box will be moving to the ______with a force of _____ ___.  60  0  70  10  70 N NOT  unbalanced right  70  N

14. Remember graphing quadrants? • Going up is _________. • Going down is _________.  positive negative  Figure the net force the same way: Give arrows going up a positive number and arrows going down a negative number.

15. 10 N • Example #3: 20 N • Figure the net force: _____ + _____= _____ • The net force is _____________ • The net force is _____ zero, therefore it is _________________. • The box will be moving ______with a force of _____ ___.  -10  20  10  10 N NOT  unbalanced up  10  N

16. 20 N • Example #4: 30 N 80 N 20 N • Figure the net force: _____ + _____ + ____ + ____= _____ • The net force is _____________ • The net force is _____ zero, therefore it is _________________. • The box will be moving to the ______with a force of _____ ___.  20  -50  -20  -80  30  -50 N NOT  unbalanced left  50  N

17. Balanced: • The forces on the object are equal in size • The net force iszero. • The forces are in opposite directions. • The object experiences nochange of motion. • The object is either at rest or moving at a constant speed. • Balanced forces have the same effect as no force. (Remember with no force, the object will keep doing what its already doing.)

18. Example #1: 20 N 20 N • Figure the net force: _____ + _____= _____ • The net force is _____________ • The net force _____ zero, therefore it is _________________. • The box will either be at _____ or _________ at a ______________ speed.  20  -20  0  zero N IS  balanced rest  moving  constant

19. 5 N • Example #2: • Figure the net force: _____ + _____= _____ • The net force is _____________ • The net force _____ zero, therefore it is _________________. • The box will either be at _____ or _________ at a ______________ speed. 5 N  -5  5  0  0 N IS  balanced rest  moving  constant

20. 10 N • Example #3: 10 N 10 N 10 N • Figure the net force: _____ + _____ + ____ + ____= _____ • The net force is _____________ • The net force _____ zero, therefore it is _________________. • The box will either be at _____ or _________ at a ___________ speed.  10  0  -10  -10  10  0 N IS  balanced rest moving  constant

21. Example #4: 40 N 10 N 10 N • Figure the net force: _____ + _____ + ____ + ____= _____ • The net force is _____________ • The net force _____ zero, therefore it is _________________. • The box will either be at _____ or _________ at a ___________ speed. 40 N  40  0  -40  -10  10  0 N IS  balanced rest moving  constant

22. In outer space, there are no unbalanced forces. There is no air therefore no air resistance (which is a type of friction). There is also no gravity. Therefore, with no unbalanced forces, a space ship launched with a certain speed and direction will keep going at that same speed and in the same direction forever!

23. On Earth, however, there are always unbalanced forces, such as air resistance and gravity. Therefore an object can’t keep moving forever on Earth.

24. Examples of the 1st Law: 1) If you slide a book across thetable it will come to a stop due to the force of friction acting upon it. 2) If you throw a baseball upward it will eventually slow down and come back down due to the force of gravity acting upon it.

25. 3) A soccer ball will stay sitting at rest until an unbalanced force like a kick changes its motion. 4) A vacuum chamber on earth will have no air, therefore no airresistance, but will still havegravity. This allows us to see that objects fall to the ground at thesame rate without air resistance, like a bowling ball and a feather! https://www.youtube.com/watch?v=E43-CfukEgs (4.41 length)

26. 5) Wearing a seatbelt in the car protects you from your own inertia. If you are in a car going 65 mph and the car suddenly stops, your body keeps moving at 65 mph until it comes in contact with a force to stop it. | Better a seatbelt than the windshield!

27. 6) When you top a hill and godown the other side quickly, you can get a “floaty” feeling. This is because your body wants to keep moving upward, but it’s being forced downward. 7) When you are forced up a hill quickly you can get a “heavy” feeling. This is because yourbody wants to keep moving downward, but it is being forced upward.

30. Add the following slides next year?

31. Force Diagram Problems • When solving problems with multiple forces, you first have to add up all the forces to get a net force. • An arrow going right or up represents a positive number. • An arrow going left ordown represents a negative number.

32. Force Diagram Problems • Shorter arrows represent smaller forces. • Longer arrows represent larger forces. • Remember, two negative numbers added together = a negative number. 10 N 100 N

33. Results • While we don’t write the negative sign on the number of the force, it does tell us about the movement of the object. • A positive net force results in the object moving to the right or upward. • A negative net force results in the object moving to the left or downward.

34. The sign also helps us with graphing. • A positive net force means positive acceleration and would be graphed on a speed-time graph like this: Speed (m/s) Time (s)

35. A negative net force means negative acceleration, or deceleration, and would be graphed on a speed-time graph like this: Speed (m/s) Time (s)

36. A net force of zero means the object is either at rest or moving at a constant speed and would be graphed on a speed-time graph like this: Speed (m/s) Time (s)

37. Problem #1 27 N (ball) 2 N (friction) • Net force = _____

38. Problem #1 27 N (ball) 2 N (friction) • Net force = 25 N • Balanced? _____

39. Problem #1 27 N (ball) 2 N (friction) • Acceleration? ____ • Net force = 25 N • Balanced? No

40. Problem #1 27 N (ball) 2 N (friction) • Acceleration? Yes • Movement? _____ • Net force = 25 N • Balanced? No

41. Problem #1 27 N Speed (m/s) (ball) Time (s) 2 N (friction) • Acceleration? Yes • Movement? yes • Net force = 25 N • Balanced? No

42. Problem #1 27 N Speed (m/s) (ball) Time (s) 2 N (friction) • Acceleration? Yes • Movement? yes • Net force = 25 N • Balanced? No

43. Problem #2 10 N (contact) 10 N (friction and drag) • Net force = _____

44. Problem #2 10 N (contact) 10 N (friction and drag) • Net force = 0 N • Balanced? ____

45. Problem #2 10 N (contact) 10 N (friction and drag) • Acceleration? _____ • Net force = 0 N • Balanced? Yes

46. Problem #2 10 N (contact) 10 N (friction and drag) • Acceleration? No • Movement? ______ • Net force = 0 N • Balanced? Yes

47. Problem #2 10 N Speed (m/s) (contact) Time (s) 10 N (friction and drag) • Acceleration? No • Movement? constant or no • Net force = 0 N • Balanced? Yes

48. Problem #2 10 N Speed (m/s) (contact) or Time (s) 10 N (friction and drag) • Acceleration? No • Movement? constant or no • Net force = 0 N • Balanced? Yes

49. Add balancing problems with the formula