1. NEWTON’S THIRD LAW OF MOTION
Physics

2. “HOW I FINALLY TAUGHT THE BIG GUY A LESSON.”
Story Time:
“HOW I FINALLY TAUGHT THE BIG GUY A LESSON.”

3. In Words: EVERY action has an EQUAL and OPPOSITE reaction.
(Equal in magnitude/number, opposite in direction.)
ALL forces
in strength
in direction

4. Action equals opposite reaction.
using math:
FAB = - FBA
Force of A acting on object B
Force of B acting on object A
Action equals opposite reaction.

5. EQUAL

6. All action / reaction pairs are exactly equal in strength, but one object is usually better at dealing with the force.
For example, when someone gets punched in the face the hand is hit just as hard as the nose is. The fist can handle it. The nose cannot.
EQUAL
Poker analogy here: Look at pretend card say “I raise you 5 N.” The other person looks at pretend cards and says “I see your 5 N and I raise you 10 N.” The first person says: “I fold” because they can’t handle the force.
Can do the Force Plates with Logger Pro here – where the strongest person in class pushes against the least strongest. Let one win then let the other and notice that the force is equal on the graph.

7. Bob crashes his motorcycle head on with a semi truck. BAM
Bob crashes his motorcycle head on with a semi truck. BAM!! Which was hit harder, Bob or the semi?
They were hit the SAME!!
Bob’s frail little body couldn’t handle it.
The truck could.
EQUAL

8. Karate Kate chops a block of wood in half with her hand. Hi Yah
Karate Kate chops a block of wood in half with her hand Hi Yah!! Which was hit harder, the wood or her hand?
They were hit the SAME!!
The wood couldn’t handle it.
Her bones could.
EQUAL

9. Tug of War Free Body Diagram
pulling tension is equal and opposite
A pushes off ground
B pushes off ground
Have the class do a tug of war out in the hall. Winning team takes off their shoes (they need to be in socks) then have them tug of war again. Team with socks should loose this time. A tug of war is won by other outside forces (the friction from the shoes against the ground).
B wins because they get more outside help from the ground and A can’t handle that much force without moving.

10. Arm Wrestle Free Body Diagram
B pushes to the left
A pushes B to the right
(equal and opposite)
A pushes off ground to the right
B pushes off ground to left.
Have them arm wrestle sitting down. Then have them arm wrestle standing up. Then have them do a one legged arm wrestle. There is equal force in the hands but it is your legs that help you win an arm wrestle.
A wins because he gets more outside help from the ground and B can’t handle that much force without moving.

11. opposite

12. Sentence:
If A does this to B, then B does the same thing to A but in the other direction.
We will keep the same verb.
Order of nouns switch and the direction reverses.
OPPOSITE

13. If a gun shoots a bullet FORWARD, then the bullet will also shoot the gun
BACKWARD.
(Called a kick back)
OPPOSITE

14. OPPOSITE If the Earth pulls a skydiver DOWN with its gravity then
The skydiver pulls the Earth UP
(Gravity pulls BOTH ways. The earth is just so big it doesn’t move very much.)
OPPOSITE
image from:

15. If Bob’s foot kicks a soccer ball FORWARD with 20 N of force, then the soccer ball
kicks Bob’s foot BACKWARD with 20 N.
OPPOSITE

16. If a tow truck pulls a car FORWARD with 1200 N of force, then the car
pulls the truck BACKWARD with N.
(Luckily the tow truck has more help from static friction than the little car so he wins.)
OPPOSITE

17. If a magnet pulls a paperclip UP with 2 N of force, then the paperclip
pulls the magnet DOWN with 2 N.
OPPOSITE

18. If Bob’s teeth push IN on a juicy apple with 7 N of force, then the apple
pushes his teeth OUT with 7 N.
(His teeth can handle it the apple can’t. Crunch!!)
OPPOSITE

19. If Bob’s teeth push IN on a metal nail with 50 N of force, then the nail
pushes his teeth OUT with 50 N.
(This time, Bob’s teeth will probably break before the nail does. Hello chipped tooth!)
OPPOSITE

20. Bob is trying to do a pull up on a bar
Bob is trying to do a pull up on a bar. If Bob pulls DOWN on the bar with 700 N of force, then
the bar pulls Bob UP with 700 N.
OPPOSITE
(Should really be called a pull down instead of a pull up)

21. Bob is trying to do a push up
Bob is trying to do a push up. If Bob pushes DOWN on the floor with 400 N of force, then
the floor pushes bob UP with 400 N.
OPPOSITE
(Should really be called a push down instead of push up)
Bring up and demo: walking, ice skating. You push in the opposite direction you want to go.

22. Why don’t pair forces cancel each other out?
Let’s look at the action reaction pairs
Even though they are equal and opposite, the normal force and gravity are NEVER pair forces with each other!!! n n
table pushes book up
Book’s gravity pulls the Earth up
Book
Table
(They should have done the reading for the other information.)
book pushes the table down W W
Earth’s gravity pulls the book down

23. One More Example n n f f T T W W A box being pulled by a rope.
Let’s look at each Force
A box being pulled by a rope.
Notice how each force comes from something different.
They are not action reaction pairs!
Ground pushes the box back n n
ground pushes the box up
human pulls the box forward f f T T
Box W W
Earth’s gravity pulls the box down

24. The Weakest Link
The strongest force that can ever be exerted between two objects is always limited by the
Once you reach the limit of the weakest object it will either _________ or ____________
weakest object
break
move

25. Examples
Hardest you can push in an arm wrestle depends on the weakest contestant.
Hardest you can pull in a tug of war depends on the weakest team.
Hardest you can push on a window depends on the strength of the glass (not your arm.)
Hardest you can push on a paper is pathetic it runs away before you generate any pressure.
Paper Demo: Punch a paper. Paper won’t push back, but punch paper on a wall as hard as you can and you break your hand. Paper won’t push back but the wall does.

26.   DAY 3 HOMEWORK:
TO BE COMPLETED IN THE PACKET AND DUE NEXT TIME!

27. HOMEWORK ANSWERS

28. 1. What is Newton’s Third Law of Motion?
EVERY action has an EQUAL and OPPOSITE reaction.

29. 2. If a golf club hits a golf ball forward with 55 N of force, then _________________________.
THE GOLF BALL HITS THE GOLF CLUB BACKWARD WITH 55 N OF FORCE.

30. 3. Bob is trying to climb up a rope
3. Bob is trying to climb up a rope. If Bob pulls down on the rope with 650 N of force, then _________________.
THE ROPE PULLS UP ON BOB WITH 650 N OF FORCE.

31. 4. Bob smashes a watermelon with his hammer
4. Bob smashes a watermelon with his hammer. Both of the following statements are true. “Today a watermelon was hit by a hammer.” “Today a hammer was hit by a watermelon.” Which one was hit harder? Why did the watermelon explode while the hammer escaped unhurt?
They were both hit the SAME!
The watermelon was the weakest object and couldn’t handle the force, but the hammer could.

32. 5. There is a proverb that says “Whether the china (glass plate) hits the rock or the rock hits the china, it is going to be bad for the china.” Explain how this relates to Newton’s Third Law.
The china plate is the weakest object and will break even though both the plate and the rock feel the same force.

33. 6. Explain how runners and swimmers both use Newton’s Third Law to move their bodies forward.
It is just like walking or ice skating. They push in the opposite direction they want to go.
Demo this.

34. 7. Brain Teaser 1: An astronaut is floating in space with nothing except his suit and a fire extinguisher. (Why? Why not?) He starts to drift away from his spaceship. How can he get safely back? (You can’t “swim” back to the ship by flapping your arms because there is no air up there silly!)
Like the movie WALLE. Point the fire extinguisher in the opposite direction you want to go and then shoot it.

35. 8. Brain Teaser 2: James Bond is in a canoe in a lake of acid. (Why
8. Brain Teaser 2: James Bond is in a canoe in a lake of acid. (Why? Somebody is trying to kill him.) He has no paddles. How can he get the canoe safely back to shore?
Walking the opposite way from the shore in the canoe can get him back to the shore safely.
Breathing
Demo the breathing.

36. 9. Brain Teaser 3: There is an Asian story about a greedy man on a bridge that notices a bag of gold sitting on the ice below him. He jumps down to get it, but finds that the ice is so slippery he can’t get back to shore and he freezes. If the man hadn’t been so greedy, how could he have gotten safely back to the shore?
He could have pushed the gold in the opposite direction of the shore.
Pushing the gold away from the shore would cause him to move toward the shore.

37. 10. Brain Teaser 4: A horse is pulling a wagon across the prairie
10. Brain Teaser 4: A horse is pulling a wagon across the prairie. If the horse pulls the wagon forward just as hard as the wagon pulls the horse backward, then how on earth does the horse ever get the wagon to move?
The horse gets more outside help from his legs (like the tug of war and arm wrestle examples).