1. Newton’s Third Law
Newton’s Third Law Examples
Quiz
Outline

2. Action  Reaction Newton’s 3rd Law
NIII definition
Newton’s 3rd Law
“If object A exerts a force on object B, then object B exerts a force equal in magnitude and opposite in direction on A”
Action  Reaction

3. Two Rules for Newton’s 3rd
Every Action force has a Reaction Force. Period. Always.
Action and Reaction forces ALWAYS act on different objects.

4. Action Force Reaction Force Example : block Fmy hand Fblock block
Action-Reaction
Example : block
Fmy hand ON
block
Action Force
Fblock ON
my hand
Reaction Force

5. Example: A ball and the earth:
Fball on earth
Fearth on ball

6. The Math
FA on B = - FB on A B
Fapp A

7. QQ58: N3 misconception
Newton’s Third Law
A precocious child is given a wagon for her birthday. She refuses to use it. She says, “Newton’s Third Law says that no matter how hard I pull, the wagon will exert an equal but opposite force on me. So I will never be able to get it to move forward!”
Is she right?
Does Newton’s third law work for wagons?

8. Moving Objects How can we walk ? – static friction Physicist
QQ58: N3 misconception
Moving Objects
How can we walk ? – static friction
Physicist
Push on the earth
Earth pushes back

9. QQ58: N3 misconception
Moving Objects
If Newton’s 3rd law always holds, how can objects move ??? Why does a car move forward ? v
Wheels push backward on earth
Earth pushes forward on wheels

10. Forces vs Accelerations
QQ58: N3 misconception
Forces vs Accelerations
Newton’s 3rd law states that the forces are equal.
However: Since F=ma
a=F/m Therefore, given same F but different masses, the
accelerations are NOT equal.

11. Forces vs Accelerations
QQ58: N3 misconception
Forces vs Accelerations
Given an earth/ball system, the magnitude of the forces are equal, so
Fearth on ball = Fball on earth
mbg=meae So, ae=(mb/me)g = 2x10-24 m/s2
The ball, of course, falls down with 9.8 m/s2

12. Do for next class: Read: Sections 8.1, 8.2, 8.3
Try problems: 8.1, 8.5, 8.6, 8.8