1. Lec 05: Newton’s Third Law
Applications of Newton’s Law

2. How Hard Does the Wall Push?
Interaction between 2 objects
Person pushes Wall
Each object experiences it
The direction is opposite for each object
Wall pushes Person
Both experience same strength of push

3. Newton’s Third Law As two objects interact,
each experiences a force equal in strength but opposite in direction
Person pushes Wall
Wall pushes Person

4. Being with the Force
As Earth pulls you towards its center with a force of 500 N,
Do YOU exert a force
on Earth?
Yes, but very small
Yes, exactly 500 N
No, you are too small
I don’t know

5. Bus-Bug Collision
A high-speed bus colliding head-on with a flying bug. Which is greater, the force on the bug or the force on the bus?
Bus
Bug
Both are the same
I don’t know

6. Bus-Bug Collision
A high-speed bus colliding head-on with a flying bug. Why does not the bus “feel” it? F m
= a
Newton’s Second Law
Which one has a bigger acceleration?
Bus
Bug
Both are the same
I don’t know

7. Puzzles
P1. You trip over and fall into a pool. Not knowing how to swim, you begin to sink. You grab yourself by the hair and pull upward, but it doesn’t seem to work. Which of the Newton’s Laws can explain why it doesn’t work.
P2. Two people of equal mass on slippery ice push off from each other. Will both move at the same speed?
(a) Yes (b) Yes, but only if both push equally (c) No
P3. A bird flies off upward toward the sky. What force is responsible for the upward motion?
P4. You get angry at your friend and punch him in the face. Why does YOUR hand hurts, if you were the one punching?

8. Newton’s Three Laws
A Net Force is required to change the velocity (motion) of an object
The net force is proportional to the acceleration (F = ma)
Acceleration
Net Force =
Mass
Two interacting objects experience opposite but equal in strength forces (action and reaction)