1. Equal & Opposite Forces
Newton’s third Law
Equal & Opposite Forces

2. 3rd Law
Forces are not a thing of themselves, but are part of a mutual interaction
Forces only exist in pairs
Third Law says….
“For every force there is exist another force that is equal in magnitude but opposite in direction”
You cannot push on something without it pushing on you by the same amount in the opposite direction

3. Action & Reaction
Action forces produce the interaction, and the reaction force must be in existence at the same time, have the same size, and be opposite in direction
You apply a 10 N eastward force to wall, wall applies a 10 N westward force to you

4. What is the reaction to the Force of Gravity?
ACTION- Earth’s Gravity pulls person down
Earth’s gravity pulls objects towards it
So if all forces have are part of a pair, what is the reaction force to this??
Object’s gravity pulling Earth up!
An object pulls upward on the Earth as much as the Earth pulls it downward
Object’s mass is much less than Earths, so object falls down rather than Earth rising up to object
REACTION-Person’s gravity pulls Earth up

5. Draw all forces on the car/Earth system as it is driving down the road
The road’s friction on the car’s tires
**Action/Reaction pairs have matching colors in drawing
Normal force of the Earth pushing up on the car
Earth’s Gravity pulling the car down
The road pushing the car forward
The car’s tire pushing on the road
The car’s gravity pulling the Earth up
Contact force of the car pushing down on the Earth
The cars friction on the road

6. Elevators & Normal Force
Elevator Acceleration Down
Elevator at rest
Elevator accelerating Up
FN=900 N
FN=890 N
FN=910 N
W=900 N
W=900 N
W=900 N
FNET=+10 N
FNET=-10 N
FNET=0 N

7. Normal Force “Normal” means perpendicular
Force of the Earth (ground) pushing up on an object when they are in contact with each other
If object is on a horizontal surface and no other vertical forces are applied, then Normal Force is Equal to the weight (force of gravity) of the object
Normal Force is NOT the reaction to the force of gravity acting on an object
Responsible for how heavy we “feel”
AKA “Apparent Weight”

8. Astronauts & Reaction Forces

9. How do objects move? What pushes them?
Car??
Rocket??
Person??

10. Examples

11. Swimming, rowing
Swimmer pushes backward on the water…… the water pushes swimmer forward
The oar pushes water backward… water pushes oar and boat forward

12. Stuck in Space??
An astronaut in space…not moving with just a spacesuit on and he’s not tethered to the ship. If he is only a few meters away from his ship and there are no other people to help him how could you get back to his spaceship?
Oh No!!

13. Forces on different masses
Cannon and cannonball
Do both the cannon and the cannonball experience the same force the cannon is fired?
Yes, the force on each are the Action/Reaction pairs
Does the cannon or the cannon ball have a larger acceleration?
The ball, b/c it has a small mass. It is easier to accel. a small mass
a = F/m

14. Do the forces cancel??
If every force produces a force that is equal and opposite, why don’t all the forces cancel out and always just have a net force = 0??
If the apple pulls on the cart, and the cart pulls back on the apple by the same amount in the opposite directions, wouldn’t the two forces just cancel each other?
No, the action force,
And the reaction force
Are acting on diff.
Objects and therefore will not cancel