1. Newton’s Third Law
Physical Science 11/1/11

2. The heavy weight champion can hit the massive bag with considerable force.

3. But with the same punch he can only exert a tiny force on the tissue paper in midair. Why is this?

4. Newton’s 3rd Law For every action there is an equal and opposite reaction
These forces are equal in magnitude and opposite in direction .
The reaction and action forces make an interaction.
Action and reaction means that the forces act in pairs. Either force can be the action or the reaction.

5. Again, Forces come in pairs
Object A exerts a force on B
Object B exerts a force on A
There are two objects in an interaction

6. Action and Reaction on Different Masses
Newton’s 2nd law requires us not only to look at force but also mass.
The force exerted against the
recoiling rifle is just as great
as the force that drives the
bullet. Why then, does the
bullet accelerate more than the
rifle?

7. Examples
While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly.
Which of the two forces is greater: the force on the firefly or the force on the bus?
The force is the same for both. Bug has smaller mass and unable to withstand acceleration.

8. Examples
Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle.
The acceleration of the recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet

9. The force on the gun equals the force of the bullet
The force on the gun equals the force of the bullet. Acceleration depends on both force and mass. The bullet has a greater acceleration due to the fact that it has a smaller mass.

10. Which exerts more force, the Earth pulling on moon or moon pulling on the Earth?
The pulls are the same but in opposite directions