C3 - Mass Matters!!!

What are Newton's 1st, 2nd, and 3rd law of motion?

* In the first law, an object will not change its motion unless a force acts on it.

* In the second law, the force on an object is equal to its mass times its acceleration.

* In the third law, when two objects interact, they apply forces to each other of equal magnitude and opposite direction.

Remembering the third law:

For every action there is an equal and opposite reaction.

In the first picture, do you see the runner’s back foot pushing backward on the track (action)? The track interacts by pushing on the runner’s foot, propelling him forward (reaction). In the

second picture, the ball forces the bat in one direction (action), and the bat forces the ball in the opposite direction (reaction). The force of the ball can sometimes break a bat!

Mass matters! The biggest misunderstanding about Newton’s third law is the incorrect assumption that equal and opposite forces result in equal motion. Objects do exert the same amount of force in action-reaction pairs, but the final effect of the forces depends on the mass of the objects.

The Major Misconception of Newton's Third Law is that Equal and opposite forces do not result in the same motion.

For example, your foot exerts an action force on a kicked soccer ball, while at the same time, the ball exerts an equal and opposite reaction on your foot. You stay put, but the ball flies off. Why? You have much more mass than the ball, so the ball accelerates. The same force that sent the ball into the air is not enough to accelerate your massive body (Newton’s second law; F=ma).

Action-reaction force pairs are NOT cause-and-effect

interactions. Naming which force is the action or reaction is arbitrary (done without much thought), because the interaction happens at the same time. One force does not come first, nor does the other force follow. The action-reaction name is really a misnomer, because both forces are part of a single interaction. A single event takes place in which force pairs happen at the same time, have equal size, and are in the opposite direction. Either force can be labeled as the action force or the reaction force.