Question

Explain the flaw in the following reasoning - When a baseball is hit with a bat, there are equal but opposite forces on the bat and ball. The forces therefore cancel out and no motion results. Why is this statement wrong?

Answer 1

This is very prevalent mistake while using Newton's third law of motion.

The complete statement of third law is: For every action there is equal and opposite reaction and the action and reaction act on different bodies. So they won't cancel out each other. Generally the later part is missed by many people.

For example here, if action is acting on the ball, the reaction will act on the bat. So the action force acceleratea the ball while the reaction force gives acceleration in opposite direction to the bat.

If we take both bat and ball the part of system, then both the action and reaction will act in the system and cancel out each other. And that is also true in reality. The center of mass of bat and ball system will not accelerate.