Newton’s 3rd Law

Newton’s Third Law states that when one object puts a force on another, the second always exerts an equal and opposite force back on the first. We call these forces an action force and a reaction force.

Action and reaction forces always act on different objects. If they were to act on the same object the net force on objects would always be zero. This would result in no acceleration.

A simple example of an action-reaction pair is a book on the table. The weight force of the books is supported by the upwards reaction force of the table. 

Newton’s third law states that for every action, there is an equal and opposite reaction.

Mathematically, we represent Newton’s 3rd Law as follows: { F }_{ AB }={ -F }_{ AB }

From the equation we can note:

  • The forces are equal
  • The forces act in opposite directions
  • A exerts a force on B and B exerts an equal and opposite force on A

Example 1:

Describe which are the action and reaction forces in the following examples.

a) a tennis racquet hits a tennis ball:

action: tennis racquet hitting ball. reaction: ball pushing back on racquet

b) a girl kicks a soccer ball

action: girls foot kicking ball. reaction: ball pushing back on foot

c) a rock resting on the ground

action: weight of the rock pushing on ground. reaction: ground pushing back on rock