Newton's Third Law 

To every action there is always opposed an equal reaction; or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts. 

Let two bodies A and B that are interacting. We define F_{A} as the force applied to body A by body B and F_{B} as the force applied to body B by body A. These forces will be equal in magnitude and opposite in direction. 

In mathematical terms, it is expressed as:


F_{B} =  F_{A}


or F_{A} + F_{B} = 0


Newton's Third Law of Motion states that any time a force acts from one object to another, there is equal force acting back on the original object. If you pull on a rope, therefore, the rope is pulling back on you as well. 

For example, if the road exerts a forward frictional force on an accelerating car's tires, then it is also a frictional force that Newton's third law predicts for the tires pushing backward on the road. 