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Tips for Studying the Third Law of Motion

By Kathy Foust

This science study guide explains Newton's Third Law of Motion so that it can be easily understood and used by physical science students.

Newton's Third Law of Motion is probably one of the most commonly known laws of motion. It must be understood in order to understand how forces in the world around us respond to each other. The law states:

For every action there is an equal and opposite reaction.

It can also be written as "For every force there is an equal and opposite force." or "Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object". It may also be written as action = reaction.

Let's take a look at the terms of this law.

  • Action: When using Newton's Third Law of Motion, the term "action" refers to an applied force. Without force there is no motion and without motion there is no action.
  • Reaction: The reaction refers to the response of force to the first action.

Examples to Illustrate

Look at the examples below to further understand the Third Law of Motion. To do the experiments, have an adult actually perform them as you sit in the passenger seat.

  • Consider a car. What force drives it? The force produced by the engine propels the car forward using. The force is constant as long as the car is running, but is not applied until the car is placed in a gear.
  • If a car is sitting on a level surface, which is to say the car is not going uphill or downhill, place the car in neutral and see if it moves. The transmission is not engaged, so the car is not actually using all the force it has. The car will most likely not move at all in this scenario. The force from the idling motor is not unbalanced enough to create motion. In other words, there is no thrust or external unbalanced force to propel the car.
  • If a car is facing forward going up a steep hill, place the car in drive but do not step on the gas. If the hill is too steep, the car will roll backward because there is not enough forward thrust to keep the car moving forward or sitting still. If the thrust of the motor is great enough, the car will move forward slightly. Try this same experiment going downhill.
  • This law is probably most commonly demonstrated using rockets. The force of the the rocket's exhaust gases and the rocket's mass push against each other and create acceleration. The exhaust gases accelerate one way while the rocket accelerates the other way.

Hopefully this study guide has helped you to understand the basic idea behind Newton's Third Law of Motion. For review of other aspects of motion, use these motion study guides.