Newton’s Third Law

Newton’s third law of motion states that when two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction. From this law comes the popular phrase: ‘Every action has an equal and opposite reaction.

According to Newton’s third law, the forces must always:

  • act between two different objects
  • be of the same type (for example, if they are both gravitational forces)

In the diagram below, Isaac Newton is pushing a wall.

When Newton pushes on the wall, the wall also pushes back on him and this is the reaction force. Newton’s third law of motion states that forces always act in equal. but opposite pairs. So, when he pushes on the wall, the wall pushes back on him with a force equal to the force he exerted.

You may wonder how objects move if every action has an equal and opposite reaction. This is because the forces are on different objects. For example, if you push a boulder above your head, the boulder will exert an equal force downwards. However, this downward force will cause you to push down on the Earth with the same force.

Your body does not move downwards because you are pushing down on the Earth, which is a large, immovable object, and pushing the object upwards. If there were no Earth beneath you, then you would be pushed downwards as you tried to lift the object.

More Examples

  • When we jump, our legs apply a force to the ground. The ground exerts an equal and opposite force, propelling you into the air.
  • When rowing a boat, the paddle pushes the water backwards, which makes you move forward.
  • When a hammer strikes a nail, it drives the nail downward into a piece of wood. In response, the nail exerts an upward force on the hammer, which is the reaction force.

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