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:
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.