Motion and Its Causes

Force and Newton’s Laws of Motion

Aren’t there three Laws of Motion? What is Newton’s Third Law?

Forces are interactions between objects. You need more than one object to have a force. Therefore, if two objects interact, each exerts a force on the other. If you push on a friend’s hand, his or her hand pushes back on you. If you stand on a floor, you exert a force on the floor and the floor exerts a force on you.

Newton’s Third Law of Motion states that these two forces are equal in magnitude but opposite in direction. If you exert an 800-newton force downward on the floor, the floor exerts an 800-newton force upward on you.

Note that Newton’s Second and Third Laws are different. The Third Law describes forces on two different objects. Those forces can then be used with the Second Law to find out how the motions of the objects are changed.


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