Newton’s Third Law of Motion, Essay Example

Newton’s Third law of motion states that “for every action, there is an equal and opposite reaction.” The 400N force exerted by the horse on the wagon is equal but opposite in direction to the 400N exerted by the wagon, i.e. equal in magnitude but opposite in direction. However, to expect the two forces to cancel and produce zero acceleration, demands the need to differentiate between motion (momentum) and Velocity.

Literally, the two forces would be expected to cancel out and produce an acceleration of zero (no movement). However, Newton’s Third law expects that the 400N force exerted on the wagon, equally (but in the opposite direction) draws the horse towards the wagon, eventually obstructing the progress of one as much as it advances the progress of the other.  Therefore, the horse impinging on the wagon and by the 400N force moves it from rest, will itself (due to the equal force) undergo an equal but opposite change in its momentum. The changes made by the actions of the horse and wagon are equal not in velocities but in momentum. As a result, the less massive wagon experiences a greater acceleration due to Newton’s Second law.

Mathematically, if body A (horse) exerts a force on body B (wagon), denoting this force as F_AB, Newton’s Third law of motion states that

F_AB = – F_BA  Where by subscript AB means force on object A by object B, while subscript BA means force on object B by object A

Hence 400 N = – 400N

Although, both forces exerted by the horse, and the wagon, upon each other being equal, the difference lies in the direction (denoted by the negative sign).