Blog / Newtons Third Law

Newton’s Third Law

Written by Chau Pham on Feb 26, 2020

If object A exerts a force on object B, according to Newton’s Third Law, object B will exert a force on object A that is equal in magnitude and opposite in direction

F(a on b) = – F(b on a)

Take a swimmer who uses her feet to push off the wall in order to gain speed. The more force she exerts on the wall, the harder she pushes off. This is because the wall exerts the same force on her that she forces on it. She pushes the wall in the direction behind her, therefore the wall will exert a force on her that is in the direction in front of her and propel her forward.

When a rocket launches into outer space, it expels gas backward at a high velocity. The rocket exerts a large backward force on the gas, and the gas exerts an equal and opposite reaction force forward on the rocket, causing it to launch. This force is called thrust. Thrust is used in cars and planes as well.


Key Points

  • F(a on b) = – F(b on a)
  • This pair of equal and opposite forces act on 2 bodies, hence they don’t cancel each other out.

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