Newton’s 3rd Law

Newton’s 3rd Law states that “for every force there is an equal an opposite force”.

To determine the reaction force, switch the subject and direct object in the sentence (and you thought this was a science class, not an english class) and the direction.


  • The earth pulls down on Mr. Lynch (via gravity).
  • The reaction force is: Mr, Lynch pulls up on the Earth (via gravity).

This leads to some interesting observations:

  • I actually pull up on the Earth! The force on the Earth is the same as the force on me! Note that my acceleration is much greater, because I have much less mass than the Earth.
  • Don’t be tricked. You may get a problem with a bug being hit by a car. You will be given a lot of extraneous information: The mass of the bug, the mass of the car, the speed of the car, etc. However, the force on the bug and the force on the car are equal in magnitude (and opposite in direction).

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