Newton's third law

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PS: Apologies for reproducing the stereotype, but we found it funny.

Background

This joke should be seen in contrast to the Newton's third law of motion which states that "for every action there is an equal reaction". A more rigorous definition is

When a first body exerts a force $\vec{F}_1$ on a second body, the second body simultaneously exerts a force $\vec{F}_2 = - \vec{F}_1$ on the first body. This means that $\vec{F}_1$ and $\vec{F}_2$ are equal in magnitude and opposite in direction.