The first important concept in collisions is called momentum. Momentum is what is transferred from one body to another when they run into each other. The other important thing to note is that Momentum is always conserved. This means that all of the Momentum transferred from one body is absorbed by the other body.

Momentum [kg m/s] = Mass times velocity m*v
In a collision, we have that the Force equals the change in Momentum divided by the change in time, or F=dM/dt. This leads us to the second important concept, called Impulse. The Impulse is simply the Momentum delivered in a specific time.

Impulse [kg m/s2] = Momentum delivered in a specific time. M*dV/dt

As you can see from above, dV/dt is really Acceleration, so this can also be stated as F=MA. In fact, this is known as Newton’s Second Law. All Momentum is delivered only if the two bodies are allowed to remain in contact for an infinite amount of time. For our purposes, when hitting another person, we have only a short amount of time in which to transfer our momentum.