In Football Newton's 3rd law of motion is in action. When a running back is running head on against a tackler who is running just as hard and fast the outcome may vary. In games there are times where the running back gets hit so hard that he fumbles and other times the running back pancakes the tackler. One of the biggest factors is the mass because the forces are creating equal and opposite reaction. Force is applied and transmitted back. The player with more mass will generally hit harder.