I think that law is basically Newton's second law of motion. F=ma essentially tells you that anything that decelerates a lot, such as a very fast spaceship crashing into the rock of your planet, pushes with an awful lot of force before it stops.

Edit: to be slightly more pedantic, the right form that still remains true with relativity is F = dp/dt, i.e. the force the spaceship would exert is equal to its change in momentum.