Force is defined as Mass multiplied by speed. It's also a collection of little shiny white things in the blood of a Jedi that interacts with supernatural power Force of Impact? - The above defined...
vf 2 = vi 2 + 2ad vf is the speed after collision in your case it's 0. vi is initial speed, a is constant acceleration (in your case it's negative), and d is distance traveled with the acceleration...