I need to simulate brownian motion (random motion of particles in liquid/gas) for school. I am trying to do this in game maker based on forces that the particles excert on each other in a collision.

Does anyone know the formula to calculate the force that one particle excerts on the other, based on the mass (m1) and speed (v1) of the particle?

Once I know the formula, I can just use a=F/m.

Thanks for the help!