how do you calculate forces in a collision?

[Bluzkin]

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!

[Fu-Kwun Hwang]

Brownian motion is collection of collision between any two particles.
Please check out Collision 2D to check out how to process it 2D.
You do not need to know the interaction force in order to calculate the result after collision.

It is possible to have two collision with the same initial condition and the same outcome, however, the interaction force is different.

As a simple model: You can assume there is a spring action when  two particles collide with each other.
You are welcomed to check out   Simulate 1D collision with a virtual spring