I have worked out how to do what I asked originally, if anyone is interested.
so if you want to get a realistic collision force for damage you might want to multiply by the mass if you have objects of different sizes but more importantly you can multiply the velocity by the impact normal and get the length of that so for example in a car game you would take more damage from hitting a wall you are driving towards than if you glance off a wall running almost parallel to the direction you are travelling.