heavy cavalry engaging heavy cavalry

[Macedon]

No disrespect Martin, I still think it is momentum you need regarding this issue. The law you refer to has to do with the force that is being exerted on a body in order to accelerate it, which would be relevant if we wanted for some reason to compute the force exerted on a body (the horse or the target) should we know its acceleration. Plus, acceleration has nothing to do with end speed since you can obtain the same end speed by exerting less force for more time. It is the force exerted on the target by the horse-rider set we need here and for this we need the total mass and the speed upon impact. The acceleration here would be irrelevant.