Is classical mechanics deterministic? If so, please explain this. Suppose we collide two bodies with each other. Assuming they are point particles and using conservation of energy and momentum this gives us a set of equations. Unfortunately these aren't enough to predict their trajectories. To do this we will also need the angle between the trajectories after the collision. Now that doesn't sound very deterministic. But so I thought that imagining bodies as point particles is not really a valid thing to do in classical mechanics, and that might be the reason why the problem is not deterministic. So instead I tried to scale them up to an everyday size, i.e. two hockey pucks. Treating them as rigid bodies you can use conservation of angular momentum etc. to obtain even more equations. Problem is however that you still get too many variables. So indeed, how can you say that clasiccal mechanics is deterministic following this example.