Given a certain situation, how do you figure out the force that will be exerted onto something (you can only use F=ma to find the average net force over a certain time period, and only after you know the acceleration)? For example, if you have a non accelerating object colliding with another non accelerating object, given their mass and velocity, how do you find the acceleration that the objects will experience upon collision? According to Newton's Third Law of Motion, momentum is always conserved, but how do you figure out the rate at which the two objects will change their velocity (the objects won't immediately reach their final velocity)? Can you figure out the amount of time it takes for the objects to reach their final velocity before empirical observation? How do you figure out what their final velocities will be?