What happens to the force by the person pushing the crate ( F applied ) as the frictional force increases? What can they do to change this force?

Why is there friction between objects

Consider a system to be two train cars traveling toward each other. what is the total momentum of the system before the train cars collide?   kg • what must the total momentum of the system be after the train cars collide?   kg •  

The strong nuclear force acts over a small distance than the electrostatic force. true or false