I understand that physics is mostly based on empirical evidence, but sometimes it's interesting to know how some things are explained. My questions about friction are: 1. Why does friction increase as velocity increases? 2. Why does it take time for friction to become equal to the force? For example, let's discuss a block that is accelerating. If I start pulling it with 10 N of force, eventually friction will oppose it with 10 N in the opposite direction, but not instantly. Of course, I don't think objects could move if friction was always equal to force so there must be an explanation for why there is a delay. Also, is there an equation to describe the relationship between friction and time?