I have a problem in analyzing movement with friction involved. I do not know how to correctly analyze systems as such and what is the correct view. I have tried to answer the several question below. I have the following diagram. There is friction between all parts. 1) What force do we need to apply in order to give acceleration to the system when it is moving at constant velocity(so ∑F is not 0 on A)? I would say it must be greater the Fk, since Fk magnitude is a constant value. Is it correct? 2) Let's say the system is moving at constant velocity. What are the forces applied on B? Is there a static friction force? I find it hard to imagine what is happening to object B. ∑F is 0 on B too since it moves with constant velocity. The only one who can apply force on it is A. if A was to apply force on B, then there would be also an opposite force applied by A (so ∑F is 0 on B) so it is a contradiction thus no force is applied on B. However, lets look at the molecule level. B's molecules are moved by A molecules. So A must apply force on B! Like in this picture: I'm really confused. 3) System is accelerating. B stays stationary above A. What are the forces applied to B and A? According to Newton second law, now force is applied on B. The only one who can apply force on B is still A. I would guess that A is applying force in the direction of the acceleration and there is also static friction that is opposed to that force, also from A (since B does not move above A). I find it really confusing that A is applying two different forces to B. At the molecular level it seems easier to view it, it is very similiar to question 2. The forces applied on A are Fk kinetic from both B and the ground and I just don't know which forces are applied to B. Are my views wrong? 4) System is accelerating. B is moving backwards (opposite of acceleration direction) above A (A reference frame). What are the forces on A and B? What acceleration should be applied in order to make this situation? I would say the forces on A are the forces from que 3. Now there must be kinetic friction on B. This force is opposite of acceleration. However at the molecular level I would say A is trying to pull B with it, so it also must be a force in the direction of the acceleration. I really can't get how to write the equations. Any help will be much appreciated.