Answer
(a) In the frictionless case, the two blocks accelerate to the right. The friction between the two blocks exerts a force on block A, which accelerates it to the right.
(b) In the case with friction, the blocks do not accelerate because the net force on block B is zero. Therefore, there is no friction force between the two blocks, which accelerates block A to the right. In this case, there are only two forces acting on block A: the weight of block A directed down and the normal force from block B pushing up on block A.
Work Step by Step
(a) In the frictionless case, the two blocks accelerate to the right. The friction between the two blocks exerts a force on block A, which accelerates it to the right.
(b) In the case with friction, the blocks do not accelerate because the net force on block B is zero. Therefore, there is no friction force between the two blocks, which accelerates block A to the right. In this case, there are only two forces acting on block A: the weight of block A directed down and the normal force from block B pushing up on block A.
