____ | B | |__| | A | |__| Suppose that there is a net force from left hand side pushing the 'A' box to right hand side. During the pushing, 'B' box moves in right same as 'A'. According to my textbook, there is zero work done on 'B' because the force(weight) on the 'B' is at right angle to the motion. However, friction(reaction) should be existed from 'A' on 'B' so that 'B' moves from at rest.This friction is along the motion direction. Since friction is a force, so work done should exist. But why does it wrong? Anyone can help me?