The work done by frictional force is negative as it opposes the relative motion? Please explain when will the work done by friction be (i) zero (ii) positive?
Zero= When there is no force acting upon it there is no friction e.g an object which is stationary and has no force causing it to accelerate, so it therefore doesn't have any forces (friction) acting upon it giving it a value of 0 and therefore would have zero work done by friction. Then positive is like if I had two wooden blocks I put one on top of the other, and then made the one on the bottom accelerate. Although one at the bottom would be negative the one on top would be positive as it has no friction acting upon it and is only accelerating.
If you observe the application of friction from a frame fixed to any one of the surfaces on which friction is acting, work can never be positive.
Friction, by its nature, always acts opposing the relative motion between two surfaces. work done by friction can be zero, when there is no relative motion. But positive work cannot be done under any circumstances, if you see it from a reference frame fixed to one of the two surfaces. In some cases people have cited the example of pulling of a sheet, " if you try to pull top sheet off table while dishes still rest on it, the dishes will begin to move as a result of the friction forces from the sheet." but here also, there is no relative motion at the point of application of force. Which makes the work done still zero.
But if you change the reference frame, then friction can do positive, negative, or zero work depending upon what reference frame you chose. Then the sheet example also holds good.