A disc of mass m slides with zero initial velocity down an inclined plane set at an angle alpha to the horizontal; having traversed the distance l along the horizontal plane, the disc stops. Find the work performed by the friction forces over the whole distance, assuming the friction coefficient k for both inclined and horizontal planes.
The Attempt at a Solution
I am confused because in the solution the don't count in the work of gravitational force. I thought that the summed work is equal to the change in energy, so from that I would derive that the frictional forces work is equal to the change in energy minus gravitational work