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## Homework Statement

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.

## Homework Equations

## 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