Problem 1.122

A disc of mass \(m=50 \mathrm{~g}\) slides with the zero initial velocity down an inclined plane set at an angle \(\alpha=30^{\circ}\) to the horizontal; having traversed the distance \(l=50 \mathrm{~cm}\) along the horizontal plane, the disc stops. Find the work performed by the friction forces over the whole distance, assuming the friction coefficient \(k=0.15\) for both inclined and horizontal planes.