Work done by friction at constant speed on inclined plane. Work energy theorem friction concepts.

New videos every week! Subscribe to Zak's Lab    / @zakslab   Questions or requests? Post your comments below, and I will usually respond within 24 hours. In this work energy theorem friction concept question, we indirectly calculate the work done by friction at constant speed down an incline. The block moves at constant speed on inclined plane, so the kinetic energy is constant. Without knowing the angle of the incline, it is impossible to compute the work directly with a force and displacement. We quickly get the work done by friction by using the work-energy theorem and the path-independence of the work done by gravity. The work done by gravity is just mgh, and the net work is zero because the block moves at constant speed down the incline. Thus, the work done by friction at constant speed down the incline is just -mgh.