Here's the question: "An object of mass m is traveling on a horizontal surface. There is a coefficient of kinetic friction mu between the object and the surface. The object has speed v when it reaches x = 0 and encounters a spring. The object compresses the spring, stops, and then recoils and travels in the opposite direction. When the object reaches x = 0 on its return trip, it stops." "Find k , the spring constant. Express in terms of mu, m, g, and v." I'm stuck. So I used the conservation of energy law + the friction work. They don't want x in the terms of the answer. How can that be possible when x is part of the elastic potential energy. Any helps would be appreciated.