I have this homework question: A sled is pulled with a horizontal force of 15 N along a level trail, and the acceleration is found to be 0.40 m/s2. An extra mass m = 4.3 kg is placed on the sled. If the same force is just barely able to keep the sled moving, what is the coefficient of kinetic friction between the sled and the trail? Important equations: Fk = uk *N F = ma Attempt to solve: Sum of all forces = ma sum of all forces = Fpull - Fk Fpull - Fk = ma Fpull - ma = Fk Fpull - ma = uk *N (Fpull - ma)/mg = uk I'm unable to solve since I don't know what the original mass was, since it says 4.3 extra mass was added. Can someone help me out??