"A box slides down a 30.0 degree ramp with an aceleration of 1.20m/s^2. Determine the coefficient of kinetic friction between the box and the ramp." I know that the coefficient of kinetic friction is found by kinetic friction / normal force, but the only problems I've delt with in the past gave you the mass of the object. In those cases, I would find the force of gravity then use it to find the force upwards using the sin of the angle of the incline. From there, I would find the normal force subtracting force upwards from the force of gravity - then finally multiplying the mass times the acceleration to find the kinetic friction. I know how to work through it one way, but I can't figure out how to work backwards given the acceleration and no mass. Could someone please point me in the right direction?