1. The problem statement, all variables and given/known data A 2.5 kg box in on a 53 degrees inclined plane. mu-kinetic = 0.25 and mu-static=0.5. Find it's acceleration if it is initially standing still. 2. Relevant equations F = ma 3. The attempt at a solution The X component of the gravity is = 2.5*g*sin(53) Normal force = 2.5*g*cos(53) Maximum static force = mu-static*N = 0.5*2.5*g*cos(53) To my understanding, since it is not moving, I will use mu-static because once the force becomes greater than maxmum static force by an infinitely small number, it is accelerating close to the accelerating value found using maximum static force. http://img252.imageshack.us/img252/8263/frictionto4.jpg [Broken] F = ma [2.5*g*sin(53)] - [0.5*2.5*g*cos(53)] = 2.5*a g [sin(53) - 0.5*cos(53)] = a a= 4.87 m/s^2 however, by looking in the answer booklet, this is in correct, and the acceleration is found by using mu-kinetic g [sin(53) - 0.25*cos(53)] = a a= 6.35 m/s^2 it really makes more sense in my head using mu-static instead of mu-kinetic can somebody show me why it is wrong?