1. The problem statement, all variables and given/known data Hello guys :) I'm a physics student from Israel and I couldnt find a decent physics forum to help me answer some hard to solve exercises (btw, forgive my english if I fail to express myself well). The exercise: (hopefully this works..) http://img143.imageshack.us/img143/8707/88816184zp7.jpg [Broken] http://g.imageshack.us/img143/88816184zp7.jpg/1/ [Broken] The mass of object A is 2kg The mass of object B is also 2kg The mass of object C is yet unknown The friction coefficient between object A and B is μ=0.808 (no friction between A and the rail) 1) I need to find the maximum value for the mass of object C so that all 3 object will accelerate in the same acceleration (answer:Mc=3.5kg) 2) Now it is known that the mass of object C=4kg. They are asking for the acceleration of object A,B and C (answers: the acceleration of objects A and C is 2.67 m/s^2, acceleration of object B is 2 m/s^2) 2. The attempt at a solution 1) What I tried to do is solve the problem by using 3 equations: object B- T=fk (fk=friction) => fk=μ*N=0.808 * mgCos30=14N so: T=14N objects A+B- T-MgSin30=MgSin30*a when M=4kg and T=14 so that: a=-0.3 m/s^2 object C- Mc*g - T = Mc*a when Mc=The unknown mass of object C and a=-0.3 so that: Mc=1.5kg But, the right answer by the book is 3.5kg 2) Tried to solve but with no luck. Can someone please help me understand what is it that I'm doing wrong? I'm in a point where I'm completely helples, I have no clue..