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## Homework Statement

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..

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