# Homework Help: How to find the the acceleration of the block M?

1. Feb 8, 2014

### avistein

1.The problem statement, all variables and given/known data
Find the acceleration of the block of mass M in the situation shown.All surfaces are frictionless

2.The attempt at a solution
2Mg-2T=2Ma
T=Mg-Ma
and T-mgsin30=ma

But a is cancelled .what to do?

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Last edited: Feb 8, 2014
2. Feb 8, 2014

### jordanhuh

T=Mg-Ma

Mg-Ma-Mgsin30=Ma

2Ma = 1/2 Mg
a= 1/4 g

Not cancelled?

3. Feb 8, 2014

### avistein

Sorry, but answer given is g/3.

4. Feb 8, 2014

### voko

Acceleration of which block is denoted by $a$? Are the accelerations the same for both blocks?

5. Feb 8, 2014

### avistein

Oh,sorry.the acceleration of block 2M is a ,so acceleration of M is 2a.

6. Feb 8, 2014

### voko

Do you get the answer with that correction?

7. Feb 8, 2014

### avistein

Yes sir,after Bvu's help,it is solved.

Last edited: Feb 8, 2014
8. Feb 8, 2014

### BvU

Don't understand. In the expression T-Mgsin30=Ma a is clearly the acceleration of block M, which is what is asked for. So keep it and change the acceleration of the other block to a/2, like:

(2M)g - 2T= (2M) (a/2). Bingo!

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