# Rotation dynamics pulley concept confusion plz help

1. Jul 17, 2016

### Clara Chung

1. The problem statement, all variables and given/known data
A light string is passed over a pulley and two masses m1 and m2 are suspended from the two free ends. The pulley is a uniform circular disc of mass M. Find the linear acceleration of the two masses. Friction may be neglected.

2. Relevant equations
I through center of the disc = MR^2 /2
The answer is a= (m2-m1)g / (m1+m2+M/2)

3. The attempt at a solution
I drew 3 free body diagrams separately, and get T-m1g = m1a..(1)
m2g-T=m2a...(2)
1/2 MR^2 (a/R) = 2TR (2 tension acting both clockwise if I assume m2 is on the right hand side and it is heavier than m1) ...(3)
Plz point out what's wrong in each equation 1,2,3, thxxxx

2. Jul 17, 2016

### Safakphysics

You are wrong in 2TR because the tension direction opposite and also the rope's tension isn't same in this equation. I will add true equations in few minutes

3. Jul 17, 2016

### Safakphysics

m_2.g-T_2=m_2.a

T_1-m_1g=m_1.a

If i use torque

T_2.R-T_1.R=M.R^2.\alpha\div{2}

\alpha.R=a

4. Jul 17, 2016

### Clara Chung

Thank you. I got it.

5. Jul 17, 2016

### Safakphysics

You are welcome