# Homework Help: Finding acceleration of an Atwood's machine

1. May 8, 2014

### santoki

1. The problem statement, all variables and given/known data
Two masses are suspended from a pulley:

[PLAIN]http://i.imgur.com/B4x8pzx.png[/PLAIN]

mass of pulley = 0.20kg
constant torque = 0.35Nm due to friction between the rotating pulley and its axle
m1 = 0.40kg
m2 = 0.80kg

What is the acceleration of the suspended masses?

2. Relevant equations
Fnet = I$\alpha$

3. The attempt at a solution
I tried to make sense out of this problem using the equation from above initially, but my acceleration was way too big. I know the acceleration is supposed to be somewhat big anyway but it was too big. So instead I searched for helm on google first and found an equation which told me to do this:

I = (1/2)mr2 = (1/2)(0.20)(0.5)2 - 0.025kgm2

a = [g(m1 - m2) - $\tau$/r]/(m1 + m2 + I/r2)
a = 9.8(0.80+0.40)-(0.35/0.15)/(0.80+0.40+(0.0025/(0.15)2)
a = 1.23 m/s2

If this is right, how can I clean it up to make it relevant to how Fnet = I$\alpha$ is supposed to be set up?

2. May 8, 2014

### paisiello2

Fnet I$\alpha$