# Mass of cylinder

1. Dec 17, 2012

### doub

1. The problem statement, all variables and given/known data

A light string is wrapped around a solid cylinder and a 300 g mass hangs from the free end of the string, as shown. When released, the mass falls a distance 54 cm in 3.0 s. Calculate the mass of the cylinder?

2. Relevant equations

not sure

3. The attempt at a solution

I have gotten as far as to determine the Tension on the string of -2.9N. Without known the radius of the cylinder I feel that the mass cannot be determined.

Granted these are study question but if the final is going to be like this...yikes :surprised

2. Dec 17, 2012

### tiny-tim

hi doub!
the motion depends on what i call the rolling mass of the cylinder, I/r2, which is always half the actual mass!

carry on … you should find that the radius cancels out

3. Dec 17, 2012

### doub

The best I can get is

$\alpha$ = (T1 + T2)/I

= (Fr - Tr)/1/2 mr2

= (mra -(ma+mg)r)/ 1/2 mr2

don't see how the radiii cancel each other though

4. Dec 17, 2012

### tiny-tim

sorry, i don't understand any of this

you should have an F = ma equation and a τ = Iα equation