# Homework Help: Stuck on a Torque Problem Help is appreciated!

1. Jul 4, 2012

### cngest

Stuck on a Torque Problem!! Help is appreciated!

1. A mass of 45.5kg is attached to a light string (negligible) that is wrapped around a cylindrical spool of radius 10cm and moment of inertia 4.0kg*m^2. The spool is suspended from the ceiling, and the mass is then released from rest a distance 3.5m above the floor. How long does it take to reach the floor?

2. Sum of the torques = Inertia * angular acceleration

3. I calculated the mass of the spool to be 400kg by using I=mr^2. After that, I'm directionless! Thanks in advance for your help :)

2. Jul 4, 2012

### azizlwl

Re: Stuck on a Torque Problem!! Help is appreciated!

Distance is given and you are required to find time.
You can use SUVAT equation to find the time.
Another value you need to find is the accceleration.
Downward acceleration of the mass is also related to rotational acceleration of the spool.

3. Jul 4, 2012

### cngest

Re: Stuck on a Torque Problem!! Help is appreciated!

that helps a bit but i'm still confused on how to calculate angular acceleration since i don't know ω...

4. Jul 4, 2012

### azizlwl

Re: Stuck on a Torque Problem!! Help is appreciated!

You have find α not ω.
Just assume downward acceleration equal to a.
Then draw a fbd of the mass.

Assume a force is pulling 2 bodies connected by a string. But here the second body is a spool.

Last edited: Jul 4, 2012