# Homework Help: Rotational Mass

1. Nov 29, 2007

### rtran

I read the rule and ran a search with no luck.

How do i find the Tension in a cable?
I got a free falling cylinder with mass m and radius r

according to the formula a=torque/inertial and the tangential force is=to torque.
the torque =inertial*r

So i got angular acceleration = F*r/(m*r) and Tension=mg-ma

2. Nov 29, 2007

### Staff: Mentor

Please define the problem completely. Don't know what you mean by a "free falling cylinder".

3. Nov 29, 2007

### rtran

I read the rule and ran a search with no luck.

How do i find the Tension in a cable?
I got a free falling cylinder with mass m and radius r attach to a stationary string

according to the formula angular acceleration=torque/inertial and the tangential force is=to torque.

the torque =inertial*r or m*r^3

So i got angular acceleration = F*r/(m*r) and Tension=mg-ma

How exactly do i setup this equation to find the tension?

4. Nov 29, 2007

### rtran

sorry the rest must have timed out

A free falling cylinder is attached to a stationary string.

I got as far as torque =F*r=mgr which also equal to mr^2*angular acceleration.
The tangentail force =Tensioner which is ma , but I can't seem to put everything togather.

5. Nov 29, 2007

### Staff: Mentor

I merged the two threads that you started... (I will move this to the Intro Phys section)

Start by identifying the forces acting on the cylinder. Compare translational acceleration and rotational acceleration, applying Newton's 2nd law to each.

I don't understand what you did here. What's the torque about the center of mass? What's the rotational inertia of a cylinder?