# Angular acceleration and torque

1. Feb 13, 2005

### Rave Grrl

I need some help, I've never dealt with a torque problem that uses friction or mechanical energy. This is the problem:

Attached to each end of a thin steel rod of length 1.00 m and mass 6.70 kg is a small ball of mass 1.10 kg. The rod is constrained to rotate in a horizontal plane about a vertical axis through its midpoint. At a certain instant, it is rotating at 39.0 rev/s. Because of friction, it slows to a stop in 32.0 s. Assume a constant frictional torque.

(b) Compute the retarding torque due to friction.

(c) Compute the total energy transferred from mechanical energy to thermal energy by friction.

(e) Compute the number of revolutions rotated during the 32.0 s.

Then I am told to calculate the mechanical energy loss, which we have never done.

2. Feb 13, 2005

### StatusX

You know how to calculate the force due to friction, right? So just use T=rXF. This will give you a differential equation for angular velocity, which you then set to zero to solve for how much time it rotated for. The energy questions are easy, just use conservation of energy.

3. Feb 13, 2005

### Staff: Mentor

Use this information to figure out the angular acceleration ($\alpha$).

Now apply Newton's 2nd law for rotational motion to find the torque:
[tex]\tau = I \alpha[/itex]
(You'll need to figure out the rotational inertia of the system.)