# Find mom of inertia and frictional torque

1. Jan 6, 2013

### physicsgirl4

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

2. Relevant equations

3. The attempt at a solution

Last edited: Jan 6, 2013
2. Jan 6, 2013

### I like Serena

Welcome to PF, physicsgirl4!

That's the right value!

But the wrong unit...
You converted revolutions to radians, and it's an angular acceleration instead of a angular velocity.

You seem to have lost a pi here...
But the value is not correct yet anyway, since you haven't taken friction into account yet.

Let's set up the equations first:

In the first stage:
$$T_{net} = T_{ext} - T_{fric} = I \cdot \alpha_1$$
$$T_{ext} = 60 N m$$
$$\alpha_1 = {800 \cdot 2\pi \over 60 \cdot 12} {rad \over s^2}$$

In the second stage:
$$- T_{fric} = I \cdot \alpha_2$$
$$\alpha_2 = - {800 \cdot 2\pi \over 60 \cdot 100} {rad \over s^2}$$

This is a set of equations, can you solve it?