# Easy question that I'm not getting?

1. Sep 24, 2008

### dominus96

Easy question that I'm not getting??

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

A small ball with mass 1.40 kg is mounted on one end of a rod 0.850 m long and of negligible mass. The system rotates in a horizontal circle about the other end of the rod at 5020 rev/min.

Calculate the rotational inertia of the system about the axis of rotation.

2. Relevant equations

I=MR2/3

3. The attempt at a solution

Inertia for a rod spinning at one end = MR2/3, so I did that and got .337, but that wasn't the correct answer. What am I doing wrong?

2. Sep 24, 2008

### dominus96

Re: Easy question that I'm not getting??

Come on, SOMEBODY has to know this =/

3. Sep 24, 2008

### alphysicist

Re: Easy question that I'm not getting??

The rod has (essentially) zero mass, so when you apply your equation you will find that the rotational inertia of the rod is zero.

But the system consists of more than the rod. What is the rotational inertia of this small ball about the axis of rotation?

4. Sep 24, 2008

### dominus96

Re: Easy question that I'm not getting??

Not quite sure what you mean. There is one rod and it has a mass attached to it, so don't you just use that as the mass in the inertia equation?

5. Sep 25, 2008

### dominus96

Re: Easy question that I'm not getting??

Wait, i think I get it. So do you do MR2/3 + MR2?
The inertia of the rod is 0, so the answer is just MR2? That would be 1.0115, am I right?