# Conceptual - lab

1. Mar 11, 2008

### frasifrasi

So, in the lab that my class just finished for angular momentum, we had a rotaring hollow cylinder rotating and we had to drop an iron disk square on top of it to measure the momentum before and after to verify conservation.

The professor inquired how the result would be affected if we had dropped the disk off center, but never explained it. I realize the result would be impacted but am not visualizing in what way or how. Can anyone help me understad this?

Thank you.

2. Mar 11, 2008

### Staff: Mentor

Hint: How does the rotational inertia of the combined system (cylinder + disk) depend on where the disk is placed?

3. Mar 11, 2008

### frasifrasi

I am not sure. I would guess the velocity will be higher because the mass is less concentrated?

4. Mar 11, 2008

### frasifrasi

anyone?

5. Mar 12, 2008

### Staff: Mentor

When the disk is added, the rotational speed goes down because the rotational inertia of the system goes up. The rotational inertia of the system about the given axis equals the sum of the rotational inertia of the cylinder (which doesn't change) plus the rotational inertia of the disk. How does the rotational inertia of the disk about the given axis depend on its distance from the axis?

Hint: Whenever mass moves away from the axis, does that increase or decrease the rotational inertia?