# Moment of Inertia Question

1. Oct 30, 2012

### utkarshakash

1. The problem statement, all variables and given/known data
A closed cylindrical tube containing some water (not filling the entire tube) lies in a horizontal plane. If the tube is rotated about a perpendicular bisector, the moment of inertia of water about the axis
a)increases
b)decreases
c)remains constant
d)increases if the rotation is clockwise and decreases if it is anticlockwise

2. Relevant equations

3. The attempt at a solution
Initially the water is at a greater distance from the perpendicular bisector. But after rotating the cylinder it comes near to the axis. So in the latter case the MI should decrease as it is directly proportional to the distance from the axis.

2. Oct 30, 2012

### Saitama

Start by drawing two figures, one when the cylinder is not rotating and the other when the cylinder is rotating. Take care about the position of water in both the situations.

3. Oct 30, 2012

### utkarshakash

I still can't bring out the mistake

4. Oct 30, 2012

### Saitama

When you rotate the cylinder around the given axis, the water will try to stay near the circular base of the cylinder on left and right.

5. Oct 30, 2012

### utkarshakash

In this case also after rotating the distance of water from the axis decreases.

6. Oct 30, 2012

### Saitama

It increases, thats why i asked you to draw two figures.

When the cylinder is not rotating, the water is near the axis too but when it is rotating, most of the water is on the sides i.e water is away from the axis now. How MI is defined?

7. Oct 30, 2012

### utkarshakash

OK I have understood now