# Hollow flywheel

1. Feb 8, 2009

### wolram

If i had a driven flywheel with a hollow outer rim, and could rapidly fill it with water while in motion, would i gain any energy?

2. Feb 8, 2009

### Tominator

If you will rapidly fiill it with water while moving, in the first moment water would be still, than friction would cause the water to start moving but the friction would also slow down the flywheel. Means no. You won't gain any energy.

3. Feb 8, 2009

### cjl

Actually, you would lose energy. The momentum would remain the same, and the net energy would decrease. As Tominator said, the flywheel would slow down as the water accelerated, and the final state would be significantly slower than the initial state.

4. Feb 8, 2009

### rcgldr

Assume that the flywheel has water in it but it's walls are frictionless so the water isn't moving. Then assume vanes in the flywheel are deployed to accelerate the water up to flywheel speed. Ignoring losses to heat, it's a closed system, so energy and momentum are preserved.

5. Feb 8, 2009

### wolram

Ah, thanks every one.

6. Feb 8, 2009

### cjl

True. I was thinking of kinetic energy, which would decrease (and heat would increase).