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AP® Physics C: Mechanics

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Spinning to Dry Off: What Happens to Angular Momentum?

APPHMC-HP42U@

A wet horizontal disk is rotating on a frictionless axis. As it rotates it dries because small water droplets are thrown off the edge of the disk.

Which statement is true about the rotational velocity of the disk?

A

The rotational velocity of the disk decreases because the water droplets remove some of the rotational kinetic energy as they fly off.

B

The rotational velocity of the disk decreases because the water droplets that fly off have momentum, so when they leave the disk they reduce the angular momentum of the wet disk.

C

The rotational velocity of the wet disk decreases because in order to fly off there must be a force pushing the water droplets off the disk. According to Newton's third law there must also be an equal and opposite force pushing back on the disk, which will slow its rotation.

D

The rotational velocity of the wet disk stays the same because the water droplets that fly off have momentum, so when they leave the disk they reduce the angular momentum of the wet disk. This also reduces the effective mass of the wet disk, so the rotational velocity does not change.

E

The rotational velocity of the wet disk increases because the water droplets that fly off are effectively removing mass from the wet disk. Since angular momentum must be conserved, as the mass decreases the rotational velocity must increase to compensate.