A child of mass 30 kg stands beside a circular platform of mass 95 kg and radius 2.4 m spinning at 2.3 rad/s. Treat the platform as a disk. The child steps on the rim.

Question

a) What is the new angular speed?

b) She then walks to the center and stay there. What is the angular velocity of the platform then?

c) What is the change in kinetic energy when she walks from the rim to the center of the platform?

Answer 1

Ip = I of platform =(1/2)(95)(2.4^2)

Angular momentum = Ip(2.3)

IT STAYS THAT FOREVER

New In = Ip +30*(2.4^2)

In omega = Ip (2.3)
omega = (Ip/In)(2.3) (Part a)

b)
Her I = mr^2 is zero
BACK to original I and 2.3 rad/s

c)
(1/2) In omega^2 - (1/2)Ip(2.3^2)

Answer 2

Thank you so much!

Answer 3

You are welcome.
Remember that figure skater spinning slowly with arms extended, then suddenly speeding up when she pulls her arms in :)