Suppose an ice skater goes into a spin with her arms extended and with an initial angular velocity of 20 rpm. Pulling her arms tightly in decreases her rotational inertia by 2/3.

A) what is her new angular velocity?
B) If it takes her 2 seconds to change her angular velocity, what is the average angular acceleration?

I need the equations along with the process.

Question

Suppose an ice skater goes into a spin with her arms extended and with an initial angular velocity of 20 rpm. Pulling her arms tightly in decreases her rotational inertia by 2/3.

A) what is her new angular velocity? 
B) If it takes her 2 seconds to change her angular velocity, what is the average angular acceleration?

I need the equations along with the process.

anonymous · Answer

Angular momentum = Rot. Inertia x Angular velocity
or sometimes written as:
L = I x w

Since the angular momentum is constant, if the rotational intertia decreases by 2/3, the angular velocity has to increase by a factor of 3/2.

anonymous · Answer

Ok. How does that aid me in finding the answer?

anonymous · Answer

The new ang. velocity = 3/2 x 20 = 30 rpm

anonymous · Answer

the average acceleration would be the new velocity - the old velocity divided by the time

anonymous · Answer

But I noticed your question said a decrease BY 2/3 which would mean the angular velocity is three times faster.

anonymous · Answer

i'm sorry can you put this into equation form? Your answer 30 rpm is incorrect

anonymous · Answer

w = 3 x 20 rpm = 60 rpm

anonymous · Answer

How did you get 3 or rotational inertia?

anonymous · Answer

for*

anonymous · Answer

The inertia decreased BY 2/3, which is the same thing as dividing by 3. 
This means the ang. velocity has to increase by multiplying by 3.