# physics !!

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A student on a stool rotates freely at 180 rounds per minute. The student holds a 1.00 kg mass in each outstretched arm, 0.8 m from the axis of rotation. The combined moment of inertia of the student and the stool is 6.00 kgm2 which remains constant. How far should the student pull his arms inward so that the rotation becomes 4.00 Hz?

• physics !! -

180 rounds per minute = 3.00 Hz
Angular momentum of each mass is:
L = (m r^2)ω

Mass and Angular momentum are conserved, so:
r0^2 ω0 = r1^2 ω1
or
r1 = r0 √(ω0/ω1)

r1 = 0.800 √(3.00/4.00)
r1 ≈ 6.93m

• physics !! -

how u calculate i calculate get 0.693m .
can u explain to me ??how u did this ~ why the 3Hz /4Hz?

• physics !! -

My apology for the typo. I must have miskeyed during calculation, but I should have realised that seven meter was a bit of an arm stretch.

So yes, r1 ≈ 0.693m

Because angular momentum and mass are conserved, the angular velocity is inversely proportional to the square of the radius. Thus to increase the frequency from 3.00Hz to 4.00Hz requires decreasing the radius from 0.800m to 0.693m.
(r1/r0) = √(ω0/ω1)
(6.93m/0.800m) = √(3.00Hz/4.00Hz)

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