A student sits on a rotating stool holding two 4.0-kg objects. When his arms are
ID: 1490551 • Letter: A
Question
A student sits on a rotating stool holding two 4.0-kg objects. When his arms are extended horizontally, the objects are 1.0 m from the axis of rotation and he rotates with an angular speed of 0.75 rad/s. The moment of inertia of the student plus stool is 3.0 kg · m2 and is assumed to be constant. The student then pulls in the objects horizontally to 0.44 m from the rotation axis.
(a) Find the new angular speed of the student. rad/s
(b) Find the kinetic energy of the student before and after the objects are pulled in. before J after J
Explanation / Answer
Key concept : angular momentum conservation
Li = Lf
I*w = If*Wf
( 3+ 2* 4*1^2 )0.75 = ( 3+2*4 ( 0.44^2) *Wf
Wf= 1.81367 rad/s ............answer
KEbefore =
0.5*( 3+ 2* 4*1^2 )0.75^2 = 3.093 J
KEafter
( 3+2*4 ( 0.44^2) *1.81367^2
= 14.96 J
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