Question

# A student sits on a rotating stool holding two 2.7-kg objects. When his arms are extended...

A student sits on a rotating stool holding two 2.7-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 0.43 m from the rotation axis.

(a) Find the new angular speed of the student.

(b) Find the kinetic energy of the student before and after the objects are pulled in.

 before J after J

Page no. 1

Given information,

i) mass of objects = m = 2.7 kg

ii) distance of objhect before objects are pulled in = r1 = 1.0 m

iii) angular speed before objects are pulled in w1 = 0075 rad / s

iv) Moment of inertia of the student and stool is Is = 3 kg . m2

v) Distance of object after objects are pulled in = r2 = 0.43 m

To find,

i) angular speed after objects are pulled in

ii) Kinetic energy of the student before and after the objects are pulled in.

Here , no external torque is acting on the body

Hence angular momentum L = Iw is conserved.

Initial moment of inertia of the system is

Ii = Is + 2 mr12 this term is added because student is holding two objects of 2.7 kg masds at certain distance ( r1 = 10 m )

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