Question

A horizontal disc rotating freely about a vertical axis through its centre makes 90 revolutions per minute. A small piece of wax of mass m falls vertically on the disc and sticks to it at a distance r from the the axis. If the numberof revolutions per minute reduce to 60, then the moment of inertia of the disc is ?

Answer 1

The moment of inertia of the disc is 2mr²

Explanation:

• Let I be the moment of inertia of disc
• Moment of inertia of wax = mr²
• After wax falls the net moment of inertia = I + mr²

• Since there is no external torque acting on the system.
• So the angular momentum of the system will remain constant.

Iω = ω₁(I + mr²)

I x 90 = 60(I + mr²)

1.5 I = I + mr²

0.5 I = mr²

I = 2mr²

Hence the moment of inertia of the disc is 2mr²

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A disc of mass 1 kg and radius 0.1 m is rotating with angular velocity 20 rad/s. what is angular velocity if a mass of 0.5 kg is put on the circumference of the disc ?

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Answer 2

Answer:

Explanation:

Please check the attachment