Question

9. A uniform disc rotating freely about a vertical axis passing through its centre makes 90 rpm. A small piece of wax of mass 10g falls vertically on the disc and sticks to it at a distance of 5 cm from its axis. If the frequency is thus reduced to 60rpm, find the moment of Inertia of the disc.

Answer 1

Answer:

Explanation:

Here, n1=90rpm,n2=60rpm

Let I1= moment of inertia of disc.

∴I2=I1+mr2

From the principle of conservation of angualr momentum,

I2ω2=I1ω1or(I1+mr2)2πn2=I1×2πn1

or I1+mr2=I1n1n2=I19060=3I12

or 32I1−I1=mr2,I1=2mr2gcm2