Question

A disc of moment of inertia is rotating in horizontal plane about an axis passing through a centre and perpendicular to its plane with constant angular speed . Another disc of moment of inertia having zero angular speed is gently placed coaxially on a rotating disc. Now both the discs are rotating with constant angular speed . The energy lost in the process is:

Answer 1

Explanation:

Initial angular momentum of the system Li=I1w1+I2w′   where w′=0

∴ Li=I1w1+0=I1w1   

Thus initial kinetic energy of the system  Ki=21I1w12+21I2(w′)2

⟹  Ki=21I1w12+0=21I1w12

Final angular momentum of the system  Lf=(I1+I2)w2

Using conservation of angular momentum :  Li=