 #1
Saptarshi Sarkar
 99
 12
 Homework Statement:
 A thin uniform disc of mass M and radius R is rotating in a horizontal plane about an axis passing through its centre and perpendicular to its plane with an angular velocity ##\omega##. Another disk of same dimensions but mass M/4 is placed on the first disk coaxially. What is the angular speed of the system is now
 Relevant Equations:

##K.E. = \frac 12I\omega^2##
##L= I\omega##
I first tried to get the solution by conserving the rotational kinetic energy and got ##\omega'=\frac2{\sqrt5} \omega##.
But, it was not the correct answer. Next I tried by conserving the angular momentum and got ##\omega'=\frac 45 \omega##, which is the correct answer.
Why is the rotational kinetic energy not conserved here? Where is the energy being used up?
But, it was not the correct answer. Next I tried by conserving the angular momentum and got ##\omega'=\frac 45 \omega##, which is the correct answer.
Why is the rotational kinetic energy not conserved here? Where is the energy being used up?