Q 11-06-037NEETJEE MainMedium
A disc of moment of inertia $2\ \text{kg m}^2$ spinning at $6$ rad/s is gently placed on an identical disc at rest on the same axis. They soon rotate together. The kinetic energy lost is
Answer: (C) $18$ J
$$\omega_f = \frac{2 \times 6}{2 + 2} = 3\ \text{rad/s}$$
$$K_i = \frac{1}{2}(2)(36) = 36\ \text{J}, \qquad K_f = \frac{1}{2}(4)(9) = 18\ \text{J}$$
Lost $= 18$ J, turned into heat by friction between the discs.
Solution by Sreeraj P, M.Sc Physics