A cord of negligible mass is wound round the rim of a flywheel of mass $20 ext{ kg}$ and radius $20

Question

A cord of negligible mass is wound round the rim of a flywheel of mass $20	ext{ kg}$ and radius $20	ext{ cm}$. A steady pull of $25	ext{ N}$ is applied on the cord. What is the kinetic energy of the wheel when $2	ext{ m}$ of the cord is unwound? (Assume the wheel starts from rest)

Professor PC Thomas Classes · Answer

The work done by the pull in unwinding $2	ext{ m}$ of the cord is $W = F 	imes s = 25	ext{ N} 	imes 2	ext{ m} = 50	ext{ J}$. According to the work-energy theorem, since there is no loss of energy due to friction and the wheel starts from rest, the kinetic energy gained by the wheel equals the work done, which is $50	ext{ J}$.