Question

A disc of mass 2 kg and radius 10 cm is rotating about an axis tangent to it and parallel to its diameter with an angular velocity of 10 rad /s. The kinetic energy of the disc is

• A. 2.5 J
• B. 1.5 J
• C. 1.25 J
• D. 2.25 J

Answer 1

Answer: (C)

1.25 J

Answer 2

Moment of inertial of the disc about the axis of rotation is I = $\frac{mR^{2}}{4}$+mR² = $\frac{5}{4}$mR²

∴ Kinetic energy of the disc = $\frac{1}{2}I\omega^{2}$ = $\frac { 1 } { 2 }$ $\frac{5}{4}$mR²ω² = $\frac{5}{8}$×2×(0.1)²×(10)² = 1.25 J

(3) is correct.