First slide

Dynamics of rotational motion about fixed axis

Question

A flywheel is a uniform disc of mass 72 kg and 50 cm. When it is rotating at the rate of 70 rpm its kinetic energy is

Easy

A

142J
B

242J
C

342J
D

300J

Solution

$K . E . = \frac{1}{2} {Iω}^{2}$
The moment of inertia of flywheel
$I = {MR}^{2} / 2$
$\begin{array}{l} ∴ K . E . = \frac{1}{2} \times (\frac{{MR}^{2}}{2}) \times (2 πv)^{2} \\ = π^{2} {MR}^{2} v^{2} \\ = (3 \cdot 14)^{2} \times 72 \times (0 \cdot 5)^{2} \times (70 / 60)^{2} \\ = 242 J \end{array}$

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