First slide

Torque

Question

A flywheel, in the form of a solid disc, of mass 25 kg has a radius of 0.2 m. It is making 240 rpm. If the torque is due to a force applied tangentially on the rpm of the fly wheel, what is the magnitude of the force to bring it to rest in 20 sec ?

Moderate

A

8.48 N
B

3.14 N
C

1.57 N
D

6.28 N

Solution

$I = \frac{1}{2} \times 25 \times {(0 .2)}^{2} kg - m^{2} = 0 .5 kg - m^{2}$
$α = \frac{ω_{0}}{t} = \frac{π \times 240}{30 \times 20} rad / s^{2} = 1 .256 rad / \sec^{2}$
$∴ τ = F R = I α$
$\Rightarrow F = \frac{0 .5 \times 1 .256}{0 .2} N = 3 .14 N$

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