First slide

Power

Question

A quarter horse power motor runs at a speed of 600 r.p.m. Assuming 40% efficiency the work done by the motor in one rotation will be

Moderate

A

7.46 J
B

7400 J
C

7.46 ergs
D

74.6 J

Solution

Motor makes 600 revolution per minute
$∴ n = 600 \frac{revolution}{minute} = 10 \frac{rev}{\sec}$
$∴$ Time required for one revolution $= \frac{1}{10} \sec$
Energy required for one revolution = power × time
= $\frac{1}{4} \times 746 \times \frac{1}{10} = \frac{746}{40} J$
But work done = 40% of input
$= 40 % \times \frac{746}{40} = \frac{40}{100} \times \frac{746}{40} = 7 .46 J$

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