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

• A. 7.46 J
• B. 7400 J
• C. 7.46 ergs
• D. 74.6 J

Answer 1

Answer: (A)

7.46 J

Answer 2

Motor makes 600 revolution per minute

∴ n = $600 \frac { \text { revolution } } { \text { minute } } = 10 \frac { \mathrm { rev } } { \mathrm { sec } }$

∴ Time required for one revolution $= \frac { 1 } { 10 }$sec

Energy required for one revolution = power × time

=

But work done = 40% of input

$= 40 \% \times \frac { 746 } { 40 } = \frac { 40 } { 100 } \times \frac { 746 } { 40 } = 7.46 \mathrm {~J}$