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

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

Rajasthan PETRajasthan PET 2010

Solution:

Motor makes 600 rotation per minute
$ \therefore $ $ n=600\frac{rotation}{minute}=10\frac{rev}{\sec } $
$ \therefore $ Time required for one rotation $ =\frac{1}{10}s $
Energy required for one rotation = power $ \times $ 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}=0.40\times \frac{746}{40} $
$ =7.46J $