First slide

Power

Question

A machine rated as 150W, changes the velocity of a 10kg mass from 4ms^–1 to 10ms^–1 in 4s. The efficiency of the machine nearly is

Easy

A

70%
B

30%
C

50%
D

40%

Solution

$\large {\mathop{\rm P}\nolimits} = \frac{{\frac{1}{2}m({v^2} - {u^2})}}{t} = \frac{{\frac{1}{2} \times 10 \times (100 - 16)}}{4}$
$\large P = \frac{{5 \times 84}}{4} = 105Watt$
$\large efficiency(\eta ) = \frac{{105}}{{150}} \times 100 = 70\%$

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