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

Given,

mass m =10kg

$v_f=10m/s \& v_i=4m/s$

Here,

The work done by the machine =$\frac{m{v}^2}{2}-\frac{mv{\rho }^2}{2}$

$∆w=\frac{1}{2}\left[10\times {10}^2-10\times {\left(4\right)}^2\right]=\frac{1}{2}\times 10\times 84=4205$

Average power applied by machine = $\frac{∆w}{time taken}$

$=\frac{420}{4}=105W$

$Efficiency=\frac{applied power}{rated power}=\frac{105}{150}=0.7=70\%$

Hence the correct answer is 70%.