Question

A man weighing 60 kg lifts a body of 15 kg to the top of a building 10 m high in 30 minutes. His efficiency is:

• A. 10%
• B. 20%
• C. 30%
• D. 40%

Answer 1

Answer: (B)

20%

Here, a man weighing 60 kg lifts a body of 15 kg to the top of a building 10 m high in 30 minutes. initially, while lifting the body(doing work) the power is
$P_i=\frac{mgh}{t}$

$P_i=\frac{(60+15)\left(10\right)\left(10\right)}{\left(30\right)\left(60\right)}$

$P_i=\frac{7500}{1800}$

After reaching to the top the power is

$P_f=\frac{mgh}{t}$

$P_f=\frac{\left(15\right)\left(10\right)\left(10\right)}{\left(30\right)\left(60\right)}$

$P_f=\frac{1500}{1800}$

The efficiency of the man is the ratio of final power to the initial one
$\eta =\frac{P_f}{P_i}$

$\eta =\frac{\frac{1500}{1800}}{\frac{7500}{1800}}\times 100$

$\eta =\frac{1}{5}\times 100$

$\eta =20$%