Question

A machine, which is $75$% efficient, uses $12J$ of energy in lifting up a $1$ kg mass through a certain distance. The mass is then allowed to fall through that distance. The velocity at the end of its fall is (in m/s)

• A. $\sqrt{24}$
• B. $\sqrt{12}$
• C. $\sqrt{18}$
• D. $\sqrt{9}$

Answer 1

The correct option is C $\sqrt{18}$

Energy stored $E=\frac{75}{100}\times \left(12\right)=9J$
As $E=\frac{1}{2}m{v}^2$
$v=\sqrt{\frac{2E}{m}}=\sqrt{\frac{2\times 9}{1}}=\sqrt{18}m/sec$