Question

A bullet of mass 50 g is fired with a speed 100 m/s on a plywood and emerges with 40 m/s. The percentage loss of kinetic energy is : 

(1) 32% 

(2) 44% 

(3) 16% 

(4) 84% 

Answer 1

Correct option is :  (4) 84% 

\(\mathrm{K}_{\mathrm{i}}=\frac{1}{2} \mathrm{m}(100)^{2}\)

\(\mathrm{K}_{\mathrm{f}}=\frac{1}{2} \mathrm{m}(40)^{2}\)

\(\% \text { loss } =\frac{\left|\mathrm{K}_{\mathrm{f}}-\mathrm{K}_{\mathrm{i}}\right|}{\mathrm{K}_{\mathrm{i}}} \times 100\)

\(=\frac{\left|\frac{1}{2} \mathrm{m}(40)^{2}-\frac{1}{2} \mathrm{m}(100)^{2}\right|}{\frac{1}{2} \mathrm{m}(100)^{2}} \times 100\)

\(=\frac{|1600-100 \times 100|}{100}=84 \%\)