Question

a bullet of mass 20g is to pass 2 points 30m apart in 4s .Find its kinetic energy

Answer 1

velocity of bullet = 30/4 = 15/2

k.e = 20 *15*15 /(2*2*1000) =1.125 j

where k.e is kinetic energy

j is joules

velocity = distance / time

Answer 2

Kinetic energy of bullet is 0.5625 Joules

Solution:

Mass (m) = 20 g = 20/1000 kg

Distance (d) = 30 m

Time taken (t) = 4 seconds

To find: Kinetic energy of the bullet =?

Formula to be used:

$\begin{array}{l}{\mathrm{V}=\frac{\text {Distance travelled}}{\text {Time taken}}} \\ \\{\mathrm{K.E}=\frac{1}{2} m v^{2}}\end{array}$

$\begin{array}{l}{\mathrm{V}=\frac{\text { Distance travelled }}{\text {Time taken}}=\frac{30 \mathrm{m}}{4 \mathrm{sec}}=\frac{15 \mathrm{m}}{2 \mathrm{sec}}} \\\\ {\mathrm{K.E}=\frac{1}{2} \mathrm{mv}^{2}} \\\\ {\mathrm{K} . \mathrm{E}=\frac{1}{2} \times \frac{20}{1000} \times \frac{15}{2} \times \frac{15}{2}} \\\\ {\mathrm{K.E}=\frac{9}{16}=0.5625 \mathrm{Joules}}\end{array}$