Question

A bullet of 5gm moving at 200 m/s strikes a fixed plank of thickness 0.2 m and passes through it losing half of its kinetic energy. If it again strikes an identical plank and passes through it,the ratio of thermal energies produced in the plank is?

The Options Are-
1) 1:1
2) 1:2
3) 2:1
4) 4:1

Answer 1

(3)

Explanation:

Let the bullet have initial kinetic energy `=KE_i`

As it passes through the plank it looses half of its kinetic energy.

Energy Lost `=(KE_i)/2` .......(1)

Bullet strikes identical plank. Assuming that it it looses again half of its kinetic energy.

Energy Lost this time `=((KE_i)/2)/2=(KE_i)/4` .......(1)

Now assuming that all the loss of kinetic energy in both cases contributes towards production of thermal energy in plank only,

Ratio of thermal energies produced in the plank`=((KE_i)/2)/((KE_i)/4)=2`