Work, Power and Energy Part 1

Let K be the initial kinetic energy and F be the resistive force. Then according to work energy theorem,
W=∆K
i.e., 3F=‌

1

2

mv2−‌

1

2

m(‌

v

2

)2......(i)
Let the bullet will penetrate x‌cm more before coming to rest.
∴Fx=‌

1

2

m(‌

v

2

)2−‌

1

2

m(0)2.......(ii)
Dividing eq. (i) and (ii) we get,
‌

x

3

=‌

1

3

or x=1‌cm