Question

A bullet of mass 25 g has a velocity of 600 ms^-1. What is the kinetic energy of the bullet? If it penetrates 50 cm into a target, find the resistive force offered by the target.

Answer 1

Given, mass m = 25g = `25/1000  "kg" = 1/40  "kg"`

initial velocity u = 600 m/s

final velocity v = 0 m/s

Distance traveled, s = 50 cm = 0.5m 

Now, K.E of the bullet = `1/2` mu^₂ = `1/2 (1/40) (600)`²

or, K.E = 4500 J

let F be the resistive force

According to work energy theorem,

work done = change in K.E

or, work done = `1/2` m(v-u)^{2 =} `1/2 (1/40)` (0 - 600)² = -4500 J

Also, we know that: work done = force x displacement

`therefore` -4500 = F x 0.5

or, F = `-4500/0.5` = -9000 N

Hence, the resistive force is 9000 N.