Question

A bullet of mass 20 grams is fired into a wall with velocity of 400 ms 1. If the bullet penetrates the wall to a depth of 1cm, find the resistive force of the wall.

Answer 1

momentum = m v = .020*400 = 8 kg m/s

stops in 1 cm = .01 meter
average speed during stop = 200m/s

so time to stop t = .01/200 = 5*10^-5 seconds

Average force = change of momentum/time
= (8/5)10^5 = 160,000 Newtons

Answer 2

Is this the only way?

Answer 3

Nope,
say F = ma
a = 400/t
so t = 400/a

d = Vi t -(1/2)a t^2
d = 400 t - (1/2)(400/t)t^2
d = 400 t - 200 t = 200 t
t = d/200 = .01/200 = 5*10^-5 s again
a = 400/t = 8*10^6
F = m a = .02*8*10^6 = 160,000 N
again :)

Answer 4

Ty :)