Kinetic Energy?

Question

A 15g bullet strikes a metal plate on an armoured car at a speed of 350m/s. The bullet penetrates the armour 3.3mm before coming to a stop.
a) Calculate the average net force acting on the bullet while it is in the metal.
b) Calculate the average force exerted on the metal by the bullet.


Answers:
a) -2.8 x 10^5N
b) 2.8 x 10^5N

Answer 1

a) Kenetic energy equal to work done on penetrating the metal
Ke=W
0.5mV^2=Fs
F= 0.5mV^2/s
F=0.5 x 015 x (350)^2/ 0.0033
F=2.8 E+5N
Since it is against motion of the bullet the force exerted by metal on bullet is
F=- 2.8 E+5N

b)The average force exerted on the metal by the bullet is a reaction force
R= -F=2.8 E+5N