# Force and impulse

Joined Dec 29, 2004
83
Hi,
Please can I have some help with this problem:

A machine gun in automatic mode fires 20g bullets with vbullet=300m/s at 60 bullets/second.
a- If the bullets enters a thick wooden wall, what is the average force exerted against the wall?
b- If the bullets hit a steel wall and rebound elastically,what is the average force on the wall?

What i did is
for a:
I computed the impulse: I= pf-pi= m*v-( 0)=6 N.s
to calculate the average force: I/(time for one bullet to be fired)= 6/(1/60)=360N

Am I right?
However I have no idea for the second question??
B.

#### n9352527

Joined Oct 14, 2005
1,198
The momentum is a vector quantity, so if the bullet rebounded elastically the magnitude of the velocities after and before the impact would be equal. The delta momentum's calculation has to take this into account, e.g. if the angle of impact and rebound to the steel plane is 90 degree then the delta momentum is equal to twice the initial bullet momentum.