Answer
$5\ N$
Work Step by Step
Magnitude of the average force is $F_{avg}=\frac{\Delta P}{t}$
(where $\Delta P$ is the change in momentum for one bullet and t is the time between the bullets).
First, we find $P$:
$\Delta P =m(\Delta v) = (0.003\ kg)(500\ m/s -(-500\ m/s)) =3\ kg.m/s$
Given that the collision rate is 100 bullets / 1 min,
for 1 bullet, the time is 0.01 min =$ 0.01\times 60\ s =0.6\ s$.
Therefore;
$F_{avg}=\frac{3\ kg.m/s}{0.6\ s} = 5\ N$
