Spider-Man leaps in front of Lois Lane to save her from a volley of bullets. In a 1-minute interval, an automatic weapon fires 155 bullets, each of mass 8.5 g, at 4.15 x 10^2 m/s. The bullets strike his chest, which has an area of 0.70 m^2. Find the average force exerted on Spider-Man's chest if the bullets bounce back after an elastic, head-on collision.