I thought it was pretty amazing that the cell phone was able to stop the bullet. I was curious about how much force the cell phone exerted on the bullet and the deceleration experienced by the bullet.
In order to determine this, I made several assumptions. I found the average mass of a bullet to be 200grains or .01kg. I also determined that the average muzzle velocity is 850ft/s or 259m/s. The article stated that the robber shot at the clerk on his way out of the store, so I assumed that the bullet traveled 5m before hitting the clerk. A bullet traveling at 259m/s would travel 5m in .02s.
So, we have the following information:
m = .01kg
v0 = 259m/s
vF = 0m/s
t = .02s
We now have enough information to calculate the net force exerted on the bullet by the cell phone.
Force = change in momentum / time
change in momentum of bullet = mFvF - m0v0
=(.01kg)(0m/s) - (.01kg)(259m/s)
Fcell phone = (-2.59kg m/s) / .02s
Now that have calculated the net force, we can figure out the deceleration experienced by the bullet using:
F = ma
-129.5N = (.01kg)a
a = -12950m/s2