# Finding time in ballistic problem

The problem is like this:

A bullet with mass 15g moving at a speed of 425 m/s is brought to a stop in 3.5 cm in a block of wood. Find the force on the bullet, assuming that it is constant.

So I know that: F = m*a

m = 15

g=9.81

a = v/t

v = 425 m/s

t = ?

My question is: how to calculate time? My guess is, if the velocity is 425 m/s, and the bullet stumble upon a block of wood in 3.5 cm, I could transform the 425 m/s to cm/s (42500 cm) and then get the time, but what I get is a very tiny number, so I think I am doing it wrong...

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The time should be a tiny number, and the acceleration a very big number. Also, its easier to use work, W = F*d, where W is the initial kinetic energy of the bullet, d is the distance traveled. –  user1631 Dec 30 '11 at 21:40
F=W/d. Where W=1/2mv^2. W=1354687.5 N d=0.035m F=1354687.5/0.035=38705357,14N Just to be sure -- is this right? It's like, a huge number for me... –  Ron Dec 30 '11 at 22:02