How to find velocity of a bullet given energy?

[rockninjaalex]

Homework Statement

A bullet with a mass of 15 grams is fired from a rifle with a barrel that is 85 cm long.
a. assuming that the force exerted by the expanding gas to be a constant 7200N, what speed would the bullet reach?

Homework Equations

Ek= 1/2mv^2
Work= Force x distance

The Attempt at a Solution

Originally I thought that 7200 N were equal to Energy Kinetic so I solved like that
7200N=.5(.015)v^2...
but that seems to be incorrect.

So I found the equation for work
So I tried doing
work= 7200N x .85 meters work = 6120 J
Then setting that equal to Ek so...
6120J= 1/2(.015)v^2
v= 903 m/s
But that also seems to be incorrect.
Am I missing a step? Or am I using the wrong equation?

Please Help me, thank you in advance.

 

[bjornebarn]

Work is energy, so in the case where the work accelerates a bullet, the work is the same as the change in kinetic energy. W=ΔE_k

Thus, your calculations should be correct... Are you sure all the values are correct? What is the answer supposed to be?