What is the force exerted on a bullet leaving a rifle?

[physicsquest]

Homework Statement

A 5.0g bullet leaves the muzzle of a rifle with a speed of 320 m/s. What force (assumed constant) is exerted on the bullet while it is traveling down the 0.82 m long barrel of the rifle?

Homework Equations

$$\Sigma$$F=ma

The Attempt at a Solution

 

[Donaldos]

It might be simpler to use the work-energy theorem here:

$$W=\Delta E_k$$