Finding force without acceleration

[RedDanger]

Homework Statement

A machine gun fires 22 g bullets horizontally at 260 m/s at a constant rate of
40 bullets/s. (a) If the bullets embed themselves in a thick wooden door, what is the average force
exerted on the door? (b) If the bullets hit a steel door and rebound elastically, what is the average
force exerted on the door?

Homework Equations

F = ma
Vf = Vi + at

The Attempt at a Solution

The door shouldn't experience any force at all, because the bullets are not accelerating horizontally. Is this correct?

 

[zhermes]

The door shouldn't experience any force at all, because the bullets are not accelerating horizontally. Is this correct?

Not at all correct. The bullets are going at some velocity, and then they are stopped by the door right? Thus they have some acceleration.

You should think about this in terms of `impulse' (I)
$$I \equiv \Delta mv = F \Delta t$$
The impulse, which is defined as a change in momentum, is the force applied times the time over which it is applied.

 

[RedDanger]

Okay, so for the first part I solved for F and got 229N. For the second part, the amount of force exerted on the door should be the same because neither the mass of the bullets nor their final velocity changes due to the type of collision. Is this correct?