Momentum conservation and Newton's 3rd

[mrcotton]

Homework Statement

Explain why a soldier holding the machine gun will experience a force from the gun
whilst it is firing bullets

Homework Equations

Momentum is always conserved in closed systems

F= Δmv/Δt

The Attempt at a Solution

The system has zero momentum then when the bullet is fired we must have an equal but opposite momentum.

The force on the gun is the bullets change in momentum divided by the time that the momentum change occurs in.

So why can't we just say according to Newtons 3rd that there must be an equal and opposite force of the gun on the soldier?

Any help appreciated
D

 

[nasu]

What makes you think that we can't say that?

 

[Simon Bridge]

I think you'd need to be more careful than that - this is an exercize given as part of your education so you have to demonstrate that you understand the physics involved.

So - there is a force of the gun on the soldier - what is it equal and opposite to?
(you have to actually say...)

The conservation of momentum answer is just quicker.
(though you didn't complete that answer either...)

Wite out both complete answers side by side and see which one you prefer.

Anyway - you may happen to know that the question is part of an exam set on "conservation of momentum" - just like you know you are expected to do the problem in Newtonian mechanics without having to be told ;)