1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Momentum conservation and Newton's 3rd

  1. Nov 5, 2013 #1
    1. The problem statement, all variables and given/known data

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

    2. Relevant equations

    Momentum is always conserved in closed systems

    F= Δmv/Δt

    3. 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 cant 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
  2. jcsd
  3. Nov 5, 2013 #2
    What makes you think that we can't say that?
  4. Nov 5, 2013 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    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 ;)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Momentum conservation and Newton's 3rd
  1. Newton's 3rd (Replies: 13)