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!

Bullet velocities

  1. Dec 9, 2007 #1
    Hello to all. I am merely looking out of interest peeked by a friend who asked me,mistakenly thinking I am smart enough to answer, this particular question. (probably very simple for you guys !) Here goes.
    If you are flying a jet at say 600 mph and it has a weapon that fires a bullet forward at 600 mph,does the bullet come out of the barrel and just stop? or is it now traveling at 1200 mph upon exit?...how about if the gun fires rearward? Sorry if this seems well...I don't know ....but I have to ask ... Thanks ...

  2. jcsd
  3. Dec 9, 2007 #2


    User Avatar
    Science Advisor

    Since all speeds are relative and the speeds you are talking about are very small compared to the speed of light, ordinary addition applies. The bullet (forward or backward) will be travelling 600 mph relative to the jet. To get the ground speed of the bullet as it leaves the barrel, just add in the jet speed (+ or - as needed). The net result is 1200 mph forward and 0 mph backward.
  4. Dec 10, 2007 #3
    the moment the bullet comes out of the barrel is the moment when it starts decelerating right
  5. Dec 10, 2007 #4


    User Avatar
    Science Advisor

    If it is subject to outside forces. Ordinarily these include air resistance and gravity.
  6. Dec 10, 2007 #5
    Yes. And the fact that it comes out in one instance at 1200 means that it will slow down much faster than the one at 600 due to wind resistance. It will STILL go farther than the one at 600, but not twice as far. I'd SWAG it at about 72% farther, but would want to see some hard data.

    Just a thought though in the same vein, a bullet has a given length. If it is moving so fast that the harmonic of it's wavefront is such that it is less than the length of the bullet itself I would not be surprised to find that in turbulent air the bullet itself might be induced to tumble even spinning in a fashion akin to gymbal lock. Which, obviously, would drastically reduce its effectiveness, range, and accuracy.
  7. Dec 11, 2007 #6


    User Avatar
    Homework Helper

    No sane person would design an aircraft gun that would risk the aircraft flying into it's own bullet trail. The muzzle velocity of aircraft guns is very high. Also when in a dog fight, if the bullets are fired with significant upward component, or the pilot isn't sure, the pilot is supposed to keep pulling back on the stick after firing has stopped to avoid flying into a stream of bullets.
  8. Dec 11, 2007 #7
    To my understanding ,as per an old work friend of mine that was in WWII, that problem existed back then ,even w/ prop driven aircraft.He claims that they could do this in a
    'diving ' dog-fight situation. I never really believed that claim but now I find it interesting, it being mentioned here at all. Interesting thread indeed .
    I should try to locate and call my friend,it's been a long time.He may even be gone by now. ...Jj
  9. Dec 11, 2007 #8


    User Avatar

    I thought what was a problem was firing while keeping the propeller engines? Someone devised a way to time the MG to fire between each turn of the main fan.

    Linearly, you'd add the velocities and assuming they don't catch up, no problem. The problem only comes if you have to describe the force on the bullet as a vector.

    600mph relative to the plane, 1200mph relative to an observer on the ground.
  10. Dec 11, 2007 #9
    ok i got it until the harmonic part

    if a pilot is shooting and the bullets are going a lot faster then why should he have to pull up
    and does any one know what speed they go at?
  11. Sep 22, 2011 #10
    One thing to consider in this discussion:

    The bullet after it leaves the muzzle is slowing down...

    The aircraft has the ability, via its propulsion, to keep its speed constant, speed up or slow down…

    If the bullet has sufficient drag, upon leaving the muzzle, it may slow down sufficiently for the aircraft to catch it or pass through the bullet grouping…
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook