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I need someone to answer this physics questiopn for me please.

  1. Feb 27, 2007 #1
    I need someone to answer this physics question for me please.

    I have no education in physics, save basic high school sciences, so I must apologize for this question if it sounds stupid to you. My brother and I have been quarrelling over this all day.

    He and I were playing the video game "Mega Man X" for the old super nintendo the other day. I was at a part in the game where the protagonist hops onto a mine cart and goes flying thorough a mine shaft. The protagonist is equiped with a projectile weapon on his arm and as he fires it while riding the mine cart, the projectiles travel very slowly away from him whereas the projectiles travel much faster away from him while he's standing stationary. I've established that the projectiles have a set speed whether they are being fired from a stationary postion or from the speeding mine cart. My little brother said that that would happen in our universe and him being an idiot I wasn't quick to believe him.

    Suppose I were driving down a straight highway with no one around for hundreds of miles (thus making this experiment safe) and I had a handgun in my left hand. Let's also suppose that I could shoot the gun in an exact straight line realtive to the car, but next to it. I don't know exactly how fast a bullet will travel but let's also suppose that the car was going almost as fast as the bullet, let's say ten miles an hour slower than the bullet.

    Here's my question; will the bullet travel away from the car at ten miles an hour or will the bullet travel away from the car at the speed of the vehicle plus how fast the bullet would travel if fired stationary? Or is there another possibility I have not taken into account?

    Do you understand the question? It's my fault if you don't, like I said I have next to no science education, except for basic high school classes and an infatuation with physics books. I also haven't put too much thought into the wind resistence and gravity....I wasn't exactly sure how to factor it in there. Again I apologize if this is a stupid question.
    Last edited: Feb 27, 2007
  2. jcsd
  3. Feb 27, 2007 #2


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    A bullet with mass will travel away from the cart with the sum of the speeds.


    If you were firing a laser, the speed of laser beam would not be effected by the cart.
  4. Feb 27, 2007 #3
    So we're both right...huh, thanks for the answer, friend
  5. Feb 27, 2007 #4


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    "stupid" this question may sound, but these are the kind of stuffs that gave us Einstien's relativity (quite non-trivial). Well, the speed that you are talking about is too slow for "real" relativity to be relevant, so we may stick to classical physics and Galilean relativity (that is time is an absolute quantity and that it is the same in all reference frames). btw, when you were asking that question, you should have specified "which reference frame should the speed of the bullet or car is measured in"... note it is all about reference frame (ie. coordinate system) I guess, implicitly, you are refering to the ref frame of the ground. (ie. the coordinate grid is not moving with the car or the bullet, but moving with the ground).

    Anyway, so the car and gun is moving with respect to the ground ie. to the coordinate grid, so when you fire the bullet the bullet will have a speed equal to its "normal" speed + the speed of the moving car/gun. NB, this is only true if all speeds involved is much less than the speed of light. but in the classical limit, it is correct.
  6. Feb 27, 2007 #5


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    Maybe you should try agian, this time make it a bit more complicated. :rolleyes: Note that your final answer is the same as my first single sentence reply.
  7. Mar 2, 2007 #6
    As an extension to Integral's answer, if the vehicle speed is constant then it will ultimately appear as though the bullet begins travelling slower than you due to the fact that air friction will slow the bullet down. I've heard that jets which fire convential bullets in dogfights have to roll out of the way in order to keep from running over their own shots.
  8. Mar 3, 2007 #7


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    Since you can see the projectile in the game, it's safe to asssume it's not a beam of light or some imaginary object that travels at the speed of light.

    Assuming it's a bullet in your example, it leave the barrel at the same speed relative to the stationary or moving barrel, with a slight difference due to the air resistance while moving forwards at a high speed.

    Regarding jets flying into their own bullets, the bullets follow a normal glide path (almost ballistic though, bullets don't glide well). So unless a pilot followed or intercepted the path of the bullets there shouldn't be an issue. For example, it would be dumb to fire bullets straight up and then fly straight up into them.
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