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what's the bullet speed?

  1. Oct 4, 2012 #1

    I am new to this forum, so please go easy on me.
    I wonder about this...

    You are traveling in a train at the constant speed of 100 mph. Imagine that you throw a small ball at the speed let say 40 mph. So ground speed of this flying ball is 140 mph (as you already travel at 100mph + speed of thrown ball 40mph. For you in train ball travels at 40 mph, for someone outside the train, standing on ground it travels at 140 mph.

    Question...What if you have a gun and you shot from train outside in same direction as train is moving... what is the ground speed of the bullet?

    What if you will shoot in opposing direction, what will be speed of bullet?

    For the sake of keeping it simple, let's assume that bullet travel at 700mph (9mm has about this speed, all depends from weather, temperature etc..but not important)

    While shooting in the same direction as moving train the bullet should travel at 800mph?
    (here we should look at 2 options)
    1. Shooting inside the train along wagon 800mph
    2. Shooting outside the train, taking gun out through the window and shooting same direction as moving train [i think air/wind will slow down initial speed of bullet, etc)

    Shooting opposite direction will travel at 600mph?

    Just wonder about this, as .. well is interesting.
    Hope I will find my answers here.

    Thanks for reading
    Last edited by a moderator: Oct 4, 2012
  2. jcsd
  3. Oct 4, 2012 #2


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    Welcome to PF!

    Hi Marcin! Welcome to PF! :smile:

    Yes, that all looks correct …

    relative speeds add like vectors (so if they're in directly opposite directions, you subtract the magnitudes).

    (but in relativity, of course, the formula for adding relative speeds is different)
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