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Additive velocity in space?

  1. Jun 29, 2010 #1
    The title is somewhat ambiguous and I apologize beforehand for lacking any real technical vocabulary or an extensive knowledge of physics.

    This query stems from reading some discussions of a moderately popular MMO called "EVE Online." The game is set in some area of the universe where the player pilots a spacecraft and engages in interstellar combat with other players or NPCs (non-player characters).

    There was a discussion about the effect of a ship's velocity versus its angular velocity, in relation to another craft, on the sustained damage of a fired projectile.

    I understand this is all simply a few pixels on a screen being placed according to a series of calculations programmed into the servers, but it led me to wondering some things about velocity in space flight.

    If I'm not mistaken, if one were to drive on the highway and fire a slingshot from a window, the projectile would be moving (although only momentarily) at the combined velocity of the vehicle and slingshot release -- the velocities in this case seeming to be somewhat additive until gravity and air resistance brings the projectile down.

    My question is this:

    If one were to piggyback spacecraft onto spacecraft onto spacecraft, and launch each in succession (assuming that each craft has to ability to achieve the same maximum velocity), would the final ship be moving at a velocity that is equivalent to its maximum velocity multiplied by the number of vessels in the chain?

    If not, for what reason(s)?

    Furthermore, in thinking this through it seems as though that may not be possible under current conditions. Hypothetically, if one were able to simulate gravity on the craft and also allow enough space for the ship to achieve maximum velocity before exiting the initial vessel, would it then be possible?

    Sorry for the somewhat frivolous rantings of a bored law student, but sometimes curiosity just gets the better of me. No rush on an answer as I'm sure there are many more important topics being discussed besides a game-inspired physics question.

  2. jcsd
  3. Jun 29, 2010 #2
    The short answer is, no. At high speeds velocities dont add that way. That is, the slingshot projectile situation you describe is true because the speeds are so low. But as you get higher speeds deviations from that 'common sense' behavior become greater and greater. To see how to properly add velocities at high speeds look for relativistic velocity addition.
  4. Jun 29, 2010 #3


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    Science Advisor

    The maximum velocity of a space craft is determined primarily by the mass of the craft and the fuel it has to propel itself with. That's where EVE already diverges from real physics.

    You can increase velocity that way. This is how multi-stage rockets work. A big rocket carries a smaller one which carries a really small rocket that delivers final cargo to orbit. Each stage delivers the next one to as high a speed as it can, and the next stage takes over.
  5. Jun 29, 2010 #4
    Yes, they add. Not "somewhat", but really.

    No. The rule for adding velocity is not just simple addition, though that is close enough for low speeds. Even for real rockets. It starts to show differing results at a few percent of light speed. So yes, you apply the velocity addition rule N times, but no that is not real-number multiplication, because the addition rule is not real-number addition.

    As for your musings on simulating gravity, I don't see the connection.

  6. Jun 29, 2010 #5

    Filip Larsen

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    Gold Member

    While EVE online is a great MMO it suffers from the same common "star wars physics syndrome" like so many other games and movies in recent year where physical realism is almost completely replaced by artificial models that have very little in common with real life physics. In EVE, vehicle dynamics is no doubt designed primarily to have an interesting and accessible game-play with physical realism, where it appears to be present, being only paper thin. For instance, from the data and descriptions of orbital elements in star systems it appears that object are orbiting each other as should be expected, but if you look closer everything just hangs completely static at a fixed position. Or for fun, try calculate the acceleration forces a pilot must experience when he turns an Iteron V (or any other long spaceship) 180 degrees. Or wonder what kind of physics can explain a ship can accelerate with hundreds of g's but still have its speed capped at, say, 500 m/s? Oh, well, as a game its fun and quite complex in it own artificial way (like most other MMO's).
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