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Light speed limit

  1. Dec 13, 2006 #1
    Is there a name for the 'force' that would increase a ships mass as it gets near to the speed of light , hence slowing it down?
    I was thinking about a space ship in free space with no gravitational force acting on it, how fast would it have to go before its mass starts increasing?
  2. jcsd
  3. Dec 13, 2006 #2
    It is not a force but an effect of relative motion. For the observer it is the spaceship that gains mass for the spaceship it is the observer.

    From the perspective of the spaceship its mass always stays the same regardless of its relative motion.
    However for an observer who is in relative motion the spaceship's mass would increase each time the relative speed between them increased and the mass would decrease each time the relative speed between them descreased.
    Last edited: Dec 13, 2006
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