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Questions on Energy, Mass, and velocity

  1. Jun 24, 2010 #1
    Okay, these questions are relatively complex from my perspective. Bare with me if they turn out to be simple to the rest of you...

    Question one. This one will require that some assumptions be verifierified before the question will make sense.

    Premise 1: Energy possesses mass
    Premise 2: The velocity of a body is relative to the velocity of the observer. The same is true of acceleration
    Premise 3: Kinetic Energy, as a form of energy, has mass. (i have a feeling this is where my mistake lies)

    Okay, so question one. Is the mass of an object relative to the position of the observer?

    In other words, if a body is falling toward or away from an observer, the body would have a kinetic energy relative to the observer in proportion to the velocity of the body. As kinetic energy is a from of energy, and energy has mass, increased kinetic energy would have mass.

    Would this mean that the mass of an object is NOT a fixed quantity regardless of the location of the observer? This presents MASSIVE (LOL) problems, as that would indicate that the effect of gravity produced by the body would be relative to the vector of the observer.

    Question 2: As a body falls toward another body, and is accelerated by gravity, it's velocity is increased relative to the body toward which it falls. As we know that the mass of an object increases with it's velocity, this object becomes more massive as it accelerates.

    Would this object be "objectively" more massive? By that i mean, would an observer traveling the same vector observe this increase in mass, or would it be more massive only within a given point of reference?

    Okay, that's my first attempt to explain these ideas that i have in my head. I don't have the math to get it all out, and as i'm sure you all know, you can do a LOT more in your head than you can with words. Someone ask for clarification if my question doesn't make sense.
  2. jcsd
  3. Jun 24, 2010 #2


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    Relativistic/Gravitational mass depends on kinetic energy. Rest mass obviously doesn't. Make sure you check which one you need for a particular equation.

    This doesn't cause problems with gravity, because Energy-Stress tensor already accounts for the fact that energy and momentum depend on coordinate system.

    Acceleration, in General Relativity, is absolute. It is not relative, like velocity is.
  4. Jun 25, 2010 #3
    That is not true. Acceleration in GR is relative.

    What is absolute is proper acceleration.
  5. Jun 28, 2010 #4
    look Beprepared mass of a body is relativistic in einstein theory and in SR you can find a equation on how mass increases with velocity of a body. what einstein said about acceleration is that all laws of physics are same in accelerated reference frame. it is the first postulate of SR. the increase in mass (because of relativistic) isn't observe by the person who is traveling in spaceship (who try to measure the increse in mass of his ship) because the instrument from whic h he measures will be increase in its mass also.
  6. Jul 6, 2010 #5
    In which case - it cannot possibly be correct. - No?

    In which case, since there must be objects in the universe travelling close to the speed of light and for which you and I must appear hugely massive....
    Are we hugely massive? - Then no.
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