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Speed Doubt

  1. May 8, 2013 #1
    Hey folks i got a doubt, one of the theories of albert einstein say everytime we run for example our mass become a little tiny for example one person have 80 pounds when you start run you have for example 79 pounds of weight because of the speed, and when you stop running your mass go back to 80 pounds or can be minor than this like you run so much you lost weight like instead you weight 80 pounds you will weight like 79 or78?? thanks for future answers.
  2. jcsd
  3. May 8, 2013 #2


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    That's not how it works.
    Your weight does not change when you're moving; and a good thing too, because right now you're standing on an earth that is moving through space at 99.9% of the speed of light (relative to some creature on some galaxy far away, maybe).

    All those weird relativity things like mass increase, length contraction, time dilation don't happen to the moving object (as far that object is concerned, it's not moving - it's at rest and you're moving in the opposite direction!); they are what you at rest observe about objects are moving relative to you.
  4. May 8, 2013 #3
    thanks for the answer,but when you are running and you sweat a lot you can lose weight ??
  5. May 8, 2013 #4


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    You do, but you don't require a physics knowledge to know that, do you? It is also not an issue of "speed" anymore.

  6. May 8, 2013 #5
    thanks again folks ^^ so resuming speed don't have anything to do with the lost of weight who i suppose we can call mass too?
  7. May 8, 2013 #6
    Note that a object that's moving with respect to you will appear heavier, but that the effect is proportinal to

    [tex] \sqrt { 1 - \frac {v^2}{c^2}}[/tex]

    where v is the speed of the runner and c the speed of light of 3*10^m/s,
    and that this is about 5.5 * 10^-15 for v = 10 m/s, wich is much smaller than:

    - the weight loss because of transpiration, exhaling of carbon dioxide, shedding dead skin cells, hair loss.
    - local gravitational anomalies, including those of buildings, trees, cars, pavement, other persons.
    - the tidal forces of the sun, moon, most of the planets
    - variations in buoyancy because of changing temperature and air pressure.

    so it's completely impossible to measure, and completely irrelevant for human weight loss or gain.
    Last edited by a moderator: May 8, 2013
  8. May 8, 2013 #7
    Sorry, Einstein has no theory about sweat. And when you sweat you temporarily lose mostly water weight.....not really body weight such as fat or muscle....So many mixed martial arts fighters might lose 10 or 20 lbs of water weight getting ready for a fight a few days prior....then drink a lot of water and are back to their normal weight almost immediately....
  9. May 8, 2013 #8
    thanks for all who answer ^^ the lost of weight is more biologic thing like workout, but resuming is like the weight not will be change because of speed.
    Last edited: May 8, 2013
  10. May 9, 2013 #9
    [QUOTEdarksoda the lost of weight is more biologic thing like workout, but resuming is like the weight not will be change because of speed.[/QUOTE]

    You should not draw that conclusion from any replies here.
  11. May 9, 2013 #10


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    Is there any reason why we should be taking this thread seriously?
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