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Is this a viable way to increase an object's mass/energy?

  1. Feb 19, 2016 #1
    Lets say that somewhere in space there is a spaceship that has 2 identical reactors - one in the front and one in the back. The spaceship is stationary relative to earth. We fire both reactors in such a way that spaceship remain stationary relative to earth. Did its mass increased? Did its energy increased?
     
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  3. Feb 19, 2016 #2

    Orodruin

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    No, if anything the mass (and energy) decreased due to radiating away energy.
     
  4. Feb 19, 2016 #3
    ok, now consider the same ship with the same power reactors, both in the back, accelerating around the earth. Does its mass/energy increases? If yes why?
     
  5. Feb 19, 2016 #4

    Orodruin

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    No, it does not. It is also not clear what you mean by "accelerating around the earth".
     
  6. Feb 19, 2016 #5
    Dont know why my statement is confusing!?! I meant the ship is orbiting the earth not with a constant speed but with whatever acceleration the engine is capable of producing...
     
  7. Feb 19, 2016 #6

    Orodruin

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    If you accelerate you will no longer be in an orbit. If you accelerate enough, you will reach escape velocity.
     
  8. Feb 19, 2016 #7
    So relative to what is the ship accelerating when it gains speed from zero to lets say 10000 km/h while orbiting the earth?
     
  9. Feb 19, 2016 #8
    Acceleration can be measured in some arbitrary coordinate system (coordinate acceleration) or by an accelerometer on board the ship (proper acceleration).
     
  10. Feb 19, 2016 #9

    Orodruin

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    It seems that you do not get the point here. You cannot be in orbit travelling at 0 km/h (relative to the Earth). You also cannot say that you are "in orbit" when you are accelerating - it refers to a particular bound state of the system.
     
  11. Feb 19, 2016 #10

    jbriggs444

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    You are postulating magical reactors that produce energy out of nothing and generate force without reaction? Both violate the laws of physics.

    If you spit reaction mass out the back of the craft, you lose mass. If you emit energy in any form, you lose the mass equivalent of that energy. If you close all the windows and doors, plug all the nozzles and make sure that your insulation is perfect then you can run the engines as long as you please and not lose mass. But then you won't go anywhere either.
     
  12. Feb 19, 2016 #11

    Ibix

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    OP: Are you thinking of relativistic mass? That an object at initially at rest in some frame then accelerated to a high velocity increases mass because of the energy given to it by the rockets? And then wondering what happens if you fire rockets in opposite directions so that it remains stationary?

    If so, there are three issues here. The first is relativistic mass - it's an out-of-date concept that isn't really used any more except in pop sci presentations where it sounds cool. So the short answer is that the spaceship doesn't gain mass even in the moving case, despite what you may have read. The second is that the engines do no work on the spaceship if they are acting in opposition to each other, so the rocket's energy doesn't increase anyway. Both exhaust plumes are more energetic in this case than they would be if they were accelerating the spaceship; these carry away the energy. The third is that the reaction mass is being spit out of the rocket so its mass is rapidly decreasing, although you could get around this by specifying that you are interested in the "dry mass" of the rocket - the mass with its fuel tanks empty.

    There is one slight modification to the above, which is that a rocket under thrust is subject to compressional stress. This will, I think, affect its mass, although only very slightly for any plausible rocket, and it will not change over time while the rockets are at constant thrust.
     
  13. Feb 19, 2016 #12
    I was actually trying to get to this:
    Is a spaceship that accelerates away from an observer, warps timespace around it and thus its clock runs slower then the observer clock?
    If yes, isnt the ship, as long as it accelerates, gaining mass in the process? because i think ive read somewhere that for a ship that gets close to the speed of light its mass tends to infinity?!?
     
  14. Feb 19, 2016 #13

    Ibix

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    The spaceship does not warp space time. Time dilation is simply an effect of a disagreement over the definition of "now", which is a slightly more flexible concept in relativity than one might think from our every day experience. There is no warping going on.

    The ship does not gain mass as it accelerates. Older texts do say that "relativistic mass" increases with velocity, but no-one uses relativistic mass any more because it causes nothing but confusion. Modern pop sci presentations continue to use it because they can afford to be careless, and it sounds cool. It's not helpful. I seriously recommend that you forget that idea.
     
  15. Feb 19, 2016 #14

    pervect

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    If you track down what you read, you'll find that what you read about is "relativistic mass". Once you're familiar with this terminology, you can read the rather long (and somehwat repetitive) discussion about the issue, which can be quickly summarized sy saying that a lot of the lay population are very enthusiasitic about it, while those who have a more technical backround are not so enthusiastic about it. . You may eventually arrive at an understanding of why people are not enthusiastic about it, but that may take some time. Along the way you may well have to revisit some concepts like "energy".
     
  16. Feb 20, 2016 #15
    ty all
     
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