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Mass dialation for space drive

  1. Jan 21, 2013 #1
    Question to the physicists in here:

    Would it be possible to spin a metal disk such that it increases mass along the outer rim(as all objects increase mass up to infinite mass near the speed of light)? Then oscillate the disk in a tube such that it is spinning while it travels in one direction(having increased mass), stationary as it travels in the other direction(having decreased mass), so that no reaction mass is lost as in a rocket.

    In theory this would negate the conservation of momentum as the disk while spinning has MORE mass than the disk while resting. The disk while spinning would only travel in one direction while the resting disk would travel in the other during it's oscillation in a tube.

    Obviously the type of material, maximum rpm for the material, and relativity equations would apply.

    Hope to hear from someone!

    Last edited: Jan 21, 2013
  2. jcsd
  3. Jan 21, 2013 #2
    Opposite and equal reactions, remember! How will you make your disc spin in one direction without making the rocket spin in the other with the same angular momentum?

    As you then push the heavier spinning mass in one direction, the heavier spinning rocket in turn will move in the other direction. The centre of gravity will not move.

  4. Jan 21, 2013 #3
    I think that problem would be easily solved by having two discs of equal mass spinning in opposite directions--and moving them both up and down the tube together. But, I have no idea if this plan would work or not because of some more fundamental reason. Good thinking, though!
  5. Jan 21, 2013 #4


    Staff: Mentor

    This should be a big red flag. If you think you've violated a conservation law, it means you've left something out.

    In this case, as Mike Holland pointed out, you're leaving out how the disk gets spun up at one end and then spun down at the other. It has to exchange energy and angular momentum with *something* to do that. Include that in your scenario and you will see that overall momentum is conserved.

    cephron's idea of having two disks spinning in opposite directions would take care of the angular momentum part (the net angular momentum of the two disks would be zero), but not the energy part: it takes energy to spin up the disks, and they have to give up energy to spin down. Where does that energy come from, and where does it go? Again, when you include that you'll find that the conservation laws hold up just fine. There ain't no such thing as a free lunch; sorry. :wink:
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