1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Why wouldn't this space propulsion work?

  1. Oct 2, 2015 #1
    I'm pretty sure someone would have already made this if it did work, but I don't know why it wouldn't. Can someone explain why this spacecraft couldn't move forward?

    Attached Files:

  2. jcsd
  3. Oct 2, 2015 #2


    User Avatar
    Gold Member

    Can you explain why you think it would?

    How much work is done to get the ball to the far end of the tube?
    How much work is done to bring it back to its starting point?
    (Note, btw, that you have not actually completed a full cycle back to the starting state in your diagram.)
  4. Oct 2, 2015 #3
    There is acceleration when the weights are first repelled by the elecromagnet and the weights would not lose all their momentum before they reached the other electromagnet on the other end of the tube, meaning that less force would be burned in the other direction that the force that was created by the original "push".
  5. Oct 2, 2015 #4


    User Avatar
    Gold Member

    The ball, by contacting the bend, will exert a rearward force. This makes up the "missing" force in your diagram.
  6. Oct 2, 2015 #5


    User Avatar
    Gold Member

    Note that your device is technically a perpetual motion device, which is a forbidden topic here on PF.
    It's all right to ask why the design won't work (and there are several PPM principals that describe why), but be warned, this thread probably won't remain open long.
  7. Oct 2, 2015 #6


    Staff: Mentor

    Closed pending moderation.

    This isn't technically a perpetual motion machine. As described it violates conservation of momentum rather than conservation of energy. However it is close enough that we need to moderate.

    EDIT: the thread will remain closed. Dave has identified the reason that it won't work. Working out the details is a good exercise for personal study, but not a good topic for PF.
    Last edited: Oct 2, 2015
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook