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  1.  
    Well, here's the outrageous deal. Imagine shuttling a mass - say a rubber ball - between the front and back walls of a railcar. Ignoring frictional losses, it will continually bounce between the walls and the car will sit there oscillating between two extremes, but going nowhere. Total (cart plus ball) energy and momentum is always unchanged at all times, so there's no issue of conservation violation.

    Now we make the ball out of magic rubber and repeat the test. The magic lies in our ability to vary the mass of the ball at will, with the proviso (tossing a sop to the conservation crowd, we pretend) that its time-averaged mass remains constant. So - and here's the nifty part - we arrange the ball to be heavier when it strikes the front wall and lighter when it strikes the back wall.

    The result is radical - the cart accelerates forward down the track. This is propellantless propulsion.

    Yup, magic rubber is cool.
    •  
      CommentAuthorTrim
    • CommentTimeMar 31st 2014
     
    Lets hope it can be made.
  3.  
    Europe ripples its muscles once again
    http://indico.cern.ch/event/282344/
    • CommentAuthorjoshs
    • CommentTimeApr 1st 2014
     
    Posted By: Andrew PalfreymanWell, here's the outrageous deal. Imagine shuttling a mass - say a rubber ball - between the front and back walls of a railcar. Ignoring frictional losses, it will continually bounce between the walls and the car will sit there oscillating between two extremes, but going nowhere. Total (cart plus ball) energy and momentum is always unchanged at all times, so there's no issue of conservation violation.

    Now we make the ball out of magic rubber and repeat the test. The magic lies in our ability to vary the mass of the ball at will, with the proviso (tossing a sop to the conservation crowd, we pretend) that its time-averaged mass remains constant. So - and here's the nifty part - we arrange the ball to be heavier when it strikes the front wall and lighter when it strikes the back wall.

    The result is radical - the cart accelerates forward down the track. This is propellantless propulsion.

    Yup, magic rubber is cool.
    So all one needs is a magical pleading and they get magical results?
  5.  
    A specially-shaped set of cams and a material that is both inelastic and perfectly elastic for your buggybumpers can also be very helpful.

    http://www.youtube.com/watch?v=6IdfVIYFGlM
  6.  
    Lacking magic rubber, one can construct this in reality by replacing the ball with a small cart within the larger car. Sand is dropped into the cart prior to impact with the front wall and flushed from it prior to impact with the rear wall.
    In this case nothing is magic and momentum and energy are of course conserved.

    You might also think about splitting a sliding puck into two pieces, tossing a piece up in the air perpendicular to the motion prior to impact with the rear wall and having it rejoin with itself prior to impact with the front wall. This will not work to produce forward motion, and cannot on principle. It's a decent thought experiment though.
    • CommentAuthorjoshs
    • CommentTimeApr 1st 2014
     
    Posted By: Andrew PalfreymanLacking magic rubber, one can construct this in reality by replacing the ball with a small cart within the larger car. Sand is dropped into the cart prior to impact with the front wall and flushed from it prior to impact with the rear wall.
    In this case nothing is magic and momentum and energy are of course conserved.

    You might also think about splitting a sliding puck into two pieces, tossing a piece up in the air perpendicular to the motion prior to impact with the rear wall and having it rejoin with itself prior to impact with the front wall. This will not work to produce forward motion, and cannot on principle. It's a decent thought experiment though.
    Just to be clear:

    Apparatus anchored to the rail car drops and collects sand from a moving object within the car. The sand collisions are considered completely inelastic. To these eyes: momentum will be conserved. Kinetic energy will not be conserved. Rather than the rail car accelerating in any direction, the cart motion relative to the rail car will die off.
  8.  
    Yes, because you're imagining the wrong model. The sand is delivered from overhead hoppers which have no mechanical connection to the railcar, and the sand is simply dumped out via a slot/hole in the floor
  9.  
    http://physics.stackexchange.com/questions/105707/why-is-water-clear


    I had never put together the fact that the reason our visible spectrum is where it is is a direct consequence of the dielectric constant of water's frequency-dependent behaviour. Recall that our eyes are full of water. Doh!
    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    Posted By: Andrew PalfreymanYes, because you're imagining the wrong model. The sand is delivered from overhead hoppers which have no mechanical connection to the railcar, and the sand is simply dumped out via a slot/hole in the floor


    Posted By: Andrew PalfreymanYes, because you're imagining the wrong model. The sand is delivered from overhead hoppers which have no mechanical connection to the railcar, and the sand is simply dumped out via a slot/hole in the floor
    Then energy is still not conserved, and the inner car slows down even faster.
    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    Posted By: Andrew Palfreymanhttp://physics.stackexchange.com/questions/105707/why-is-water-clear


    I had never put together the fact that the reason our visible spectrum is where it is is a direct consequence of the dielectric constant of water's frequency-dependent behaviour. Recall that our eyes are full of water. Doh!
    This tells us that relatively speaking there is not that much water in the atmosphere.
  12.  
    Posted By: joshsThis tells us that relatively speaking there is not that much water in the atmosphere.
    How do you figure that?
    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    2um - 8um has very little loss through the atmosphere. If there were a lot of water, there would be a lot of loss.
  14.  
    Posted By: joshsThen energy is still not conserved, and the inner car slows down even faster.
    No. Energy is conserved because the railcar experiences constant acceleration. In order to be able to dump sand into it without slowing it down, work needs to be done in accelerating the hoppers on their overhead wire
  15.  
    Posted By: joshs2um - 8um has very little loss through the atmosphere. If there were a lot of water, there would be a lot of loss.
    Ah, gotcha. It also points to the fact that we came from the ocean - or at least strongly suggests it (as if we didn't already know but hey)
    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    Posted By: Andrew Palfreyman
    Posted By: joshsThen energy is still not conserved, and the inner car slows down even faster.
    No. Energy is conserved becayse the railcar experiences constant acceleration. In order to be able to dump sand into it without slowing it down, work needs to be done in accelerating the hoppers on their overhead wire
    I think you need to perform some sample calculations and come back. While technically energy must be conserved, your system loses a lot to heat in the various inelastic collisions of sand particles with: each other, the cart, and the ground outside.
  17.  
    I have, and it's called The Oxonian Cart, documented in the Files section of my Yahoo! group "spacedrives".

    My aim was to demonstrate a mundane system which proved that if mass is modulated, then propulsion is possible. Friction and other losses just muddy the waters when one's doing a proof of principle.
    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    Posted By: Andrew Palfreyman
    Posted By: joshs2um - 8um has very little loss through the atmosphere. If there were a lot of water, there would be a lot of loss.
    Ah, gotcha. It also points to the fact that we came from the ocean - or at least strongly suggests it (as if we didn't already know but hey)
    I thought the composition of blood being so close to sea water nailed that a long time ago. At least it did when I saw the cartoon about it when I was about 10 years old in grammar school.
  19.  
    Sure - hence the aqueous humor. Light had to penetrate at least the shallow waters
    • CommentAuthorjoshs
    • CommentTimeApr 2nd 2014
     
    Posted By: Andrew PalfreymanI have, and it's called The Oxonian Cart, documented in the Files section of my Yahoo! group "spacedrives"
    Then I think there is something major missing in your description. Dropping the sand inside the rail car requires that the cart surrender energy accelerating the sand. When you dump that sand outside the cart the energy consumed accelerating the sand is lost from the cart. The cart therefore slows down no matter what direction it is traveling relative to the rail car.

    I am also wondering why a scheme that requires this constant addition and dumping of mass from and to the environment is part of any discussion of propellant-less drives. Isn't the idea to be able to accelerate where you can't suck in propellant and you don't want to use stored propellant?

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