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Railgun and the laws of physics

  1. Apr 30, 2015 #1
    I just made a rail gun using two ring stand poles, a steel ball, and a car battery. Maximum velocity is about 5 cm/sec.

    What is strange is that it is shooting the wrong way. Defying the laws of physics and right hand rule. Any ideas ?
  2. jcsd
  3. Apr 30, 2015 #2
    How do we know that is "defying" the laws?
    You don't bother to give any details.
  4. Apr 30, 2015 #3


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    Staff: Mentor

    Usually this happens because you've made an odd number of sign errors. Professional scientists and engineers have gone through years of formal training to ensure that they know to always make an even number of sign errors. (Note that making zero errors is a special case of making an even number of errors, and is generally considered to represent an unrealistic and unnecessary ideal).

    OK, kidding, aside..... You'll have to tell us a lot more about your setup before we'll be able to tell you why your getting the result you're observing.
  5. Apr 30, 2015 #4


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    5cm/sec ?
  6. Apr 30, 2015 #5
    I would definitely like to see that setup. The only thing I can imagine is that you put the ring poles in parallel to create the rail. But what creates the force on the ball?

    Also, yeah, 5cm/s? That's 0.18km/h. Are you sure your rail isn't just not completely level and the ball just rolls due to gravity?
  7. Apr 30, 2015 #6
    So, its pretty level. The ball will not roll without a current applied. If I give it a push either direction, it rolls about the same distance before stopping, so its reasonably level.

    So, the right hand rule says the force should be away from me. I am at the end of the two rails that is attached to the battery, so:
    • The current goes out the right rail (mag field up between rails).
    • The current comes back on the left rail. (mag field up between rails)
    • Current through steel ball is from right rail to left rail.
    • Right Hand Rule (Current from right to left through ball, plus mag field up = force away from me)
    Switching the direction of current in the whole system should not have any effect, as both mag field and current through ball change directions, so force should still be away from me.

    Perhaps I have created a left handed rail gun.....

    Anyway, if I put a small wire across the two rails, the wire moves away from me as would be expected. I am looking at torques on the the ball....
  8. Apr 30, 2015 #7


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    Staff: Mentor

    What is limiting the current from the car battery? You're not shorting it out, are you? That can be pretty dangerous...
  9. Apr 30, 2015 #8
    I think that a force acting away from you can result in the ball rolling towards you.
  10. Apr 30, 2015 #9
    The cause for that above behavior is very different though. If the spool didn't have that footstand to keep it a centimeter off the paper, the effect wouldn't happen.
  11. Apr 30, 2015 #10
    I am not saying that this is the case here. I have no idea of the geometry or about where the electromagnetic force acts, relative to the supporting surfaces.
    Just a possibility.
    I was thinking about a situation with the resultant of the electromagnetic force acting somewhere below the line of contact between the ball and the supporting edges. If there are any edges.
  12. Apr 30, 2015 #11
    I think I have it figured out. The force is, in fact, in the expected direction, or away from me. But, because the ball is between two rails, the axis of rotation is at the elevation of the points of contact between the ball and the rails. Thus, the force, away from me, is acting below the axis of rotation and creating a torque that spins the bottom of the ball away from me and the ball rolls toward me. This is different than the string on the spool demonstration, where the force is above the axis of rotation.

    To test this theory out, we placed two tables 6 inches apart, and placed a bowling ball in the gap. A piece of tape attached onto the bottom of the ball and pulled toward us horizontally did, in fact, roll the ball away from us.
  13. Apr 30, 2015 #12
    Also, we think the majority of the current must be conducting over the bottom surface of the ball, and thus, this is where the Lorentz force is acting.

    I am just glad I did not accidentally create the first "left handed railgun" ever.
  14. Sep 26, 2015 #13
    Mightn't you be dealing with the Faraday cage effect since you are using a metal ball?
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