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rolling nail puzzle



A colleague teaching physics in Slovenia sends me this interesting puzzle
that he's found while demonstrating magnetic effects on a conductor to his
class. It involves fixing two parallel conducting "rails" to the table, and
placing a piece of brass pipe across them. A battery is then used to send a
large current down one rail, through the brass pipe, and back up the other
rail. He reports that the pipe always rolls away from the battery,
regardless of direction of the current, as use of the fingers of my right
and left hands convinces me should be the case.

Then he says, somebody put a large iron nail in place of the brass pipe. To
his surprise, the nail rolled the other way! In fact, he says the nail
always moves towards the battery, regardless of direction of current.

He sent me a picture of the setup, which I've placed here:
http://popeye.uwcad.it/%7Env/shared_stuff/nail.JPG

I have a vague hypothesis that it's something to do with rolling behaviour
- that the current in the nail is concentrated along the bottom - perhaps
because of lower conductivity of iron - and this results in a pull to the
right at the bottom causing rolling to the left: the bottom skin of the
nail is always accelerating in the direction of the force acting while the
nail rolls in the opposite direction. Can't quite square this with impulse
and momentum change, however.

Any thoughts?

Mark



Mark Sylvester
UWCAd
Duino Trieste Italy