Re: Help understanding magnetic resistance exercise bikes
- From: "hhc314@xxxxxxxxx" <hhc314@xxxxxxxxx>
- Date: Wed, 26 Dec 2007 11:03:15 -0800 (PST)
On Dec 24, 11:33 am, Carlos Moreno <cm_n...@xxxxxxxxxxxxxx> wrote:
I'm utterly puzzled with this --- I just took the cover from my
magnetic resistance exercise bike, taking the opportunity to
verify the inner workings of it.
Basically, I'm failing to visualize/understand where does the
energy that we give to the system goes.
We definitely give the mechanical system some energy, in
terms of mechanical work (we apply force to the pedals, and
there is movement in the same direction of the force). So,
there is a steady flow of energy from our legs to the bike;
where does this energy go??
I had always thought that these bikes would have a "generator"
of some sort (i.e., like a car's alternator --- converting
mechanical
movement to electricity), and then the output of this generator
could be connected to a resistor that consumes that energy;
in that case, the physics is absolutely trivial: the energy that
we give to the system is being transformed into heat in the
resistor and transferred out of the system --- therefore we have
to continue to give energy to the system.
But what I see is a rotating piece, no belts sliding on any surface
(i.e., nothing producing friction that would dissipate heat), and
what looks like magnets in an arc close to the outer circular
surface of the rotating piece --- no contact between any pieces.
If the magnetic field does apply some force in a direction non-
perpendicular to the movement, ok, there's a bit of work that
it's doing (so, the energy is going into "potential energy" form);
but then, it's a rotating piece, so there has to be some cycle
following the idea of winding and unwinding --- when the rotating
piece passes a certain point, now the magnet that was pushing
and resisting would be in a position in which it would push in the
opposite direction and *help* the rotating movement; thus
returning the energy that had accumulated. But then, this is
not consistent with something that has to apply constant
resistance (otherwise, so much for *exercise* bike!)
A little help, please?
Thanks,
Carlos
--
Carlos, good question.
I'd suggest that you Google the "Eddy Current Brake". See, for
example:
http://www.physlink.com/Education/AskExperts/ae527.cfm
Harry C.
.
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- From: Carlos Moreno
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