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Re: [Phys-L] 9th grade energy resource projects



On 11/26/2014 08:09 PM, Anthony Lapinski wrote:

I've always wanted to build the bicycle generator demo, where the faster
you pedal, the more lights turn on. So this relates mechanical work/energy
to electrical energy.

Keep in mind that such a thing would be at best a stunt,
and has the potential to mislead students (and visitors
et cetera). That's because a bicycle generator is not
a particularly practical way to generate electricity
... and also not a particularly accurate way to measure
torque, cadence, and/or power. A skeptical analysis
can be found here:
http://www.lowtechmagazine.com/2011/05/bike-powered-electricity-generators.html

Are there kits/parts for these online? the generator? the bulb hookup
apparatus?

There's stuff available, including do-it-yourself plans
as well as almost-complete turnkey systems. Again, keep
in mind that this stuff tends to be expensive and not
very practical. If you really wanted pedal-generated
electricity, you'd end up throwing away about 80% of
the original bike parts and using other stuff instead.

I prefer to focus attention on things that are relevant
to the real world. See below.

===============

It is not at all guaranteed that "the faster you pedal"
the more power you put out. Basic physics says power
depends on torque as well as cadence. Because of
muscle physiology, if the cadence is too high the
torque drops off dramatically.

It's even more complicated than that, because there is a
cadence for max efficiency and another cadence for max
power. The latter is quite a bit higher. For a long
road-race you want max efficiency, whereas for a sprint
race you want max power. Professional racers study this
in detail. They don't necessarily publish their results,
but there are "some" publications out there.
https://www.google.com/search?q=bike+cadence+optimum
https://www.google.com/search?q=bike+cadence+optimum+sprint+efficiency

This is 100% relevant to real world sports physics and
physiology.

I definitely do not believe everything I read on this
subject. There is probably room for original research
in this area. Beware that this is starting to look
like experiments on human subjects, and you might want
to get it reviewed and approved.

9th graders should be able to map out power as a function
of torque and cadence. This would be a big job by 9th
grade standards, but highly instructive. There are
dozens and dozens of interesting lessons bundled in
this project.

Cadence sensors are cheap, widely available, and easy
to install. I have been happy with the Cateye brand;
the Nashbar house brand has an excessively slow response
time; no comment on other brands. As for torque, I'm
not entirely sure what is the best way to measure that:

1) If you have a suitable hill available, speed up the
hill gives a rather direct measure of power. If the
hill is steep enough, aerodynamic drag is only a small
correction term.

2) If you don't have a hill, there are all sorts of plans
out there for building a stationary bicycle dynamometer:
https://www.google.com/search?q=bicycle+dynamometer

3) If I wanted to measure torque, without a hill, I would
prefer a non-stationary scheme, so I could still ride
around. On-board torque sensors are widely used on
e-bikes, typically integrated inside the bottom bracket.
Probably the easiest way to get started would be to get
an e-bike, disconnect the motor, keep the torque sensor,
and add a cadence sensor.

Mapping /efficiency/ is probably out of reach for 9th
graders, since that would involve measuring metabolism,
presumably via VO2 and VCO2. That's certainly doable,
but it would be a lot more work and a lot less fun.