Re: Perpetual motion!

On Fri, 01 Feb 2008 16:53:29 -0600, Ben C <spamspam@xxxxxxxxx> wrote:

On 2008-02-01, carlfogel@xxxxxxxxxxx <carlfogel@xxxxxxxxxxx> wrote:
On Fri, 1 Feb 2008 03:27:17 -0800 (PST),
"joseph.santaniello@xxxxxxxxx" <joseph.santaniello@xxxxxxxxx> wrote:

Hi All,

Suppose a Rube Goldberg bicycle of sorts was constructed with a gizmo
that raised some weights when the freewheel was freewheeling (down a
hill), and allowed these weight to lower when pedaling, using this
potential energy to assist in pedalling.

Would this contraption be faster over a hilly course with the system
engaged vs disengaged and just along for the ride?

Joseph

Dear Joseph,

Forget the weights--the more energy they store, the heavier they are
and the harder it will be for you to reach the top of the hill and
find out that they don't work.

You can in theory store a lot of energy in light weights by lifting them
very high.

Of course only in theory.

Just imagine storing force by winding a spring up by some brake gizmo
on the downhill. Or charging a battery.

Overall, you still lose.

You can't get more energy out than you put into closed system. You
can't even get as much energy out as you put in.

It's not a closed system though. You're losing energy to the wind. If
you can keep your speed more constant (so slower down and faster up
hills) you can have a higher average speed over the course for a given
maximum power and energy use. It's just like flattening the course out.

And then if you have to brake anyway for some reason, regenerative
braking is definitely a good idea in principle.

Dear Ben,

Try to give some figures illustrating how you're going to store and
release energy on a bicycle so that the same power from the bicyclist
produces a faster time overall.

Let's say that you start at the top of a 10-mile hill and descend at
30 mph in 20 minutes. Then you pedal back up the same 10-mile hill at
10 mph, which takes 60 minutes.

Total time, 80 minutes.

Figure out how many watts you're going to store going downhill and how
much that _must_ slow you down, and you can figure out how many
watt-minutes of energy you'll have.

Then spread that energy out over the uphill climb and see whether it
lets you reach the top in less than 80 minutes.

Most perpetual motion schemes involve forgetting the times involved or
the absolute speed gains over time rather than the more dramatic
percentage speed gain with time ignored.

Cheers,

Carl Fogel
.

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