REwald9535@aol.com wrote:
> The crank does have a noticeable mass and interia, so wouldn't the situation
> of turning a crank by hand would be a combination of Newtonian physics and
> static/dynamic friction? (interia of trying to start 50 lbs of crank, and the
> friction of the crank in the oiled bearing).
In one way yes, but in a lot of ways no.
Turning a crank takes a bit of force. However, from my
experience the crank seems to "snap" loose and once it is
moving, it's straightforward to move.
Once the crank is moving at all, accelerating it up
to a decent speed is not difficult, which goes against
an all-newtonian explanation.
I would hazard a guess that this effect has something
to do with the surface friction on all the oiled
bearing surfaces.
Another example would be to take a crank, and coil a rope
around the timing chain sprocket (like a starting rope
on a small engine)
Start the crank turning slowly in the direction that
the rope coils up, then pull on the rope so the crank slows
down, stops, and reverses direction.
If all effects were newtonian, there would be no
feel difference at the moment the crank stopped and
reversed directions, it would be perfectly smooth.
I obviously haven't done this experiment, but I
suspect you would find a "notch" where the crank
stopped, and then started to reverse. (as long as
you did it all slowly)
I've turned a few cranks in my life, my 2 Magnettes
have crank starters. Inertia does have a role when
starting the car, because you have to get the 50
pound crank up to a good speed. However, it feels
like getting the crank moving at all needs a bit
of a grunt, and I beleive that is what most relates
to this discussion.
--
Trevor Boicey, P. Eng.
Ottawa, Canada, tboicey@brit.ca
ICQ #17432933 http://www.brit.ca/~tboicey/
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