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Re: CONSERVATION OF ENERGY, work, sound, EM



William Beatty wrote [excerpt]

When an auto engine drives the tires, the work done by the engine is
somehow instantly conducted along the drive shaft. Yet all I see is a
turning shaft. By looking only at the spinning shaft, I cannot tell if
the engine is sending "work" to the tires, or the opposite: if the engine
is turned off and the car is coasting and is sending "work" from the tires
to the engine. In both cases the shaft just spins.

Ah, but what if I SUDDENLY turn the crank? Or what if the engine SUDDENLY
starts up with a jerk? Then (if I have fast eyes) I will see one end of
the shaft twist a bit while the rest stays still. A wave of "twisting"
will race down the shaft and the far end will finally begin turning. So,
the shaft doesn't simply turn, there also is a wave of "something" that
moves from one end to the other. The stuff that moves is wavelike, but I
can only see it when the speed of the engine suddenly changes.

I may regret jumping into this debate at this point, and on a small point
at that, but here goes.

William said that you can't tell that work is being done by looking at the
shaft, unless there is a change in the driving torque. Might I suggest that
a very slight modification to the situation described will allow us to see
that work is being done and even which end is providing the driving torque.
If w strip is painted along the shaft, then if the shaft is doing work as
it rotates, the stripe will be twisted into a segment of a helix along the
shaft (of course one has to have seen the shaft when it is not turning to
see that the stripe is along the shaft and not already helical. Further
more, if my spatial visualization is correct (a very shaky position), I
think the sense of the helix will be different depending on which end of
the shaft is being driven, even if the rotation is in the same sense as
seen by a observer who doesn't change position when the driving end is
reversed.

Furthermore, there will be energy stored in the shaft while it is rotating
under these conditions. And I am totally unaware of the sginificance, if
any, of this stored energy. This issue is not just academic, however. The
propellor shaft of a large ship may be 100 meters or so long, and if the
ship is steaming at high RPM, the twist on the shaft can be as much as 1 to
2 revolutions.

I'm not sure this contributes anything to the discussion, except to point
out that it is possible to determine that work is being done under steady
state conditions (of course, as long as there is some resistance to the
rotation of the driven end, as there always will be in the real world). We
don't absolutely have to wait for a change to see what is going on.

Hugh


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Hugh Haskell

<mailto://hhaskell@mindspring.com>

The box said "Requires Windows 95 or better." So I bought a Macintosh.
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