A "simple" see saw question

[Richard Brekne]

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Farrell wrote:
Professor Fowler's comments make complete sense to me. But I do not
understand some things that your folks state.

My folks ?? who and what are you talking about exactly ? I only asked
the question of the fellow... and didnt state anything.

If you then push down on one end and let go the see saw will rock back
and forth until slowly it refinds its state of equilibruim." Why would
it rock back and forth? I don't think it would. If you push down on one
end, it will continue with the rotational movement you imparted until
either friction stopped it, or until one end hit the ground perhaps. If,
as the professor says, the seesaw has the axle right through the center
of gravity of the plank, it would always be in equilibrium, and have no
need to "seek" some state of equilibrium.

Tell you what Terry... you try to build the see saw that doesnt  behave
as in the question I posed. I'm not saying you cant do it... but its not
quite as straight forward as it may seem. Do a simple experiment....
take a meter stick and bore a hole in the middle... and balance it on
the that bearing swivel thats included in the Stanwood kit. When you
have it perfectly balanced horizontally... pull it downwards and hold it
at 45 degrees for a sec.... then just let it go and see what happens.
 "I understand that friction is the thing that brings it back into
equilibruim, ..." It will always be in equilibrium, friction will only
serve to slow the rotational movement.

Well, why does the thing start rotating in the first place then.... if
you have it stopped at 45 ¤... and just let it go... why doesnt it just
stay there ?... Why does it always rock back and forth until it finds
its origional state of equilibrium.. in this case horizontal. "...but
why the rotational movement to begin with exactly?" Because a seesaw is
constrained in that manner, only allowing rotational movement. Or am I
missing something here?

The question wasnt why rotational visa vi translational.. it was why any
acceleration at all ?
 "Science answer: Gravity applied to the see saw,  gravity applied to
the weight, force/counter force etc." I don't think gravity really has
much net effect in this situation - other than to keep things from
flying away from us.

Hmm... gravity exerts the downward pulling force equally on both sides
of the see saw. Thats one of the forces working on this seemingly so
simple system. "Ya think ?? Funny how a phsyics professor seemed to miss
this..." Don't understand your thinking here at all.

I was being fasicious with Brian there Terry... :) I thought the
Professors response was interesting because his was the only one I got
back from about 15 different such folks that put his finger on the
center of gravity thing.
 Terry Farrell

--
Richard Brekne
RPT, N.P.T.F.
UiB, Bergen, Norway
mailto:rbrekne@broadpark.no
http://home.broadpark.no/~rbrekne/ricmain.html


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