19 Jul 2003 17:55:44
B.Russell Paradox
Re: Blade Info & Commentary

On Fri, 18 Jul 2003, Troy Chinen wrote:

TC >Perhaps I should clarify that I'm speaking from a physics rather than
TC >perceptual perspective (which I assumed was the issue in question when
TC >W.L. posited that he was supporting 1.4 times his weight when at
maximum
TC >tilt angle) I don't "feel" like I am pushing down on the ground with
TC >140 lbs of force

Yep, I think what we are talking about is the "apparent weight", or the
force exerted on you by the ice (I got confused because of the mention of
the force produced or generated by the legs, which is something else)

TC >> Have a look at http://smb.slac.stanford.edu/~ana/skater.gif
TC >
TC >Your free body diagram is absolutely correct, so I am lost as to why
TC >we don't agree.

Well, I drew the diagram to make a slightly different (and I guess
irrelevant) point. Somehow I thought you meant that the force creating
the turn was the sum of the downwards vertical force and the "green"
horizontal force.
In a real case, am not entirely sure that the force I drew along the body
is indeed directed in that direction (it would be if the ice was not
level) On a flat surface, the perpendicular pressure from the surface
( what is called the "normal" force and equals mg/cos(angle) on an
inclined bank and is, I think, what you and Bill were calculating) is not
the key to producing the centripetal force, but rather sidewards
friction, or resistance from the ice.

TC >Incidently, measuring force with scales is not just
TC >imaginative--it's a bona fide scientific way of measuring the force of
TC >gravity on objects. This force of gravity is usually called "weight"

Yeah, but you were not asking to actually do the experiment (which was
kind of tricky to carry out in the practice), but rather to "imagine" the
results. On could indeed lean on a wall while standing on a balance
to find out the normal force. The sidewards push from the surface could
be found out by measuring the force on whatever you are leaning on (with
another balance?) Is the resultant force directed along your legs?

Ana
--

Maintainer of the recreational skating FAQ: http://home.pacbell.net/anamga