[pct-l] Gravity

[pbaily at webuniverse.net (Phil Baily)]

In your discussion, you assume that the earth is a uniform density sphere,
and that is fine. Continuing with that assumption, yes, a car would roll to
the center if the road were really flat. On one side of the center point
(the point at which the road is tangent to the earth's surface) it would
roll in the direction toward the center point and you would feel like you
were running uphill going away from the center, but going toward the center
you would feel like you were running downhill. You would not feel like you
were running uphill in both directions. The same can be said on the other
side of the center point but the absolute direction of travel would be
opposite. If you started at either end of the road and ran all the way you
would feel like you were going downhill and then uphill. Of course, the road
would have to be very long for you to feel this effect due to the earth
being spherical. (I doubt if even the "flat" stretch of the PCT through the
Mojave is long enough. (Keeping the discussion relevant to this forum.)) I
don't really understand your sensations on Leif Erickson Drive, but my own
feelings while running on flat (or nearly flat) surfaces are strongly
influenced by what I have run just prior and by my degree of fatigue. BTW, a
laser beam would travel in a straight line independent of gravity within
your ability to measure it and therefore could be used to measure flatness.

Phil

----- Original Message ----- 
From: "Wayne Kraft" <wayneskraft@comcast.net>
To: "Pct-L pctl" <pct-l@mailman.backcountry.net>
Sent: Thursday, December 08, 2005 6:23 PM
Subject: [pct-l] Gravity


> OK, let's see if I can put this thought into words.  Gravity doesn't
> exactly operate to pull you down hill.  It operates to pull you
> toward the center of the earth.  A car rolls down a real hill because
> it's contact with the earth prevents it from falling straight down.
> If a we refer to a road that follows the curvature of the earth as
> "flat" then presumably a car stopped on this road would remain
> motionless even with the brake off.  However, if the road is REALLY
> flat in absolute terms (that is, it doesn't follow the earth's
> curvature, but is level in each direction from the center point)
> would not a car roll toward the center from either end of the level
> road?  It would seem like the car would be attracted to the point
> closest to the center of the earth.  Or would such a road appear to
> us to be curved or uphill in both directions?  Is there even a way to
> measure or conceptualize absolute level independent of earth's curve?
>
> List physicists, please address this question ASAP.  There is a
> trail in Portland here called Leif Ericksen Drive (actually it is
> kind of an abandoned dirt road) that is about 11 miles long.
> Sometimes I run it out and back for 22 miles.  There's about three
> miles in the middle that I swear is uphill both directions.  It has
> caused me to seriously question my sanity. I need to have an answer
> to this before I head back out there.
>
> Wayne Kraft
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