🚀 go-pugleaf

John M. Aldrich wrote:
> 
> Aden Grue wrote:
> >
> > This isn't really on topic (then again, this thread isn't on topic), but
> > I have an idea.  The Earth spins (I know that much).  If spin produces
> > centrifugal force, wouldn't we be pulled slightly away from the center
> > of the Earth?  And wouldn't a person at the equator weight less than the
> > same person at the north/south pole?  At first I thought that this would
> > be true, but then I thought about gravity.  If gravity curves space, the
> > Earth's gravity might curve space so that we aren't technically
> > spinning, just travelling in a straight line.  Therefore, there would be
> > no centrifugal force.  Can anyone verify my conclusion?
> 
> Bzzzzt!  Go back to your 6th grade science course!  :)  Centrifugal
> force does not really exist; it's just a manifestation of inertia.
> Remember the old experiment where you swing a ball on a string in a
> circle around your head?  The tendency of the ball is to travel in a
> straight line (inertia), but this tendency is opposed by the inward
> force of the string that is holding it (gravity).  Since the two are
> equal, the ball maintains the same distance from the center of the
> circle.
> 
> The original point was correct:  it is gravity that produces
> acceleration.  If that gravity were to suddenly be "turned off", you
> would continue to move in the same direction you were going at the
> time:  parallel to the earth's surface.  Thus, everybody on the planet
> would be instantly flung off into space.  :)
> 

Not the two people standing on the north and south poles. Since their
distance from the centre of rotation is zero, their angular momentum
is zero, and their speed, should gravity be 'turned off' would also be
zero. They might get rather dizzy though.

So, we have two extremes: a man standing at the pole who remains (just
about) in contact with the ice, and a man standing at the equator
flung, as you say, into space when the force of gravity is removed.

I suggest you are mixing up two concepts. The earth's spin does not
depend on or affect gravity. Yes, the gravitational effects of a
perfectly spherical, uniformly dense earth on other objects, such as
people, will be approximately the same. The fact the man on the
equator has angular momentum means that he tends in a straight line,
thus 'reducing' the otherwise equal gravitational effects, and he
would therefore be slightly lighter, except that to measure his weight
(not his mass), the weighing machine would have to be in the same
predicament. A weighing machine at the equator would read differently
to one at the pole, so that they again show the same weight (bearing
out the statement that gravity and spin are not connected).


Fun topic though, but...


> Inertia is the tendency of a mass to resist force applied to it.
> However, it does not itself constitute any kind of force or
> acceleration.  Centrifugal force is just a manifestation of inertia.
> 
> --

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