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Re: The old centrifugal force



It seems to me that this whole discussion is a good reason why we
should never use the phrase "centripetal force." The centripetal
component of the *net* force on an object (if the object is in
uniform circular motion, the entire net force on the object is
directed centripetally). There is no such thing as a "centripetal"
which exerts a force on an object. any centrally directed forces on
any object arise from *other* things--friction, gravity, string
tension, normal force of a track, whatever. In Ludwik's example of an
object sliding on a vertical circular track, I see exactly two forces
acting on the object, gravity, mg, and what Ludwik has sensibly
called a constraint force, C. That's it. No other forces.

The vector sum of those forces, which will change with the object's
position on the track can be broken down into two components--one
tangential to the track and one perpendicular to it, in the radial
direction. The tangential component determines how the object's speed
will vary, and the radial one will determine how its direction in
space will vary. Although mg is a constant in this problem, C is
not--it varies both in magnitude and direction, and so does the
radial component of the net force, so the centripetal part of the
acceleration in this vertical case is a variable, since the amount of
force needed to keep the object in its circular path varies with the
speed of the object.

Turn the track on its side, so the object is moving in a horizontal
plane, and then the magnitude of the net force become constant,
always pointing to the center of the circle, as long as friction can
be ignored. But calling this net force "centripetal" implies that it
is somehow "there" in addition to the other forces, and this is
simply not true. This force is only the sum of other horizontal
forces and is not a force in itself. This is true of any curvilinear
motion, but it most easily seen in the special case of uniform
circular motion.

So call it the net radial force, or the net force, or almost anything
else, but don't call it the centripetal force, because there ain't no
animal called the "centripetal."

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

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