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Re: apparent weight



William Beaty wrote:

More ingredients for the stone soup:

I was confused about this before I realized that there are two
"centrifugal forces," and only one of them is real.

One is a pseudoforce "caused" by the non-inertial frame. For example, if
I ride inside a rotating cylinder and toss a ball around, the ball's
trajectory will be very strange. It will be influenced by the centrifugal
pseudoforce. Note that this pseudoforce DOES NOT fling the ball outwards.
It can distort the ball's trajectory until it looks like something drawn
by a spirograph. It has coriolus components and fugal/petal components,
and does not obey Newton's 3rd.

As I ride inside the rotating cylinder, my back interacts with the inner
surface of the cylinder. The cylinder pushes me inwards, and I push
outwards on the cylinder. It's a simple Newtonian force pair. If I wish
to concentrate only on the centripetal component of this force, while
saying that the centrifugal component is unreal, then I VIOLATE NEWTON'S
THIRD LAW. Does any physics instructor dare to claim that a surface can
"act upon" an object, but that an object cannot "act upon" a surface? In
an accelerating elevator, is it proper to say that the elevator's floor
applies a force to the rider, but that the rider does not apply a force to
the elevator's floor?


Howdy,

The force the surface applies on your back is NOT a centripital force. It
is simply the contact force (Normal force) and there is a standard
"reaction" force ON the surface (i.e.,. not on you) of equal magnitude and
opposite direction.

There is no such thing as a centripital FORCE: m*v*v/r is really the ma
side of F=ma not the F side. What does exist is centripital acceleration,
v*v/r, towards the center of the rotation. The "centrifugal force" is
simply -ma when writing Newton's second law as F-ma=0 since you, in your
accelerated reference frame, appear to be in equilibrium. Two ways of
looking at the same problem!

I often ask my students what force keeps the astronauts in orbit around the
earth. The answer is NOT the centripital force, it's the gravitational
force the earth exerts on the astronauts.

Uh oh, I think I just started things up again.

Good Luck,


Herb Schulz
(herbs@interaccess.com)