PDA

View Full Version : Re: Centrifugal Force



Paolo De Leva
12-20-2000, 07:36 PM
Dear subscribers,

In this message, I'll only give a hint. For the rest, please wait a few
days (assuming there's still someone interested in this neverending
discussion) because I am too busy right now.
Referring to the force measured by an accelerometer, Ton wrote:

> it might be helpful to say that some of the force is "real" (gravity)
> and some of it
> is caused by accelerating or rotating the frame
> to which the accelerometer is attached.

Forces caused by accelerating or rotating the frame?

Forces caused by accelertations?

Ton is referring to inertial forces, of course (D'Alembert's principle).
Wouldn't it be much simpler to admit they are just forces causing the
acceleration?

These forces are "real" ones,
producing the acceleration of the seismic mass (the little mass contained
in the
accelerometer). This is the only point of this message.

In the following rows, I'll try to show that inertial forces don't
exist, therefore cannot be measured by a real
device, built with real material, existing in a real world.

Consider this example: the centripetal force is real.
The centrifugal force is fictious. Shouldn't we see a centrifugal
force just as something computed changing sign to a centripetal force?
Before knowing the centrifugal force, don't we first need either:
1) to measure a centripetal force, for instance with an accelerometer, or
2) to compute it from a centripetal acceleration, observed in an inertial
frame?

A real device, built with real material, such as an accelerometer, can
only measure a centripetal force, and its reaction (which has a centrifugal
direction, but it's not usually called "centrifugal").

No real device can measure the centrifugal force, for the same reason
why it cannot *directly* measure the seismic mass weight.

This is the reason: the gravitational attraction (weight) is not exerted
by the accelerometer "body" (the "box" in which the seismic mass is
embedded), hence obviously its reaction is not applied on the accelerometer
"body".

The reaction to gravity is applied on the Earth, while the reaction to
the
centrifugal force does not exist, not even as a fictitiuos force,
because the centrifugal force is fictitious.

If you like, a fictious (apparent, inertial) force is exerted by no real
object, thus its reaction is exerted on no real object.

Was I convincing enough, this time? Any feedback from other subscribers,
besides Ton?

With kind regards, (and for Ton, friendly notwithstanding our
disagreement),


Paolo de LEVA

University Institute of Motor Sciences
Biomechanics Laboratory
P. Lauro De Bosis, 6
00194 ROME - ITALY

Telephone: (39) 06.367.33.522
FAX/AM: (39) 06.367.33.517
FAX: (39) 06.36.00.31.99

Home:

Tel./FAX/AM: (39) 06.336.10.218

---------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
---------------------------------------------------------------