From: cam@castle.ed.ac.uk (Chris Malcolm)
Newsgroups: rec.motorcycles
Subject: Re: Centripetal !!
Date: 9 Sep 92 22:04:56 GMT
Organization: Edinburgh University

In article <1992Sep9.103721.418@cho006.cho.ge.com> wright_t@cho006.cho.ge.com writes:

>I've really gotten into some serious conversation
>when trying to convince some folks that there ISN'T any such thing as
>'Centrifugal' force. 'Centrifugal' is only a word used to label that
>feeling of being 'thrown' outward such as in a vehicle whose course has 
>changed direction. How or where the word came into use is beyond me. The
>correct term to use when describing 'that outward feeling' is 'Centripetal'
>which is the name for the force that is actually acting _inward_! When my
>physics professor told us about it, it really changed my whole world.

It is in general impossible for a person experiencing a constant force
in a particular direction to tell by _local_ observations whether that
force is due to gravity or to acceleration. "Local observation" means
observations within the same frame of reference, e.g., not looking
outside. For example, if you were in a lift accelerating upwards, you
couldn't tell whether the increased downwards force was due to
acceleration or gravity. This is a basic tenet of relativity theory.

Now consider being in a car running in a circle. It'a natural inertial
tendency is to continue in a straight line. What moves it out of the
straight line is the centripetal force exerted by the road on the
tyres.  The driver seems to feel a force throwing him/her outwards
from the centre. This apparent force could also be demonstrated and
measured by a spring balance. You are quite right that in fact there
is no such force as this apparent centrifugal force -- it is the
consequence of the constant centripetal acceleration. On the other
hand, it is impossible to decide from measurements taken from inside
the car that this apparent force is due to centripetal acceleration.
It could indistinguishably be due to the gravitational attraction of,
say, a nearby small black hole, acting on a stationary vehicle. It
would _feel_ and _measure_ just the same. And in this latter case
there _is_ a centrifugal force, in the literal sense of a real force
acting to drag you away from that central point.

In other words, centrifugal force is a perfectly good name for the
feeling of being thrown outwards on a curve. Why? Because it is
exactly the same feeling, and measures exactly the same way, as would
a _real_ force, such as gravity. If you really want to be picky and
say that in fact there is no such thing as centrifugal force, then you
should be prepared to take your argument to its logical conclusion,
that there is also no such thing as the force of gravity either -- as
Einstein pointed out.

But it's really convenient for us earthbound biking physicists to talk
about the force of gravity, and doing so involves us in no
contradictions or false predictions. Similarly it is really convenient
to talk about centrifugal force. That also involves no contradictions
or false predictions. Arguing that we shouldn't use such convenient
and perfectly well-behaved fictions is as silly as arguing that there
is _really_ no such thing as heat: true, but the context of that
"really" is irrelevant to the world of central heating and cookery.

You'll be telling me next that I don't really weigh 130 pounds, there
is no such thing as weight, what is really happening is that I'm being
gravitationally accelerated by 4160 poundals!
-- 
Chris Malcolm    cam@uk.ac.ed.aifh          +44 (0)31 650 3085
Department of Artificial Intelligence,    Edinburgh University
5 Forrest Hill, Edinburgh, EH1 2QL, UK                DoD #205
Newsgroups: rec.motorcycles