Re: [Phys-l] force conventions

["Jeffrey Schnick" <JSchnick@Anselm.Edu>]

On Friday, October 27, 2006 7:29 PM, John Denker wrote:

> This is pertinent to the recent discussion of centrifugal force,
> because in a typical centrifuge situation, the centrifugal force
> is exerted *by* the object upon the arm of the centrifuge.  This
> is why inexpert use of centrifugal-force ideas causes so much
> trouble for HS physics teachers.  The force is perfectly real,
> the students know it is real, and no amount of shouting from
> the teacher will make them forget it is real ... but it is just
> begging to be misinterpreted and misused.

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This is an interesting use of the expression "centrifugal force."  The
word "centrifugal" in this context means directed away from the center
of the circle on which the object is moving and the force of which you
speak is directed away from the center but it is not what I generally
think of as the centrifugal force.  Suppose I am in the passenger seat
of a car with a slippery bench seat and no seat belt.  The driver has
been driving along a straight path but veers leftward onto a circular
path.  I involuntarily slide over to the right side of the car where I
come into contact with the door and feel like I am pressed up against
the door.  It seems like I am experiencing a rightward force.  It seems
very real.  The experience of accelerating toward the door in the
reference frame of the car and then having the door press leftward
against me without me accelerating leftward relative to the car makes it
seem like there is a rightward force on me.  That apparent force is
pushing me toward the door.  The door is pushing on me with a contact
force that I will call a normal force.  It is a contact force.  The
reaction force to that force is a contact force exerted by me on the
door.  (At the atomic level it is an electromagnetic proton proton
repulsion force or an electromagnetic electron electron repulsion force
or a fermion degeneracy force--let's save that for another thread.) I do
see the logic in calling that force a centrifugal force; it is directed
away from the center of the circle that I am moving on; but I don't
think that the contact force that I am exerting on the door is what
physicists generally refer to as the centrifugal force.  I think that it
is the apparent force that seems to me to be pushing me toward the door
that physicists generally refer to as the centrifugal force.  This
apparent force, real or not, manifests itself only in the accelerated
reference frame.  It is a weight-like force that one experiences when
one is in an accelerated reference frame.  In the reference frame of the
road, both the contact force exerted on me by the door and the contact
force that I exert on the door are still there, but the force that I
think most physicists generally refer to as the centrifugal force is not
apparent in the reference frame of the road.

Jeff Schnick