Re: Centrifugal force

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

John Clement wrote:

> There is an agent for the gravitational force, namely the Earth.
> The gravitational force is an interaction between the earth and
> other objects.  In the case of the centrifugal pseudoforce there is
> no agent
> in the interaction.

One minor point: In the case of Newtonian gravity, that "agent"
somehow manages to act instantaneously regardless of distance.  This
isn't likely to bother students in any significant way, but after
studying physics for awhile it eventually dawns on people to take
seriously the idea that the real "agent" is, in fact, not a distant
massive body, but the *local* "gravitational field."  At that point,
as John Denker has repeatedly noted, it makes more sense to consider
gravitational forces and centrifugal forces (and, indeed, all
so-called "inertial forces") as being associated directly with a
frame-dependent acceleration field.

> When you say that students know there is a centrifugal force, they can not
> attribute it to an agent.  This is the BIG problem.  Students must first
> learn to attribute all forces to interactions before they can learn to
> properly analyze more complex situations.

Agreed.  That is, they should first learn to work exclusively in
"inertial frames" or, what I would call more precisely
"pseudoinertial frames" in which we DO allow "Newtonian gravitational
forces" even though we NOW know (and have for nearly 100 years) that
the existence of a gravitational field is conclusive evidence that we
are NOT working in a true inertial reference frame.

> Essentially from the point of view of inertial non rotating frames the
> centrifugal force doesn't exist, so we can deny its existence to the students.

But we would be dead wrong in doing so.  There is no need to lie to
students; we can merely point out that centrifugal forces are things
that arise in rotating reference frames and that we will not USE
rotating reference frames or accelerating reference frames until we
have learned how to apply Newton's laws in nonrotating,
nonaccelerating reference frames.

>   I would however, not say that it never exists to a student, but
> rather that if they can't identify the agent of the interaction, then the
> force does not exist.

... as long as they stick to working in our so-called "inertial
reference frames."

I agree with the rest of John's remarks on pedagogy.  IMO anyone who
thinks beginning students are not gravely tempted by ideas like
"centripetal (or centrifugal) forces arise when an object moves in a
circle" or "normal forces appear because, well, they are just ...
normal" probably isn't paying close enough attention.

--
John Mallinckrodt        mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm