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Re: Newton's 3rd law? was Re: inertial forces (definition)



At 8:26 PM -0700 10/20/99, Chuck Britton wrote:

At 7:40 PM -0700 10/20/99, Leigh Palmer wrote:

the force that acts on a body in an
Earth based lab, that I have been calling "weight", doesn't have a
third law partner. The gravitational force the body exerts on the
Earth isn't quite the same magnitude and it doesn't act in exactly in
the opposite direction. Why tell your students it does?

so we shouldn't be using an analogy between the 'force' 'caused' by a
gravitational field acting on a mass and the electrostatic force on a
charge Q in an electric field E?

does this 'force' have the same pedagogical problem in your view?

Further to my earlier reply to this item, informed by Chuck's
later comment on conservation of momentum:

I must ask, how do you account for the momentum component in the
gravitational field in the laboratory? I know of no term used in
elementary physics which does this. I am thinking of a term
analogous to the energy term mgh. It is possible to use the law
of conservation of energy in the Earthbound laboratory by
adopting this (do you call it fictitious?) energy term in the
reckoning. I don't recall hearing about a momentum counterpart.
Instead we usually apply conservation of momentum only over the
brief interval from "just before the collision" to "just after
the collision", not a very satisfactory situation in my view.
Why not invent a "gravitational potential impulse" account by
virtue of which a particle builds up credit (m*g*delta t in the
downward direction) in a momentum account by merely existing for
an interval of time?

I'm not advocating the introduction of this fiction into the
curriculum; I'm merely pointing out that something must be done
to save the appearances of the law of conservation of momentum
if one is to use it in a noninertial frame such as the Earthbound
laboratory over a finite time interval. Contrary to John Denker's
vehement protestations, tables, chairs, and even floors are
needed in Earthbound laboratories to provide the necessary force
to maintain a state of zero vertical momentum in many objects.

I will point out in passing that if you use the mgh potential
energy term in your teaching (I do), then the value of g you use
is properly *not* the GM/R^2 value. It *must* include the
centrifugal acceleration to make conservation of energy work in
an Earthbound laboratory. I hope that thought doesn't spoil it
for anyone.

Leigh