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Re: transfer of momentum



1) What is gained by replacing the concept of static
force (see my original message below) by the
concept of "flow of momentum?" When we say
"force" students intuitively think about a push or
pull of some kind. That is a good beginning. Then
we define work in term of force; then we lean on
the concept of work to introduce three kinds of
mechanical energy, etc. etc.

2) What is lost by replacing the concept of force by
the concept of "flow of momentum?" Clarity.

a) Momentum is m*v; in a static situation each
part of a system is at rest (yes, in the lab frame,
and macroscopically).

b) Fluids can flow. What does the "flow of
momentum" mean in the context of an
introductory physics course?

c) Why should a static force experienced by a
person on a floor (it has the same magnitude
as m*g) be replaced by the "flow of momentum."
Is this concept less mysterious that the concept of
force? In which direction is the momentum flowing?

Radical ways of teaching should first be tried as
simulations; for example, by writing a physics
textbook based on new ideas. Then in very small
experimental classes. Throwing away the concept
of force is dangerous; we might replace it by a new
concept and be very sorry later. I would strongly resist
if somebody tried to force me to implement a radical
change, such as replacing F by dp/dt in dealing with
static situations. I would have to see some simulations
(textbooks), I would have to hear from teachers who
experimented with new ideas on a small scale.
Ludwik Kowalski

On Tuesday, Nov 18, 2003, at 06:46 US/Pacific, Chuck Britton wrote:

Here is MY take on John's Force = Flow of momentum. (reafication???)

Each of the three strings is providing the same flow of momentum
whether there is equilibrium or NOT.

In equilibrium we require that the (vector) summ of these flow add up
to zero.
Just like requiring the three Forces to add up to zero.

Force is identical to momentum flow and vice-versa.

At 7:38 AM -0800 11/18/03, Ludwik Kowalski wrote:

A ring resting on a horizontal table is pulled radially
(away from the center) by three strings. The other
sides of stings are attached to suspended masses
(via pulleys). In other words, it is a classical force
table activity on static equilibrium. Who is transferring
momentum to whom?