Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

a fan of 'momentum first'



I have been a fan of 'momentum first' for a decade or so now.

We USE the momentum first sequence for the intro course that is LESS
mathematical that the AP-B course.

Relative velocities (1-D) are covered so they can 'easily' transform
between reference frames. (This comes back to haunt them in the 2nd
semester section on SR.)

The students then learn about CM of stationary and moving objects.
Extended objects - having translational and rotational motion. They
do some air track collisions MAINLY inelastic but SOME elastic ones
that can be timed reasonably well.

These are called 'sticky' and 'bouncy' collisions to avoid the
vocabulary AND to avoid my sloppy pronunciation of 'An elastic - - '
and 'inelastic - - "

They 'observe' conservation of momentum as an experimental result
(rather than 'deriving' it later from N2, impulse form.)

For the time being we also ASSUME the observed result that each
object in a bouncy collision departs at the same speed that it
APPROACHES the collision. Later this is related to conservation of KE.

They calculate the velocity of the CM and the approach velocity of
each object WRT the CM.
Write down the SEPARATION velocity of each object (WRT the CM) and
then transform to the 'Lab' frame for the final results. I don't
stress the WORDS, Lab Frame and Center of Mass Frame of Reference but
that's what they are doing.

Once we have Conservation of p AND collisions under our belts, it's a
natural step to N2 as _*I*_ read it

delta p = F Delta t

We use this as the DEFINITION of F (something that changes momentum.)

Yes we DO 'rearrange it' into the F=ma form for 'standard' problem
solving and so they 'know the jargon'.

At 9:09 AM -0500 on 4/9/02, RAUBER, JOEL wrote
>
My pet theory goes on to suggest that Newton's own enhanced
comfort level with the integrated form of the second law which
deals only with changES in velocity rather than changING velocity
probably extends to our students as well who would be far better
served by an introduction to mechanics which completely avoids the
use of the word acceleration until the connection of impulse to
change in momentum is more fully grasped.

John
----------------------------------------------------------
A. John Mallinckrodt http://www.csupomona.edu/~ajm
Professor of Physics mailto:ajm@csupomona.edu
Physics Department voice:909-869-4054
Cal Poly Pomona fax:909-869-5090
Pomona, CA 91768-4031 office:Building 8, Room 223

That's a very interesting thought. John have you ever tried to teach intro
physics with a momentum first, and its connection to impulse?

And only later, perhaps much later doing the usual acceleration kinematics
and the F=ma connection.

Joel Rauber

--

.-. .-. .-. .-. .-. .-. .-. .-. .-. .-
\ / \ / \ N / \ C / \ S / \ S / \ M / \ / \ /
`-' `-' `-' `-' `-' `-' `-' `-' `-'
Chuck Britton Education is what is left when
britton@ncssm.edu you have forgotten everything
North Carolina School of Science & Math you learned in school.
(919) 286-3366 x224 Albert Einstein, 1936