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Re: [Phys-l] momentum dissipation?



This obviously went to the wrong list serve -

My apologies to Phys-L readers.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of LaMontagne, Bob
Sent: Monday, April 26, 2010 4:36 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] momentum dissipation?

As someone who rides trains often (I'm a railfan and ride trains and hang around stations) I'm somewhat puzzled by the whole idea of a train that never stops. It seems to involve a lot of development effort and funds on a project that's not really needed at this time. There is plenty of time between trains arriving at stations (even on the busy Northeast Corridor where I am.) The capacity for running more trainsets has not been anywhere near exhausted. The limit right now is set by ridership. Most trains that I ride are basically empty - especially commuter trains in the non-rush times. Amtrak has certain trains that are booked and require reservations (like the Acela) but others are basically just walk-on. Trains are really servicing their current audience pretty well. Adding more trains would probably reduce overall efficiency at the moment.

Mag-Lev, bullet trains, never-stopping trains, etc., are all neat ideas but just don't warrant the development costs because of the current demands for train service. One can make all kinds of arguments as to why people should ride trains instead of driving cars, but the simple proof of why those arguments are wrong-headed is that in the real world people stick with their cars - except for those commuting situations where a train is actually (not arguably) better. Traffic jams are inconvenient, but that is countered by the flexibility that most people obtain from the automobile.

I love to ride trains, but for serious travel I use automobiles and aircraft. I would love for the US to be running so many trains that I could just hop on anytime and go anywhere I want - but I'm not willing to pay the taxes necessary to keep such a white elephant running.

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Clement
Sent: Monday, April 26, 2010 1:49 PM
To: 'Forum for Physics Educators'
Subject: Re: [Phys-l] momentum dissipation?

Let me first comment on the statement

Momentum should be covered in it's entirety before acceleration comes
up (except for a fancy word that means 'Slope of a Velocity Graph').

The way I see it, students should be able to work out any dynamics
problem using Newton's 2nd, Newton's 3rd and the definitions of velocity
and acceleration before they are ever even exposed to momentum and
mechanical energy.


The order of coverage of topics should be dictated by research into how well
the students have understood the physics. At present most of the research
based curricula that show good gain use a fairly conventional sequence and
only reverse momentum and energy to put energy last. However, this does not
mean that momentum first is inferior.

As far as I know momentum first has not been extensively researched. But
there is research which shows that interactions need to be studied before
Newton's laws. So NTN3 comes first. So the way is paved for momentum by
studying interactions after acceleration.

Unfortunately what actually works is not dictated by logic, but by what you
find out from research. Putting momentum before energy is considered
illogical by many physics teachers, yet the research shows that this
sequence works better.

Robert Karplus who was in many ways the founder of PER put interactions
first. So his sequence might be one to consider researching.

Modeling puts position, velocity, acceleration first. But this fits in well
with the philosophy of having the students do labs where they can figure out
the math relationships. Modelers have also been able in some cases to get
normalized gain over 70%. But again, we do not really know if this is the
optimum pathway to understanding. But when you achieve 90% understanding,
you have come close to saturation.

As to problem solving, it is only valuable if students use the concepts.
Traditional problem solving is often just a bag of tricks that students can
not transfer to analogous problems which are slightly different.

So does anyone know of research that validates momentum first in comparison
with traditional kinematics first? It is useless to argue over paths to
understanding that have not been tested for gain.

John M. Clement
Houston, TX

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l