Re: [Phys-L] force, momentum, torque, and angular momentum

[Peter Schoch <pschoch@nac.net>]

Before I start, I think I should mention that I make no bones about it to the students when they enter my classes -- I tell them I will beat on them mercilessly, because they need to be BETTER prepared than their counterparts at a 4 year or university when earning the equivalent grade.  This is because when they transfer they have a short amount of time to "prove themselves", and not just be another "community college transfer" -- the strategy is hard on them, but ultimately it is tremendously successful (several have gone on to do undergrad. research and publish, and I have local 4 year schools actively recruiting my students because of their preparation).  I also instituted a pre-req. for "Physics for Scientists and Engineers" of Calc. 1, out of a 3 semester Calculus sequence.  This means I ensure they get to me with a reasonable level of mathematics 'under their belt' to begin the class.  That all being said...

I define conservation with my students using work, and move to curl F=0 -- they've all had Calc I and are either in Calc II or Calc III when they get to me, and I cover "essential" math so they know what the curl is (both mathematically and physically).  When we get to energy, I show them John's type of definition and tie it back.  Wouldn't it be helpful to do the reverse in this presentation -- since you do energy first, just do work and conservation and tie it back to energy?

I am one of those people who like Taylor's development in his Mechanics text; so, I incorporate that into my freshman class.  I emphasize that F=dp/dt and introduce some problems of that kind (Jerk, finding momentum change, etc.) along with the traditional F=ma sum the forces concepts.  How do you previously introduce momentum before you get here?  I LIKE to do a conveyor belt or rocket example where the mass is not constant to show how this all ties together.  In addition, the idea of momentum "flowing" sounds a bit like "Caloric" flowing to me.  If we're trying to avoid saying heat "flows", then shouldn't we likewise avoid the idea that momentum "flows"?   While the concept of "flowing" makes it easier to teach now, when they get to more advanced levels it seems to me that they'll either have to "unlearn" the idea of "flowing" or amend it significantly to grasp more complex problems/concepts.

The bisector idea is nice, and I think I will try that with my students this Fall.  However, I also know they will not see that again in any math class they take at my school; so, the risk is to introduce something that they will perceive as 'esoteric' and only used/useful in Physics.  I try to tightly link the presentation to the math they know and the math they will learn -- which is a benefit to both myself and the math instructors.  Since we started doing that, our collective assessment results have shown improvement.

Most of my students know torque from torque wrenches, that either are in ft*pounds or inch*pounds -- generally the former.

On the whole I LIKE this presentation (as I like, and have learned a fair bit, from most of your postings and website).  Those are just my initial thoughts, as you've now given me a fair bit to consider as I begin to contemplate the presentation/sequence for my Fall courses.

Peter Schoch


On Jul 22, 2013, at 9:15 PM, John Denker <jsd@av8n.com> wrote:

> As the saying goes:
>  "Better to light a candle than to curse the damn darkness."
>  http://www.buffyguide.com/episodes/outofmymind.shtml
>
> In that spirit, I finally got tired of people complaining that 
> students don't have a good, simple, constructive, operational 
> definition of "force".  So ... I decided to write something:
>  http://www.av8n.com/physics/force-intro.htm
>
> I tried to keep it as simple as possible ... but not dumbed
> down.
>
> This is an early draft.  I've already rewritten it about five
> times, but it will need to be rewritten several more times.
> Suggestions are welcome.
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