Re: vector quantities and energy

[Roger Haar <haar@PHYSICS.ARIZONA.EDU>]

Hi,
        I think Rick has the right approack here.  One could just circle around
the definitions, but I think the students questions at some level really comes
down to way did someone define KE as a saclar rather than a vector.  The reason
becomes clear in elastic collisions that are more complex that equal masses. And
the reason is that vector_KE is not (always) conserved ( even in ideal eleastic
collisions), thus vector_KE is not really a very useful quantity to calculate
and use.

Thanks
Roger Haar


Rick Tarara wrote:

> Maybe another approach might be to consider how the quantities momentum and
> energy are transferred from one object to another.  When momentum is
> transferred through a collision, direction is very important.  A collision
> that is entirely in the horizontal plane cannot produce any vertical
> momentum--collisions on a pool table being an example (although then you
> must discuss how and why sometimes you can have a ball jump off the table).
> On the other hand, kinetic energy can be transferred into forms that clearly
> have no direction (thermal energy for example).  In fact the thermal energy
> in a room full of air depends on the kinetic energy of the molecules even
> though the average _velocity_ of these molecules is zero.  This is a
> somewhat less mathematical approach to the question.
>
> Rick
>
> ***********************
> Richard W. Tarara
> Associate Professor of Physics
> Department of Chemistry & Physics
> Saint Mary's College
> Notre Dame, IN 46556
> rtarara@saintmarys.edu
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> -----Original Message-----
> From: GARY HEMMINGER <Hemmig@D-E.PVT.K12.NJ.US>
> To: PHYS-L@LISTS.NAU.EDU <PHYS-L@LISTS.NAU.EDU>
> Date: Thursday, January 21, 1999 9:32 AM
> Subject: Re: vector quantities and energy
>
> > Since we study momentum before energy I've had kids ask me the
> > question - why isn't energy (particulary kinetic energy)  a vector?
> > It strikes me that this is a great question because it can be
> > addressed at so many levels.  Feynman would no doubt say something
> > rather different to his colleagues than I would to my weakest 10th
> > graders and so on.     How would you answer this?
> >
> > *****************************************
> >  Gary Hemminger
> >  Dwight-Engelwood School
> >  315 E. Palisade Ave.
> >  Englewood, New Jersey
> >             07631
> >  e-mail: hemmig@d-e.pvt.k12.nj.us
> > ********************************************