Re: vector quantities and energy

["Robert W. Harris" <rwharris@CATH-MEM.ORG>]

When I teach vectors I use the usual "scalars are specified by magnitude and
vectors are specified by  magnitude and direction" bit.  However, I also
make the point that the REAL difference is how they add.
I ask what is 2+2?
Someone answers 4.
Then I say 2+2=4 ALWAYS, SOMETIMES, or NEVER?
They are puzzled.
Then we go into the fact that with scalars 2+2 always equals 4, but with
vectors 2+2 may add up to zero, or 4, or anything between, depending upon
directions.  So things which add like scalars are scalars and things which
add like vectors are vectors.

Energies are scalars because we know from experiment that they add as
scalars.  It works with the kids.

This may not satisfy the theoreticians who may feel free to correct me, but
it does work for high school juniors and seniors.

Br. Robert W. Harris
Catholic Memorial High School
rwharris@cath-mem.org
http://www.cath-mem.org/cath-mem/physics/contents.htm



-----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 9:33 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
> ********************************************