[Phys-l] weeds in the garden (was: Ballistics)

[John Denker <jsd@av8n.com>]

On 10/08/2006 11:20 AM, Jeffrey Schnick wrote:
> John Denker wrote
>
> > I've had kids as young as 8 ask the key physics question:  Why does the
> > rocket need both air *and* water?  I answer that the air is needed for
> > energy, and the water is needed for momentum.  I then explain what that
> > means.....
>
>
> I laud you for teaching kids physics (and I really like the way you
> explain things to us!) 

:-)

> but, unfortunately, kids learn at an early age
> that science is about using scientific words. 

I don't see what's "unfortunate" about using scientific words, if
they are used properly.

>  I think it would be
> better to explain first and use the words "energy" and "momentum" later,
> perhaps when the children are older. 

I do explain what I mean by the words.  There's a lot covered in
the "...." in my previous message.  When you're launching rockets,
you've got an opportunity to talk for an hour.  If energy is the
right word to express what I mean, I don't see why I should look
for a circumlocution or euphemism.  Ditto for momentum, force, and
other technical terms.

Kids hear these words tossed around all the time with nontechnical
meanings.  It cannot possibly do any harm to expose them to precise
technical usage, even if they don't yet appreciate all the nuances.


> For an 8 year old: On earth, when
> we want to go forward faster we push back against something--the floor
> when we're walking, the road when we're biking, and the water when we're
> swimming.  A rocket doesn't need anything outside itself to push back
> against; it carries stuff to push back against with it.  That's why a
> rocket works in the vacuum of outer space.  The water rocket works
> better when it has more stuff than air with it to push back against.
> That's why it works better when you put some water in it.

You talk about pushing;  I talk about force.  Again:  I don't see how
you can "protect" them by "saving" the technical term for later.  The
non-technical and metaphorical usages are already out there, vigorously
competing for an unfair share of the concept space, like weeds in a
garden.  Do you have a plan for preventing such weeds?  If so, please
tell us about it.  My best plan is to plant and nurture the technical
concept, so it can occupy and defend its proper share of the concept
space.  I don't know how to defend the empty place where a concept
will later be put ... whereas I do know how to defend and nurture an
actual concept, however inchoate it may be.

> That's why it works better when you put some water in it.

That's fine as far as it goes, but the kids won't let you off the
hook so easily.  They want to know why we need some compressed air,
not just 100% water.  Energy you say?  Why not store energy in the
form of compressed water instead of compressed air?