Re[2]: A Parents' Day gem

[NGUILBER@peddie.k12.nj.us (NGUILBER)]

     Ok, folks.  Enough already.

     As much as I enjoyed it, I was a bit concerned at the outset that 
relating the "Witchcraft!" incident would incite a minor feeding frenzy 
among those phys-l'ers who never met a nit they couldn't resist picking.  
As it turns out, of course, my fears were amply justified:

> Of course, it is worth noting that a bullet fired horizontally will 
> *not*, generally, take the same time to fall to the ground as one that 
> is dropped from the same height and that any mathematical "proof" to the 
> the contrary is, therefore, simply irrelevant.  
>
>
> The parent might have been correct in her
> insistance that the dropped bullet would strike the ground before the
> fired bullet. It is very probable that she knew that the earth is
> spherical rather than flat. Thus a bullet that is fired from a level gun
> has a further vertical distance to fall before it reaches the earth's 
> surface than a bullet that is dropped straight down.
>
>
> But of course in the case of a real bullet fired from a gun, they don't 
> hit at the same time, because there are all sorts of things that get in 
> the way, like the curvature of the earth, the interaction of the spinning
> bullet with the air (I don't know exactly what this does to the bullet, 
> but its a fair uess that it does something), wind or air currents, any 
> slight misalignment of the gun with the horizontal as the bullet leaves 
> the barrel, and others that I can't think of right now.

      Let's remember the context, ok?  This is from a *conceptual 
physics* course in a *high school*, a course in which we do not treat, 
when discussing projectile motion,
      -> air resistance (including spin effects)
      -> the curvature of the earth
      -> the coriolis effect
      -> the variation of 'g' with altitude
      -> the uncertainties in position or momentum of either bullet
or any of the other subtleties my learned colleagues on the list 
(correctly) point out.  It's just constant downward acceleration with a
projectile launched horizontally, in vacuum (it is assumed), on a locally
flat earth (it is assumed).  Remember also that the parent's (father's, 
actually) objections centered around the assertion that 
> the forward momentum of the SHOT bullet will make it spend LONGER in 
> the air than the DROPPED bullet!
which is clearly wrong and clearly not along the lines of the objections 
voiced here.  So spare us, please, from the nitpicking.  I think we're 
all aware of the limitations of this example, just like we are when we do 
problems with frictionless inclines or massless pulleys.

      A better question was:

> The first question that pops into my mind is, why didn't he do the demo 
> to show that they hit at the same time (at least for low velocity 
> situations)?

      Well ... because *I* had the apparatus for that demo in my classroom
that day!    :-)


      Mike Monce summarized well:

>      As Hugh suggested, the better tatic would have been to simply 
> do the demo.  However, I really must defend the teacher in this case.  
> This person was faced with someone whose "world view" was being 
> challenged, and they were not happy about it.  It cuts to the core of 
> our struggles to turn students from Aristotle fans to a Newtonian view.  
> Sure, Newton as taught in such a high school conceptual physics class 
> only works well in a vacuum and where local curvature of the earth can 
> be ignored.  Isn't it better to get someone to understand the basic 
> concepts *before* we throw all the caveats in too?
>
>        After all, the parent went screaming "Withcraft!"  This does not
> suggest to me someone who appreciates the subtleties of spinning bullets,
> lift and drag, and curved (and rotating) coordinate systems.  While the
> parent may have been correct, it was for probably all the wrong reasons.

      It was indeed.  I marvel at a parent who pays big bucks to send his 
kid to school but who obviously abhors ideas different from his own.  This
seems to me to be yet another factor, and an important one, in why some of 
our students have difficulty learning.


      Nick

      
      Nick Guilbert

      The Peddie School
      Hightstown, NJ

      nguilber@peddie.k12.nj.us