Re: [Phys-l] Work and Energy: which first?

[John Denker <jsd@av8n.com>]

Marc "Zeke" Kossover wrote:
> As Professor Denker says, it is a matter of taste.

:-)

>  I like
> this progression since it starts with easy equipment before
> it gets to the black box stuff. 

> I start with a series of pulley activities, starting with a
> simple pulley and advancing to compound ones. 
....
> 1. How much force does it take to lift Block A (and
> pulley), if lifted slowly a distance of 10 cm? 
>
> Students measure this force with a spring scale.

Let me point out that this activity can be re-arranged to use
_even simpler_ equipment.

1'.  How massive must Block B be in order to counterbalance Block A?

2'.  Ditto, with some other mechanical-advantage ratio?

The whole exercise can be analyzed in terms of energy, and the
conservation thereof.  Force need not be mentioned.  Work need
not be mentioned.  Spring scales need not be used.

The mechanical advantages can come from pulleys, and/or from
levers, and/or from ramps, et cetera.

====================

Historical remark: The two-pan balance (for comparing one mass with
another) is quite ancient, going back at least 7000 years.  This
considerably predates the invention of the wheel.  It is still in
use in many parts of the world.  It can be made very, very precise.

Another historical remark:  The idea of setting up pulleys and ramps
so that a small object can counterbalance a big one -- and using this
to illustrate fundamental physics principles -- goes right back to
Day One of modern science: 1638,  Galileo, _Two New Sciences_ (page
216 and also page 339 in the National Edition).


   Note that I mention this *after* having argued on physical and
   pedagogical grounds that the foregoing is reasonable, and having
   said at the very start that other approaches are also reasonable
   and are a matter of taste.  I don't want anybody to think that I
   am suggesting that the energy-balance argument is recommended
   (let alone required) because of its antiquity.  Forsooth I would
   put the opposite spin on it:  I would say it is quite uncommon to
   see ancient technology and ancient pedagogy remain viable so long.