Re: [Phys-l] momentum conservation ==> action=reaction

[John Denker <jsd@av8n.com>]

On 08/01/2008 03:48 PM, LaMontagne, Bob wrote:
> Doesn't your step 4 essentially declare N2? Agreed that your eqn [1]
> leads to your eqn [2]. But it is another matter altogether to assume
> F = (d/dt)p. That seems either equivalent to accepting N2 or
> requiring a set of experiments to demonstrate its validity.


1) As always, I don't much care whether it's axiomatic so long
 as it is true.

2) In some quarters, force is *defined* to be (d/dt)p.  

In particular, there is a school of thought that starts with the
great conservation laws and studies energy and momentum for quite
a while before even mentioning force.

There are lots of elementary things you can do, including the 
simple pendulum, Galileo's celebrated _interrupted pendulum_,
ballistic pendulum, trains that collide and stick, Newton's
cradle, fluid dynamics, etc. etc. that are really much easier 
if you emphasize momentum rather than force, and either don't
mention force at all, or define it as dp/dt.

It is possible for very young kids to have a good "feel"
for what momentum is:
  http://www.av8n.com/physics/mass.htm#sec-car-push