Re: impulse/momentum

[Mark Sylvester <msylvest@XNET.IT>]

Actually, I was just indulging in a little pun over breakfast coffee...

...but in my experience, far from faltering, the juvenile mind thoroughly
enjoys following through the logic that leads to the conclusion that "the
earth moves" when I jump into the air. And this is not an article of faith.
It's the logical exploration of a model. In this case the model requires a
perfectly rigid earth, so it's not realistic, but physics is full of such
creatures and it's often useful to talk about them. Yesterday someone asked
in class what happens to conservation of momentum when you have a pendulum
swinging. I find it easier to consider first a rigid earth wobbling back
and forth with the pendulum. The pedagogical point is that there must be at
least two bodies interacting before there can be a pendulum. Realistic
modelling of the earth can be added as a footnote.

Mark

PS looking back over the thread, I see Justin Parke asks "Any suggestions
on a problem that requires a student to understand/apply impulse and
momentum considerations to "continuous" systems like ropes, water, etc?"

I like to introduce this kind of calculation with the "cement factory
problem": A skip drops sand onto a conveyor belt, at a steady rate. The
conveyor belt maintains a steady speed. They find the force needed to keep
it going. The further discussion of where the energy dissipation occurs,
and the role of friction in the process is also interesting.




At 09:37 13/12/03 -0600, Brian Whatcott wrote:
> Newton kindles the juvenile imagination when he relates the tenacious
> symphony of the spheres.
> But when  this same mass in consideration is quasi-infinite in the context,
> that looks more like an article of faith provided by an educator:
> the tender wheels of the student's traction falter at the concept,
>   do they not?
> Better to ask the young to imagine what they can imagine.
>
> A water jet impinging upon a fixed pelton wheel does not in
> fact cause some contra-rotation of the Earth mass - no indeed.
>   The force is reacted much, MUCH closer to home.
> So teachers who talk of the Earth moving, are in the position of
>   discussing Angels Dancing Upon Pin Heads - and I disapprove
> the dishonesty of it.
>
> Brian W
>
> At 03:04 AM 12/13/2003,  Mark Sylvester, you wrote:
> >  [ Brian Whatcott ]
> > >I expect I am missing something here.
> > >If I want to test a pelton cup by fixing it in a lab sink,
> > >I can see the water flow reversing, and I can see that the
> > >cup remains stationary, so if one wants to satisfy an equation
> > >by considering that the Earth mass rotates in reaction, I suppose
> > >that's OK. But isn't it a bit forced?
> >
> >"forced" by Newton III?
> >
> >Mark
> >
> >
> >Mark Sylvester
> >UWCAd
> >Duino Trieste Italy
>
>
> Brian Whatcott    Altus OK    Eureka!

Mark Sylvester
UWCAd
Duino Trieste Italy