Re: impulse/momentum

["John S. Denker" <jsd@AV8N.COM>]

Quoting Justin Parke <FIZIX29@AOL.COM>:
>  water striking a blade
> and "reflecting" in the direction from which it came.
...
> I have a strong aversion to mystifying physics by saying, "we multiply by 2
> since delta v = 2v."

That's not wrong, but better explanations are possible.

First, I would phrase things in terms of momentum (not
velocity) as much as possible.  Momentum is the conserved
quantity.  A big part of physics is knowing the conservation
laws and how to use them.

So the discussion goes like this:
 -- What's the momentum of the water beforehand?
    (draw the vector)
 -- What's the momentum of the water afterwards?
    (draw the vector)
 -- What's the difference, i.e. how much momentum
    was lost from the water?
    (draw the vector)
    How long is this vector?
 -- By conservation, if it was lost from the water it
    was imparted to the blade.

Note that this approach generalizes nicely to the case
of deflection at an arbitrary angle (not just 180 degrees)
if/when you want to get into that.