Re: impulse/momentum

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

Quoting Justin Parke <FIZIX29@AOL.COM>:

> What I am hearing is that this is a poorly designed problem.

I don't see anything wrong with the problem.

Any problem can be nitpicked.  In this case, as is so
often the case, much of what the nitpickers have said
is just plain wrong.

Obviously the turbine question involves some idealizations
and simplifications, but there's nothing unusual about that.
Are we going to nitpick every problem that uses a massless
string or a frictionless pulley?

> Any suggestions on a problem that requires a student to
> understand/apply impulse and momentum considerations to
> "continuous" systems like ropes, water, etc?

1) Light in a high-vacuum radiometer.  (Do not confuse this
with the slight-pressure radiometer.)  Twice as much
momentum transfer for reflection compared to absorption.

2) This isn't the optimum time of year, but you could do
the experiment.  Get some pinewood-derby type racers, and
set up a horizontal track using taut string, and eyelets
on the cars to hold them to the track.  Propel them by
blasting them on the back with a stream of water.  Compare
the performance using a flat back-end versus a dish-shape
that throws the stream back as suggested by the original
turbine question.  Use two parallel tracks and two water
jets, so the A/B comparison will be clear.

3) As I mentioned before: generalize to non-normal incidence.