Re: Name that force

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

On 10/29/2003 01:32 AM, Ludwik Kowalski wrote:
> Therefore I would expect more pressure on the front wall
> and not on the back wall. What am I missing?

OK, I think I know where Ludwik is coming from.

Consider the following situation:
 -- Hopper-shaped car, so no rain can hit the front wall,
    because of the sloping front wall.
 -- Almost full of water, so no rain can hit the rear wall.

.     \WWWWwwwwwwww/
.      \          /
.       \________/
.  rear  OO    OO  front ==>


In addition, somewhere hidden down in the undercarriage,
there is a half-full bottle of water.  Spring water, not
rainwater.

Here's the momentum balance:

  new rainwater slows down old rainwater
  rainwater slows down car
  car slows down bottled springwater.

That is, the momentum flow is:
  new rainwater
    --> old rainwater
         --> car
              --> bottled springwater

The rainwater will be higher in the rear as shown.
The bottled springwater will be higher in the front.

As I said in my previous note:  the key idea is to
divide the world into subsystems, and keep track of
the mass flow into/out of each subsystem, and the
momentum flow into/out of each subsystem.

Note that this is a post-Newtonian viewpoint.
Newton's laws apply to particles, not directly to
regions.  So perhaps this is a good lesson in why
we learn physics:
 -- The sophisticated approach to analyzing
the problem makes things easy.  Treat the water
as a fluid.
 -- The elementary approach (keeping track of
individual particles) is a much harder.  It
tempts you to think about all sorts of horrid
details that don't matter in the end.