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Re: Name that force



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.