N. T. wrote:

> Gerhard Fiedler  wrote:
>>=20
>> I don't understand how you get from force to momentum. In this case,
>> neither the earth nor the plane have a vertical momentum. (Imagine
>> the earth as large enough so that the non-flat shape of its surface
>> doesn't matter.) So neither goes into the momentum preservation
>> calculation.=20
>=20
> That's wrong statement "neither goes into the momentum preservation
> calculation" as the assumption "neither the earth nor the plane have a
> vertical momentum" is wrong. They have variable momentums strictly
> within momentum preservation low.

(I assume you meant "momentum preservation law".)=20

You seem to have forgotten to consider that our assumption is that the
plane flies with constant speed at constant height ("height" meaning
"distance from Earth surface").

> Imagine a plane is a molecule, air is molecule and Earth is a molecule
> too. Plane and Earth will be playing ping-pong, a ball being an air
> molecule. Clearly Plane and Earth will be experiencing variable
> momentums. You can glue more molecules into each object, this will
> affect numbers, but physics would remain the same - variable
> momentums.

We assumed a plane flying at constant height. How it got there is
another question for another day, and what it does once fuel goes out
(or shortly before :) is also another question for another day -- maybe.
But for now, I was only looking at the time where the plane flies with
constant speed at constant height.=20

The second issue to consider is the reference system. In order to talk
about speed, you need to say "speed relative to ...", where "..." is
called the "reference system". I chose the Earth as the reference
system. Being the reference system, its speed and therefore its momentum
in this system is by definition zero. Since I assume a plane flying at
constant height, its vertical speed and therefore its vertical momentum
is also zero -- in this reference system.

Gerhard
--=20
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