00:00:00Hg wrote:
On Fri, 03 Mar 2006 17:56:39 +0000, Alan Baker wrote:

Suppose we have a 1500 lb airplane in level flight at 120 mph.
What are its horizontal and vertical components of momentum?

Zero at equalibrium.

Incorrect. It has considerable horizontal momentum and no vertical
momentum.

Whereas for the hovering spacecraft both components are zero.


I thought the focus was forces.

It should be.

The hovering spacecraft has zero horizontal and vertical momentum.
It has weight, directed downwards. The engine accelerates
mass downward producing an upward force equal in magnitude
and opposite in direction to the weight of the spacecraft. This
imparts an acceleration to the spacecraft equal in magnitude and
opposite in direction from the local acceleration due to gravity.

Now of course weight is a convenient fiction. There is really no such
thing as gravitational force, what we model as a force acting at a
distance is in reality the distortion of spacetime in the presence of
mass. Perhaps other forces are similarly ficticious.

But how sure can we be that mass and velocity are any less ficticious
than force?

--

FF