On 3/15/24 3:11 AM, Luigi Fortunati wrote:
In free fall, can you go anywhere freely or are there constraints
that prevent this? Of course you can't fall straight up and you
can't fall sideways.
As I keep saying, this depends on the meanings of the words you use.
Your wishy-washy words are a major part of your failure to
understand very basic physics.
If by the "direction of fall" you mean the 3-velocity relative to
ground-based coordinates, that can be pointed in any direction. If you
mean the 3-acceleration relative to ground-based coordinates, that can
only be pointed straight down. Note the former is the usual meaning for
"direction" of any motion, including falling.
Hint: throw a ball straight up. It is moving upward, it is going upward,
and its 3-velocity (relative to ground-based coordinates) is directed
upward. It is, of course, in free fall (neglecting air resistance). So
one COULD say "it is falling upward", but that is such poor terminology
that no physicist would way that.
In free fall you can only go in one direction (the vertical one) and
in only one versus (downward).
This is just plain not true (here you use words with more definite
meanings). You can be GOING up or sideways -- that depends on the
initial conditions of your trajectory. Because "going" explicitly refers
to velocity, not acceleration. Your acceleration (relative to
ground-based coordinates) is always downward.
The elevator (in free fall) and everything inside it are forced to
fall (always) vertically and (always) downwards.
Nope. See above.
So there is a constraint.
There is a constraint on the 3-acceleration (relative to ground-based
coordinates): downward. There is no constraint on the direction of
3-velocity (relative to ground-based coordinates), because this depends
on the initial conditions; of course its direction will point
increasingly downward as time goes on.
And, in free fall, can one move in a straight and uniform motion?
Yes, RELATIVE TO COORDINATES ACCELERATING DOWNWARD WITH YOU. No,
relative to ground-based coordinates.
[This opens the door to the "local vs. global"
distinction in GR. You have no hope of appreciating
the subtleties involved until you STUDY.]
No, in free fall the motion is always accelerated.
Again, this depends on what you mean by those words. In Newtonian
mechanics this is true. But we live in a post-GR world, and presuming
Newtonian mechanics is not appropriate. In GR, of course, an object in
free fall follows a geodesic through spacetime, with ZERO proper
acceleration.
So why call it "free fall" and not "forced fall"?
Because as the moderator said, it means that no NON-GRAVITATIONAL force
acts on the object. "It's a statement about what forces are (not) acting
on the body, not about the uniqueness or non-uniqueness of the resulting
motion."
As I keep telling you, your approach of making false statements in this
newsgroup is utterly failing to teach you basic physics. You MUST get
some good textbooks and STUDY. Better yet, take a college or university
course in physics so you'll have an instructor with whom to discuss your
confusions.
Tom Roberts
Re: Free fall