Text 19492, 72 rader
Skriven 2014-11-20 16:44:23 av Michiel van der Vlist (2:280/5555)
     Kommentar till en text av Kees van Eeten (2:280/5003.4)
Ärende: Mass, weight and heaviness
==================================
Hello Kees,

On Wednesday November 19 2014 23:49, you wrote to me:

 MvdV>> No. It is not the gravity as such that creates weight, it is
 MvdV>> the opposing force that balances gravity that is the source of
 MvdV>> weight.

 KE>  There is a mutual force that attracts two bodies. That force is
 KE> dependent on the mass of those bodies and their distance. When an
 KE> object cannot accelerate due to contact with a surface, that force is
 KE> balaced by a force in the oppsite direction. That force is sensed as
 KE> weight.

Correct. Funny thing is that it applies to both bodies. When I am standing on
the earth, my weight is 80 kg. Or to be more scientifically correct: 784 N.

But... from the same POV, the weight of the earth is also 80 kG or 784 N.
Becasue that is the force opposing the earth from accelerating towords me.

Fronm the POV from my cat however the earth weighs 4 kg or 157 N.

 MvdV>> But, when on is not standing on the surface, but in free fall
 MvdV>> in vacuum above the surface, there is no opposing force and
 MvdV>> hence no weight.

 KE>  Vacuum has noting to do with it. It only removes the resistence when
 KE> you move through a medium.

whwn falling in an atmosphere, there is resistance that forms an opposing
force. Actually it is not a free fall. The ooposing force creates weight.
Depending on speed the induces weight varies from almost 0 to the same weight
as when standing on earth at terminal velocity.

 WD>>>  The astronauts in the ISS do not experience weightlessness ...
 WD>>> they are in a continuous fall.

 MvdV>> So there is no force opposing earth gravity, they are indeed in
 MvdV>> free fall and so zero weight.

 KE>  Yes there is an opposing force, they move very fast perpendicular to
 KE>  gravity.

And they /accelerate/ in the direction of gravity. There is nothing to stop
that accelartion and so the do not have weight.

 KE> This causes a centripetal force that causes them to circle
 KE>  around the object pulling them.

Thet are in free fall. But the forward motion causes then to continously "fall
around" the earth. No centripetal force.

From another POV, Gravity cause curvature in space. They follow a straight line
in that curved space.

 KE> If they move faster the centripetal force will increase and they wil
 KE> move away from earth. When they slow down they will move into the
 KE> direction of earth

The effct you describe is correct, but the explanation is shabby. They actually
move in an ellipse with the centre of the earth in one of the focal points.
Depending on speed and height, the ellipse can degenerate into a cirkel or
degenerate into a vertical line with all variations in between. By changing
horizontal speed they can change the shape of the ellipse.

When they start the engines to do that, they experience weight.


Cheers, Michiel

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