[time-nuts] FW: Pendulums & Atomic Clocks & Gravity

Didier Juges didier at cox.net
Mon May 28 07:53:15 EDT 2007

I am quite familiar with the cannon analogy. If I may use this analogy
too, please consider the following:
There must be a force balancing the force of gravity, otherwise the
satellite would not cease from accelerating under gravity alone.
Gravity exerts a force on the satellite which tends to make it fall
towards earth. This is the Centripetal force. Inertia due to the mass of
the satellite makes it resist this motion, and the tangential speed
makes it “miss” the earth. Centrifugal force is the name we give to that
resistance. When the satellite is in a stable orbit, it does not
accelerate because both forces exactly balance each other. For the
reason you pointed out, in a closed system the sum of forces must be
zero, so there must be a force balancing the gravity force. So I see we
If there was no rotation, that force would not exist and the satellite
would accelerate (under gravity alone) towards earth.
Don’t be confused by terminology. The terms centrifugal and centripetal
are just names given to other forces, not actual forces by themselves.
The centripetal force is due to gravity (but is could be
electromagnetic, or anything else. In a centrifuge, it would be the
force exerted by the rotating arm), the centrifugal force is due to
mass, radius and speed.
Didier KO4BB
-----Original Message-----
From: Ulrich Bangert [mailto:df6jb at ulrich-bangert.de] 
Sent: Monday, May 28, 2007 5:03 AM
To: 'Didier Juges'
Subject: AW: [time-nuts] Pendulums & Atomic Clocks & Gravity
I am an physicist, not an engineer.
Let me use an experiment of thought that Bill Hawkins has already used
in the discussion: Assume an cannon mounted in an certain height with
the barrel mounted tangetial to earth's surface. Fire an bullet and see
it fall to earth after an certain time of flight. Now use more gun
powder and see the the bullet fall to earth later. Use a BIG amount of
powder and see the bullet leave earth's gravity completely. Between the
extremes: Drop to surface and leaving earth's gravity completely there
is one powder loading that brings the bullet into an circular orbit at
the height of the cannon. The bullet never stops to "fall" to earth.
However the motion towards earth's cencer is compensated by the fact
that an tangential motion ALSO means to depart from the center of the
body that you move tangential to.
73 and my best regards
Ulrich, DF6JB
-----Ursprüngliche Nachricht-----
Von: Didier Juges [mailto:didier at cox.net] 
Gesendet: Montag, 28. Mai 2007 02:02
An: df6jb at ulrich-bangert.de
Betreff: Re: [time-nuts] Pendulums & Atomic Clocks & Gravity

Please go ahead, I am all ears... (in all seriousness, I am not a
physicist, just an engineer)

If earth attracts the satellite and the satellite attracts earth, how
come the satellite and earth don't get together?
What is keeping them apart?

When you say the gravity forces are of opposite direction, this is
correct. The gravity applied by earth to the satellite causes a force
vector directed towards the earth, the gravity applied by the satellite
to earth is a force vector of equal magnitude and directed from earth to
the satellite. The external result is null (as a system, there is no
"loss" of force, action = reaction).

The same holds true for centrifugal forces. The satellite affects the
orbit of earth in proportion of their respective mass, so the satellite
causes earth to move around it's theoretical orbit (if there was no
satellite). The earth movement is very small (could not be measured for
an artificial satellite, but but could certainly be calculated, the
effect of the moon on earth's orbit can certainly be measured) but it
causes an equal and opposite centrifugal force on earth, which balances
the force exerted on the satellite. 

So I believe there are 2 sets of forces (gravity and centrifugal), and
each set has a resultant that is null, as seen from the outside.
However, at the level of earth and the satellite, the gravitational
attraction is equal and opposite to the centrifugal force.

I did not know physics cared if we used inertial system concepts or
accelerated systems concepts (I do not know the difference). 

If I follow your theory, the speed of the satellite around the earth has
no effect on gravity, so the satellite should stay where it is
regardless of speed, but it does not!

Please explain this to me.

I agree that as long as the distance between a satellite and earth
remains constant, the forces must balance each other. But if it's not
centrifugal force that is balancing gravity, what is it?

Thanks in advance


Ulrich Bangert wrote: 
gravitational forces, so do objects in Lagrange points. These points 
represent areas where the centrifugal forces compensate for 
I am almost sure that this will again produce me a lot of trouble in
answering a lot of people but the idea that there are centrifugal forces
which compensate for gravity are one of the BIGGEST misconcepts that one
may have in physics at all although it is quite common and you may find
statements like that eben in (bad) physics textbooks.
Centrifugal forces are so called fictitious forces which are only
observed from within accelerated systems. Normal physics is done in
inertial systems. In an inertial system consisting of earth and an
satellite there are only TWO forces available: The gravity force by
which earth attracts the satellite and the gravitational force by which
the satellite attracts earth. They are of the same magnitude but of
opposite direction. That is the reason why the "sum of forces" is zero
for the closed system consisting of earth and satellite. There is no
place for any other force like centrifugal or so because there is no
counterforce available that would make the sum of forces zero i case a
centrifugal force would exist. In case you like to discuss it a bit
please go on but be prepared that I will to blow your arguments into
little bits. A good idea to start with is to look after what Newton's
first law is saying about the behaviour of a body for which all forces
compensate each other. Is that what a satellite does???
73 Ulrich, DF6JB 
-----Ursprüngliche Nachricht-----
Von: time-nuts-bounces at febo.com 
[mailto:time-nuts-bounces at febo.com] Im Auftrag von Didier Juges
Gesendet: Sonntag, 27. Mai 2007 16:54
An: Discussion of precise time and frequency measurement
Betreff: Re: [time-nuts] Pendulums & Atomic Clocks & Gravity
For the same reason that a satellite in free fall is still subject to 
gravitational forces, so do objects in Lagrange points. These points 
represent areas where the centrifugal forces compensate for 
gravity from 
two objects instead of one for a regular satellite. The only 
way to be 
free from gravitation is infinite distance from mass, until someone 
actually invents the famous gravitational shield :-) I hope 
it comes in 
spray form...
Neville Michie wrote:
Look up Lagrangian points on Wikipedia.
There are points of zero gravitational force, about our planet. What
is more, these points are stationary with respect to Earth, so  
Doppler effects would be zero.
As the distance from Sun to Earth to Moon varies through 
the year it  
follows that the distance from Earth of these points must 
vary on a  
small scale.
These points are good for satelites as the orbit never decays.
cheers, Neville Michie
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