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

Ulrich Bangert df6jb at ulrich-bangert.de
Mon May 28 09:22:57 EDT 2007


Didier,

let us consider the more easier case of an linear motion. Imagine an
body that can glide on an surface without any friction. Now you take a
finger of your hand and press it on one side of the body so that it
moves horizontally. Clearly your finger exercises an force on the body
that makes it accelerate. And in your finger you feel an force into the
opposite direction. 

The key question is about the physical reality of this force in the
opposite direction. 

How big is it? I guess, you would argument that it has the same
magnitude as the force that you apply with your finger but has the
opposite direction, right? Now you have TWO forces. If they have the
same magnitude but opposite direction their vectors add to zero and I am
almost sure you would argument that this makes the sum of forces zero
for the system.

Now, that you have shown that the sum of forces is zero you are in the
ungraceful position in that you must explain why the body IS
ACCELERATING at all. According to F=m*a a non-zero F is necessary to
generate a non-zero a. How do you explain? 

Please note that in physics there is one substantial thing that one must
know about forces and counterforces: They never affect on the SAME body
but always on DIFFERENT bodies. In case you do not believe take the next
textbook and read it after. For the above experiment this means: Since
BOTH forces that you are talking about affect on the same one body one
of the forces CANNOT be the counterforce to the other. If this is so
then lets search for the counterforce for the force that you apply with
your finger. In order to be able to execute this force your feet or
other parts of your body execute an force into opposite direction to the
surface of earth. These two forces are counterforces to each other
because they

a) have opposite directions

b) have same magnitude

c) apply to different bodies.

Now that we have found the counterforce that makes the sum of forces
zero for the system you need to find the counterforce to your inertial
force and you will find none. Perhaps it is helpful for your
understanding that one of the definitions for fictitious forces is that
no counterforce belonging to them can be found. And if no counterforce
can be found they have NO physical reality because otherwise the rule of
the sum of forces is violated.

There was only ONE force F acting on the body that make it accelerate
according F=m*a and this force had its source in your finger. To be able
to execute this force you "pressed against earth" into the opposite
direction and that makes the sum of forces zero. This is the only
correct physical explanation of this example. ALL forces due to effects
of inertia are fictitious forces.

73 and my best regards
Ulrich, DF6JB

P.S.

Newton's first law says that a body at which all forces compensate keeps
it current state of motion. To keep the current state of motion a body 

a) has to keep its velocity

and

b) has to keep its direction of flight.

It this what a satellite does? No, it permanently changes its direction
of flight due to earth's gravitational force. If there were compensating
forces the satellite would move away along a straight line.

Please note: I knew before that this would make a big discussion because
the misconception is spread that wide. Perhaps you even learned this at
school. When I worked as an "Wissenschaftlicher Mitarbeiter" (comparable
to an assistent professor) at the department of physics of the German
university at Bochum such miconceptions were one of our favourite
subsects of study in teaching physics.  
   

> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Didier Juges
> Gesendet: Montag, 28. Mai 2007 13:53
> An: time-nuts at febo.com
> Betreff: [time-nuts] FW: Pendulums & Atomic Clocks & Gravity
> 
> 
> Ulrich,
>  
> 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 agree.
>  
> 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.
>  
> 73,
> 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
>  
> Didier,
>  
> 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 Ulrich,
> 
> 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
> 
> Didier
> 
> Ulrich Bangert wrote: 
> Didier,
>  
>   
> gravitational forces, so do objects in Lagrange points. These points 
> represent areas where the centrifugal forces compensate for 
> gravity....
>     
>  
> 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...
>  
> Didier
>  
> 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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