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

Ulrich Bangert df6jb at ulrich-bangert.de
Mon May 28 08:24:55 EDT 2007



> -----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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