[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.
>
> Dont 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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