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

Ulrich Bangert df6jb at ulrich-bangert.de
Mon May 28 15:15:08 EDT 2007


Didier, 

> Since you know a lot more about this than I do, I will accept 
> your statement that centrifugal forces (or more generally 
> inertial forces) are fictitious, but only because you insist. 
> As long as I can predict their effect and calculate their 
> magnitude, that's all this engineer is interested in :-)

That would be totally wrong! You are just missing the possibility to
gain some new insights into nature.

> Your comparison with the linear motion is not valid.

Please note that my intention was not to immediatly compare linear to
circle motion. The example was merely to show you something about about
forces and counterforces. You did not excactly understand it so let me
try to take a second chance:

> Inertia causes real forces to be developed.

If this sentence is true then please answer again the following
question: Is there a real inertial counterforce in the linear example or
not? 

73s and my best regards
Ulrich, DF6JB

> -----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 17:59
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] FW: Pendulums & Atomic Clocks & Gravity
> 
> 
> Ulrich,
> 
> Your comparison with the linear motion is not valid. While 
> you push the body, it accelerates. The energy spent giving 
> the body increased speed (due to the excess force applied to 
> the body while there is no counter
> force) is stored in kinetic energy. Once you stop pushing, 
> the body moves straight at constant speed and there is no 
> more force either way, the state of motion does not change.
> 
> Going back to the satellite, I believe we agree in principal 
> but you are hung up on the first law definition. The 
> corollary to the first law is that objects resist change to 
> their state of motion. They resist that change via inertia. 
> Inertia causes real forces to be developed. I agree it's not 
> a fundamental force like gravity or electromagnetism, but it 
> is necessary to keep the system in equilibrium, otherwise 
> what would be holding the satellite at a constant distance 
> while it is being pulled towards earth? 
> 
> Example: a car is moving at 1m/s. You stand still on the 
> driveway (your
> speed: 0m/s.) The car hits you. Because the car is much 
> heavier (err: has more mass) than you, you now move at close 
> to 1m/s and the car has just slowed down a bit (m * v stays 
> the same). The car has exerted a force on you that gave you 
> acceleration. You have exerted a force on the car that caused 
> the car to change it's speed, you imparted on the car a 
> certain acceleration in the opposite direction. The only way 
> the car could feel acceleration is because a force was 
> exerted upon it. The force that was exerted on the car came 
> from the inertia of your body. It is a real force. Now, 
> imagine the driveway was itself on a sliding plane moving at 
> 1m/s in the opposite direction to the car. In fact, the car 
> was not moving but you were. It makes no difference, once you 
> realize the only difference is where the reference it. Two 
> forces were developed. Which one you call action and which 
> one is reaction is irrelevant. It's only a matter of 
> reference. Because you change the reference does not make the 
> force go away.
> 
> We agree on what is happening, we don't agree on what to call it. 
> 
> Since you know a lot more about this than I do, I will accept 
> your statement that centrifugal forces (or more generally 
> inertial forces) are fictitious, but only because you insist. 
> As long as I can predict their effect and calculate their 
> magnitude, that's all this engineer is interested in :-)
> 
> 73,
> Didier KO4BB
> 
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Ulrich Bangert
> Sent: Monday, May 28, 2007 8:23 AM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] FW: Pendulums & Atomic Clocks & Gravity
> 
> 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.  
>    
> 
> 
> 
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