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

Didier Juges didier at cox.net
Mon May 28 11:59:20 EDT 2007


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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