Natural Science Forum / Physics / Relativity / November 2006

| Repeat to increase bandwith.
|
| "John Kennaugh" <JKNG@kennaugh2435hex.freeserve.co.uk> wrote in message
| news:+iSs8OQ6HTbFFwjG@kennaugh2435hex.freeserve.co.uk...
|| Henri
||
|| [snip] - to save bandwidth :o)
||
|| >>>You are actually saying if the rope had equal forces acting at either
| end,
|| >>>acting in opposite directions, it (the rope) couldn't be in a state of
|| >>>acceleration...and neither could the car or trailer.
|| >>
|| >>Correct
|| >>
|| >>>...so cars with trailers can never accelerate......
|| >>
|| >>No they do accelerate because there is not an equal force at each end.
|| >>There is a force at one end and a mass at the other.
|| >>
|| >>>I think there's another factor involved here..
|| >>
|| >>There is only one force. It is provided by the engine turning the
|| >>wheels.
|| >>
|| >>A force causes a mass to accelerate as in
|| >>
|| >>          Force = mass x acceleration  -------- [1]
|| >>
|| >>What you are trying to argue is that mass x acceleration produces a
|| >>force. If you could persuade a mass to accelerate without applying a
|| >>force to it then you might be able to generate a force from an
|| >>accelerating mass but there is no way of making a mass accelerate other
|| >>than by applying a force.
|| >>
|| >>  It is actually a good example of why I don't take modern physics
|| >>seriously. It builds mathematical equation upon mathematical equations
|| >>and a mathematical equation is no good unless you know what it means.
|| >>Equation [1] is not saying that an accelerating mass is the same thing
|| >>as a force it is saying that a force causes a mass to accelerate.
|| >>Mathematically there is nothing to say whether equation [1] is or isn't
|| >>saying that any force is caused by an accelerating mass.
|| >
|| >agreed. I have been pointing this out for years.
|| >'F=ma' is NOT a definition of force even though the equation holds good.
|| >The correct equation is a=F/m, a being the dependent variable.
|| >
|| >>One of the keys
|| >>to sane science is to keep a tight grip on causality. What causes what
|| >>as in - in relativity what physically causes the light to travel at a
|| >>speed independent of the source? The current answer is 'nothing'
|| >>therefore one can quite easily see that relativity is not sane science
|| >>:o)
|| >
|| >very true.
|| >
|| >>>>>John, John, if the weight is spinning around on the end of a spring,
|| >>>>>the spring
|| >>>>>extends in order to provide enough CENTRIPETAL FORCE  to balance
|| >>>>>the outward
|| >>>>>CENTRIFUGAL force on the end of the spring,
|| >>>>
|| >>>>No just as with the trailer. You are applying a force to a mass via a
|| >>>>piece of string. You cannot apply a force through a slack string. The
|| >>>>force is centripetal and so is the acceleration. The mass accelerates
|| >>>>towards the centre of the circle.
|| >>>
|| >>>Indeed it does. The centripetal force responsible for this inward
|| >>>acceleration
|| >>>equals the spring tension.
|| >>
|| >>Correct and that is a complete and accurate description of what is
|| >>happening.
|| >>
|| >>>At the same time, the CENTRIFUGAL FORCE is responsible for pulling the
| spring
|| >>>OUTWARDS and hence causing the spring tension.
|| >>
|| >>That explanation is redundant. You have already explained in the above
|| >>why the string is in tension. It is applying a force to the mass.
|| >
|| >Not so John. The centripetal force does not CAUSE the tension in the
| spring. IT
|| >IS the tension.
||
|| OK the centripetal force acts through the spring. _ I don't see your
|| point.
||
|| >>>The relativists call this 'a reaction to the centripetal force'.
|| >>
|| >>Yes an accelerating mass balances the force causing the acceleration as
|| >>in
|| >>              force = mass x acceleration
|| >>
|| >>If you balance a force with a force nothing moves.
|| >
|| >In this case it moves tangentially but NOT redially.
||
|| If a force is balanced by an equal and opposite force there will be no
|| tendency to move.
||
|| >>>It cannot be.
|| >>>It is a force of equal status to centripetal. The two forces always
| exist in
|| >>>pairs....one cannot exist without each other.
|| >>>
|| >>>This centrifugal force in the inertial frame comes directly from the
| ball's
|| >>>resistance to an angular change in momentum.
|| >>>
|| >>>The ball exerts a centrifugal force on the spring. There is NO
| centrifugal
|| >>>force on hte ball itself.
|| >>>
|| >>>>Think of a satellite. That is constantly falling, accelerating

|| >>>>the earth. It doesn't get any nearer to the earth because in a given
|| >>>>time the amount it falls, from its tangential line, (the direction it
|| >>>>would take if it wasn't falling) brings it back to its orbital line.
|| >>>
|| >>>The satellite's rotation around the EARTH/SATELLITE BARYCENTRE causes
|| >>>the Earth
|| >>>to also revolve very minutely around that centre. The radii of the two
| orbits
|| >>>are in the ratio M/m. The satellite's centrifugal 'pull' on the Earth
|| >>>actually
|| >>>pulls the Earth INWARDS towards the barycentre.
|| >>
|| >>You are just muddying the waters. Read my description above. It is
|| >>completely explained without any centrifugal force.
|| >
|| >Not so John. You are missing the point entirely.
|| >You are not explaining the existence of the spring tension.
||
|| I don't need centrifugal force to explain the existence of gravity. The
|| spring is just another way of applying a force to a mass. It causes the
|| mass to accelerate in a straight line as in the case of the trailer (or
|| a mass dropping vertically) or it causes it to accelerate by change of
|| direction acceleration as in the case of the weight on a string (or a
|| satellite in orbit).
||
|| >....and the Earth's rotation around the barycentre is also vital to the
|| >understanding of this centrifugal force question.
|| >
|| >>>If the Earth and satellite were connected by a string, the centrifugal
| force
|| >>>exerted by the satellite on one end of the string is the same force
| that is
|| >>>transmitted BY the string to become the CENTRIPETAL force that

| the
|| >>>EARTH towards the barycentre.
|| >
|| >Nobody here can understand this...yet it is most important....
|| >
|| >Consider what happens when you twirl a ball around on a string. What is
| not
|| >appreciated is that the hand also moves in a circle around the hand/ball
|| >barycentre.
||
|| In practice yes because of wind resistance you have to keep adding
|| energy to the system. In a vacuum with frictionless bearings you could
|| have the centre of the string going around a rigidly fixed pole and once
|| set in motion it would continue indefinitely.
||
|| >The centrifugal force exerted by the ball on one end of the string is
|| >transmitted through the string to become the centripetal force that
| accelerates
|| >the hand inwards.
|| >The centrifugal forces exerted on the string by both the hand and the
| ball are
|| >responsible for the string tension.
|| >The string tension, in turn, provides the CENTRIPETAL forces that
| accelerate
|| >both the hand and the ball towartds the barycentre.
|| >
|| >HW.
|| >www.users.bigpond.com/hewn/index.htm
|| >
|| >Thank christ there is one genuine physicist on the NG.