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Is there any postulate that says that translation cant be converted into rotation?

  1. May 2, 2005 #1
    in a theterball when the ball hits the stick where does the kinetic energy of the volleyball go?

    my educated guess is that is transformed into earths rotation
  2. jcsd
  3. May 2, 2005 #2


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    Theoretically, yes; it applies torsion. Practically though, it is dissipated as heat, elastic and inelastic deformation and sound.

    Don't forget though, that this will only cancel out the force that was initially applied to the ball to get it moving from rest so, even under ideal circumstances, there will ulitmately be no net change in the rotational speed of the Earth.
  4. May 2, 2005 #3
    that sounds just logical to me the spin you get in the end is cancelled by the spin its produced in the begining

    but what would happen if i have a theterball that besides pulling the ball it also lets the ball away after it hits the stick by rebounding

    if the stick is very thick so the theterball only retracts and expands in one half

    wouldnt there be spinning forces in one half and linear forces in the other half?

    wouldnt this mean the internal forces only appear in one half producing an unbalancing effect in the system?
  5. May 2, 2005 #4
    i still dont see how a rebounding theter ball works

    the earth is still and i push the theter ball so the earth rotates the oposite direction of the push

    then the ball hits the stick braking the earths rotation initiated when i pushed first the ball

    then the ball rebounds and spins the oposite way and when it hits the stick again produces a force of rotation in the earth in the same direction to the first force of rotation when i first pushed the ball

    so once started the forever rebounding process the earth rotates a little and then stops, rotates a litle and stops and so on

    so how is posible that starting with a still earth by means of using internal forces you achieve earths rotation?

    whats wrong in my concept?
  6. May 2, 2005 #5
    The ball exerts a torque on the pole, and thus the earth, via the tether. However, the energy transferred to the earth's rotation is very, very, very small because of how massive the earth is. Take a look at this thread https://www.physicsforums.com/showthread.php?t=66697. Pay attention in particular to the posts by krab.
  7. May 2, 2005 #6
    very interesting read

    but imagine the whole earth was just 1 kg mass then the rotation would be considerable

    but i still dont see why earth seems to aquires rotation by internal forces which i think is considered imposible
  8. May 2, 2005 #7


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    There is no net gain - any gain at the end is preceded by a loss at the beginning. When you hit the ball, it goes one way and you (connected to the earth) go in the other.
  9. May 3, 2005 #8
    thats my point you hit the ball the earth rotates oposite to the ball, the ball hits the pole it stops the earth, the ball rebounds so it hits the pole again after a while but going the other direction the earth spins again, the ball rebounds again and hits the pole again stoppin the earth once again, it rebounds and it hits the pole rotating the earth once more,hits and stops hits and rotates and so on forever

    i accept and no discuss your point but what happens if you consider that the ball rebounds and spins again the other way?
  10. May 3, 2005 #9
    First off you are making no mention of relative sense (which way the earth and ball are spinning) and this is a glaring omission in your analysis. The earth and ball will spin in opposite directs. This is a first year physics concept call simple harmonic motion(do a search on this site and you'll find 854682365873268764 post covering SHM). In the absence of any form of damper the earth ball system will oscillate around an axis (back and forth if you will) forever. There is nothing mystical about this. You seem to be fixating on the end result while minimizing the fact that to put the earth ball system into SHM energy had to flow into the system---work was done on the system--- to start the process. So what if the ball bounds and rebounds forever; it does so because you started the process by hitting/throwing the ball; moreover, after the process begins, the net momentum of the system should be zero (unless I too am in left field) because the the Earth/mass will spin in one sense and the ball/mass will spin in the opposite sense.

    This really isn't a hard concept IMHO.
  11. May 3, 2005 #10
    the ball spins one sense first and when it rebounds spin the oposite sense coiling in the other direction

    so first you spin the earth in one sense when you first hit the ball, the ball rebounds so it brakes the earth, rebounds again making the earth rotate again in the same initial sense it started spinning when it hits the pole again(while coling oposite the the first original sense),rebound stops, rebound rotates, rebounds stops, rebounds rotates and so on
  12. May 3, 2005 #11
    You need to be more methodical in you description. Here, I'll give you a hand:

    this means clockwise(<) for the purpose of this analysis and this means anticlockwise(>).

    You are on terra firma feet planted. You strike or push a significant
    mass (say the moon) that is tethered to the north pole.

    1) You push the moon < and the earth goes >. You've done work on the system. You've added energy.
    2) The moon winds around the pole in the < direction while the earth spins in the > direction.
    3) The moon eventually strikes the pole both the earth and moon stop spinning at that point.
    4) The rotational energy in the system has now been converted to internal energy of the moon. Striking the pole caused the moon to compress distorting the shape of the moon and thus the crystal structure of the moon. The internal energy of the moon is not at lowest potential anymore---Gibbs is mad about this (see Gibbs free energy).
    5) The moon reshapes itself (well assume a coefficient of restitution of 1 and that no vibrations occure after the rebound) pushing off the pole again with the exact energy that you initially put into the system. You can see this yourself. Get a ball point pen like this http://www.lapassione.co.uk/uploads/0/1052869606CR71300 Ballpoint Pen - diag - Small.jpg push the clicker down onto a table and quickly release the pen. The pen will jump.
    6) Pushing off of the pole caused the ball to spin in a new direction > and at the same time pushed the earth to spin in the opposite direction <.

    The earth does not ratchet--If you father told it will then I'd have to question his credentials or you need to sit down with him and hash all of this out. The earth does not spin in the > at all times.

    You can see this yourself if you want to build a contraption . A stick a string and a superball. Tie the superball to the string to the stick. Spin(rotate the ball about the stick not the ball about the string that is) the super ball. watch it wrap around the stick. Feel the twist of the stick in your hand. Watch the ball hit the stick. Feel the stick twist in your hand. Watch the super ball rebound. Feel the stick twist in your hand.

    Report the results when your done(From the sounds of things you will be surprised and everyone here will be vindicated).

    [edit] Someday---when I grow up---I'll learn how to spell. Not today though I see.
    Last edited: May 3, 2005
  13. May 3, 2005 #12


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    Staff: Mentor

    You seem to be starting to get it, aviator - so what's the point? This is just like a spring-mass system where energy is constantly transforming between potential and kinetic. Its really fairly trivial.
  14. May 3, 2005 #13
    i realize of my mistake now but i had just thought of it during this thread by the way thanks a lot faust

    so all you can get from a still earth and moon is potentiall energy or they spinning in oposite direction

    now consider the spinning vectors of earth and moon as pencils which points aim in oposite directions, one pencil for the moon and another for the earth

    now holding each pencil in each other they twist around their axes in such a way that now both points aim same direction meaning thay spin in the same direction

    wouldn i have got a spinning uniformity of earth and moon by internal forces starting from a still eath and moon and aplying a intial work that would have been transformed into earth and moon rotating in certain direction?
  15. May 3, 2005 #14
    The pencils will never point in the same direction though. The length of the pencils will get longer or shorter; however, the direction of the pencils will always be opposite (excluding the point where the system is momentarily stopped at which point the pencils do not exist).

    So, the answer to your question(if I'm reading it correctly) is no. The best you can do from the scenerio given is the produce a system with no net rotational or momentum. The rotation of each object will always oscillate about the axis (the sum of all points along one cycle of a sine wave is zero) so there is not net rotation. The momentums will cancel as well. The momentum of the earth in < direction is exactly canceled by the moon in > direction. If you look at the system from the sun then all you will see is the moon bouncing back and forth the same amount with the same instant velocities at the same points. Likewise, the earth will only oscillate about it's axis (though not nearly as much as the moon will due to the relative differences in mass).

    There is no translation here (translation is linear movement). All that you have done is added rotational energy.
  16. May 3, 2005 #15
    Tell you what, do the experiment and post your results. Just do the experiment.
  17. May 4, 2005 #16
    the object will always move around the center of mass

    you cant move the center of mass but you can rotate it through the use of precession

    but i still dont see the reaction to the force of precesion at least no oposite to the rotation that produces the precesion

    maybe precesion is reactionless as rotation is in some cases
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