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Cavendish Experiment

  1. Dec 7, 2013 #1
    Oh.. Many questions revolving on my mind..

    (1) what are the conditions or in which situation can we do cavendish experiment?

    (I mean there should be no air or no gravity?)
     
    Last edited: Dec 7, 2013
  2. jcsd
  3. Dec 7, 2013 #2
    How the two object (two lead sphere in experiment) are attract each other?

    But in our practical life two object never attracted to each other.
     
  4. Dec 8, 2013 #3
    Some one told me that : all objects attrac other objects, its just earth attracts more so you dont realize it, apparently.


    So i have doubt that when cavendish done this experiment than how two object attact each other ?
     
  5. Dec 8, 2013 #4

    jtbell

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    Use Newton's law of gravitation to calculate the gravitational force exerted by a 1-kilogram object by another 1-kilogram object, 0.1 meter away. Compare this to the gravitational force exerted by the earth on a 1-kilogram object (at the earth's surface).
     
  6. Dec 8, 2013 #5
    So how the cavendish done this experiment. because when he has done this experiment than earth gravity is also acting on the other object, so how one object is attracted by other.

    You can see the demo of experiment


     
    Last edited by a moderator: Sep 25, 2014
  7. Dec 8, 2013 #6
    But not in the horizontal plane. That's the purpose of the torsion balance. The test mass is only affected (gravitationally) by objects that are not directly above and below it.
     
    Last edited: Dec 8, 2013
  8. Dec 8, 2013 #7

    Philip Wood

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    Cavendish did the experiment in a shed on his estate in Clapham in South London (then in the countryside). Air currents were a nuisance, and he viewed the torsion balance through a window using a telescope, to avoid disturbing the air in the shed.
     
  9. Dec 9, 2013 #8
    Your mean to say that if there was no air then larger object attracted by small object. And we can realize the gravitational force. Am I right?

    But I am thinking that there is also earth gravity that playing a role. So there is no air but there is earth gravity that attract both the object. so there should not be attraction between the two object.
     
    Last edited: Dec 9, 2013
  10. Dec 9, 2013 #9
    The point of the experiment was to measure the attraction between the two objects. The experiment confirms that they do attract each other as expected. It's not clear to me why would you think they shouldn't attract each other. One important point of Newton's law of universal gravitation is that it is universal - that is, it applies to all objects.
     
    Last edited: Dec 9, 2013
  11. Dec 9, 2013 #10

    Philip Wood

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    Air doesn't get in the way of the gravitational forces between the fixed and moving spheres. Nor does it contribute a significant force on the spheres, because of its very low density, and in any case the apparatus was inside the spherical shell of the atmosphere.

    Moving air would have affected the torsion assembly, because the torsion fibre had to be very delicate - very small torques would twist it.

    Sorry if previous post cased confusion.
     
    Last edited: Dec 9, 2013
  12. Dec 9, 2013 #11

    ChrisVer

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    The essence is in an answer already given. The earth wouldn't cause motion across the horizontal plane.
     
  13. Dec 11, 2013 #12
    I am asking you one questions.

    CASE : 1

    If You have one iron ball and another is tennis ball. if you put two balls near each other. then why tennis ball not attracted by iron ball.

    No. Tennis ball not attracted by iron ball. what is the reason.

    CASE : 2

    From a rigid support a long rod is suspended using a thin metallic wire. Two small equal lead sphere are attached at the ends of the rod. Two other equal large lead spheres are brought near the small spheres on opposite sides at equal distances. The forces on the small sphere due to the larger sphere are equal in magnitude and opposite in direction.

    And here you can see the attraction or no attraction.
     
    Last edited: Dec 11, 2013
  14. Dec 11, 2013 #13

    Bandersnatch

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    Yes, the balls are attracted to each other. It's just that in typical circumstances there's too much friction to notice.

    A tennis ball(60g), placed 1mm away from a 1kg iron(or whatever) ball is attracted to it with the force of gravity approximately equal to [itex]F_G=4.2*10^{-6}N[/itex] . Assuming rolling resistance coefficient of the tennis ball to be 0.005, the rolling friction force is roughly [itex]F_{rf}=3*10^{-3}N[/itex], which is about 1000 times too high to let the ball move.

    Cavendish suspended the metal balls on a thin string, so as to make sure the friction would be negligible, and the force of gravity would move the balls.
     
    Last edited: Dec 11, 2013
  15. Dec 11, 2013 #14

    Bandersnatch

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    On a second thought, that's actually a gross overestimate of the force, as it assumes both balls to be a point masses, which they obviously aren't.
    For an average 1kg iron ball(density 7.5g/cm^3), which should be about 6cm in diametre, similar to the size of a tennis ball.
    The force of gravity would be then another 4000 times lower, totalling about 0,000000001 Newtons.
     
  16. Dec 11, 2013 #15
    If cavendish suspended the tennis ball on a thin string and one iron sphere brought near to it. then tennis ball will attract to iron ball ?
     
  17. Dec 11, 2013 #16

    Bandersnatch

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    In principle, yes. But you need to use a symmetrical setup as in the youtube links you provided earlier, with two balls connected by a rigid bar, suspended in the middle.
    With just one ball on a string as in a pendulum, you won't be able to see the effect of gravity between the balls(unless you've got VERY accurate instruments), as the gravity of Earth won't let the tennis ball move a whole lot.

    Also, note that the balls(and everything else) always "attract" each other. It's just that usually the force is too low to cause visible displacement.
     
  18. Dec 11, 2013 #17
    But cavendish done this experiment. Gravity also playing a role and he noticed the attraction.

    What kind of instruments used by cavendish ? And in which situation he has done?
     
  19. Dec 11, 2013 #18

    jtbell

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    Wikipedia has a page about Cavendish's experiment, with a description and pictures of the apparatus. A Google search should turn it up.
     
  20. Dec 11, 2013 #19

    Bandersnatch

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    Like this one:
    Only without the lasers:
    http://en.wikipedia.org/wiki/Cavendish_experiment
    or here:
    http://www.youtube.com/watch?feature=player_detailpage&v=4JGgYjJhGEE#t=69

    Notice that he made sure to eliminate the influence of Earth's gravity on the displacement of the rod by arranging the experiment to only cause the displacement in a plane perpendicular to Earth's gravitational force - the Earth pulls the balls down, but they are so arranged to be unable to move up or down.
    If you had just a pendulum(1 ball on a string), there'd be a component of Earth's gravitational force acting on the displaced ball in the horizontal plane, and since Earth's gravity is so much stronger than what you're trying to measure, it would dominate the experiment and you'd see nothing.
     
    Last edited by a moderator: Sep 25, 2014
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