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Newtonian Physics tabletop demo

  1. Apr 6, 2005 #1


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    I wish your help in creating a tabletop Newtonian physics demo.

    My friends do not believe that the centrifugal force is imaginary. They believe that, if you swing a pail of water in a circle over your head and let the rope go, the pail will sail *away* from you (as opposed to tangential to the circle).

    (see diagram)

    I am sure that the point of ambiguity is that path A is perceived as if it is really path B, that the confusion lies in not knowing exactly where the snip happens. It is key to understanding that the rope and the path of the bucket are exactly perpendicular to each other a thte point of snippage. This would demonstrate that there is no outward force acting on the bucket - only inertia keeps it moving.

    I wish to demonstate this in a simple and elegant way.

    It needs to have
    - frictionless rotation
    - a way of cutting the centripetal force bearer (i.e. the thread) while clearly not affecting the path or imparting any other forces
    - precision, inasmuch as I can show the point of snip, and the precise resultant trajectory

    I saw this in a high school film, where they used a (frictionless, dry ice-powered, air hockey puck-like) device tied to a post by a thread, and they used a candle to cut the thread. I do not have this equipment.

    My next thought was a bicycle wheel set on its axle, with a marble in a box, and a trigger. This would work, but it's large, heavy and dirty.

    Can anyone think of a simple, elegant tabletop mechanism I can setup to demonstrate this?
    Last edited: Nov 28, 2006
  2. jcsd
  3. Apr 6, 2005 #2


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    On the end of the swing-arm, mount a flat plate just above the level of the table. Put a small dimple in it. Put a marble in the dimple. When the centrifugal force gets too high (I hate it when people say its imaginary...), the marble will roll out of the dimple.

    If you want the marble to roll out at a specific point, put a groove in the plate and have something sticking up from the table (like a nail). When the nail goes through the groove, it'll pop the marble out of its little dimple.
  4. Apr 6, 2005 #3


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    And they have damn good reasons for it... :wink:

  5. Apr 6, 2005 #4


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    Yeah - what swing arm?

    Got some ideas about building a simple low-friction rotating device? A lazy susan just isn't frictionless enough and a bicycle wheel is too cumbersome.

    But yeah, the idea of knocking the ball out should do, so long as they're convinced it's not imparting or cancelling any forces. (They hate to be wrong, and they can get desparate.)
  6. Apr 6, 2005 #5

    Doc Al

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    How about the old tennis ball on a string? Have someone spin it around and let it go when it lines up with some target. Not exactly table top, but easy to do. Everyone can take turns until they are convinced that the ball does not travel to the target. (Have the doubters stand tangential to the circle.)
  7. Apr 6, 2005 #6


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    What about having an arm that is attached to a torsion spring. On the end of the arm is a basket of some kind to hold a marble. Somewhere in the path of the arm, you place a removeable stop.

    - Wind up the arm and hold in place.
    - Put the stop in place.
    - Let go of arm.
    - Arm hits stop, ball goes flying.
  8. Apr 6, 2005 #7
    Doc Al,

    That would work if "the doubters" thought the ball would fly off radially. Just set up two targets outside the circle of rotation, spaced at 90 degrees. He tries to hit theirs, and they try to his (using eachother's iPods for targets and a string with a big rock would be exciting!).

    But according to Dave's diagram, it looks like they say the mass flies off somewhere in between radially and tangentially. Assuming some error, that's usually what it will do, so Dave would be at a big disadvantage.
  9. Apr 6, 2005 #8

    Doc Al

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    Good point.
  10. Apr 6, 2005 #9
    first i would say centrifugal cause of inertia which resisting the change in velcocity it's the same when you in a car then you slowed down surprisly your body will still moving forward , so the centrifugal acceleration is illusory cause of inertia

    heaven eye
  11. Apr 6, 2005 #10
    Just tell them that if the centrifugal force exists, why does the objects do a revolution a round one point? If there was an external force, the object would certainly not follow this motion. Just take a needle and do the string thing. It will almost not budge, since it has a very little inirtia...
  12. Apr 7, 2005 #11

    thats because it has a centural force(pulling force of your hand, gravitation force) is in the opposite of centrifugal (acctually its (i dont know if it was even a force) resist for the velocity which accuring in the circular motion) , whatever if you took string and tide it with a stone then taught it from other side and turned it around you it will rolling around and there is two proofs that there is a centrifugal exist :-

    1) you feel that the stone want to skip from your hand and each time turning it faster then you have to exert more force (touch it strongly).

    2) if thats string made of spring it will be longer each time you roll it faster.

    i want say also thats circular motion has two acceleration in the same time:-
    1- at = the tangent acceleration if you multiply it with its mass it will has force ( centrifugal) . (thats explain why you feel the stone trying to skip out from you).

    2- ac = centural acceleration and its vector to the center of a circle ( to you !!!!!!) (thats explain why moon doesn't skip from the earth

    also i want to say (ac) (in acircular motion) causing of moving objects (with vector and thats another story) and (at) is why moon doesn't skip from the earth

    well you might ask two questions:-
    1) you said (ac)is making moon doesn't skip from the sun what about the earth >>>>>>>>answer is on the earth objects are not segregated of other effects (such asd earth gravitaion (g)).

    2) so what about the moon ( you mean thats there is a gravity in the space??>>>>>>>> well yes and there is (moon distance is not so long) and there is two forces as i think so:-

    f=(G mmoon mearth )/r^2
    (r is the radius from the earth core to the moon core (or to the moon surface actually i dont remember)

    finally i would say for your last question saying thats it has a little inertia >>>>>> well little inertia if the object was small such as stone but if it was a huge inertia such as moon inertia do you think that is it a small inertia (i let this question for you :biggrin: )

    thank you be cause you asked me and know that iam trying my best and iam 16 years old !!!!!

    my regards
    heaven eye
  13. Apr 7, 2005 #12
    1) Wrong. The reason why you need to exerct more force is only due to a combination of gravity and air resistance. While air resistance slows down the object horizontally, gravity drains it downward. That is why when you are turning a string you can't do it without your hand diagonally, relatively to the angle of rotation, moving it (thus a vertical and a horizontal component).

    2) Wrong again. The moon does distance itself from earth constantly. And no the tangential vector is a constant speed. You see, acceleration in a circular motion does not mean that the speed changes, but only the direction. If the vector itself alone would move (it's impossible but let say the object is stationary and the vector moves around it), in this case the speed would change. But since the speed is always the tangential to the object, its magnitude does not change.
    Last edited: Apr 8, 2005
  14. Apr 7, 2005 #13


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    Thanks for the "just tell them" solutions and the "follow this logic" solutions, but there's no point. They need to see it. Unequivocally.

    Any demo where I'm just letting something go at the right point is not going to work. It's way too imprecise. There's no way to show
    - that, in the act of releasing it, there was no extra force imparted, or
    - that it was released at the right point. This is the most likely cause for contention. (See attached updated diagram.) They can easily insist that it followed path B instead of path A.

    Note also that a tennis ball on a string will not demonstrate this well at all. To keep the tennis ball at speed, you cannot remain still, with the tennis ball rotating about your hand, you must drag the rotational point *ahead* of the tennis ball to keep it going. That's easily enough imparted force to destroy the experiment.
    Last edited: Nov 28, 2006
  15. Apr 7, 2005 #14
    Ok I just tought of an experiment that might prove the inexistence of the force. Rotate an object on a string with a little velocity. In the first cycle notice the radius of a certain point. Do the same with the second cycle as well as for the third. You shoud see that the interval is constant. Do the same with a greater initial speed. If you get the same position difference, the centrifugal force does not exist as it would have displaced the object further since it would have had a greater magnitude. This will prove that it is a matter of inirtia and not of force.
    Last edited: Apr 7, 2005
  16. Apr 8, 2005 #15
    thank you very much werg22 for discussion and sorry all if my info was all wrong

    werg can i ask you question what about at (tangent acceleration ) does it change its direction constantly such as velocity

    thank you ...... and thank you all again

    my regards
    heaven eye
  17. Apr 8, 2005 #16
    Relativly to the object, no it always be the same force that directs it inward. Relativly to the center, yes, it moves the same as the tangent speed and the object.
  18. Apr 8, 2005 #17
    DaveC426913, have you considered the experience I proposed? If you have the technical requirement the experience will surly work...
  19. Apr 9, 2005 #18

    how is it inward and tangent in the same time!!!!!! :bugeye:

    thank you again mr.werg22
  20. Apr 9, 2005 #19
    There is an acceleration toward the center, and a tangent vector. There is no tangent acceleration.
  21. Apr 17, 2005 #20
    during a circular motion,the speed may be constant,but the velocity is constantly changing.the vector sum of the velocity points towards the center hence the acceleration is also towars the center.hope that helps.
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