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Easy thought experiment imposible fro me please help

  1. Apr 29, 2005 #1
    i have a ball over a frictionless table tight to a cable tight to a frictionless axe in the middle of the table

    if the ball weights 1 kg, the rope is 1 m long and the linear speed of the ball is 100 m/s the tension on the rope will be 10000N (v*v*m/r)

    if i want the rope tension to reduce to 100 N keeping the speed of the ball what kind of outwards spiral do i get? can i aply any formula?

    if i want the tension to be constantly 20000N what kind of inwards spiral do i get?

    thanks in advance to anybody who helps me

    :smile:
     
  2. jcsd
  3. Apr 29, 2005 #2

    FredGarvin

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    Science Advisor

    Use the equation you have already stated and resolve it for rho:

    [tex]F_c = m \frac{v^2}{\rho} [/tex]

    [tex]100 N = (1 kg) \frac{100 \frac{m}{s}}{\rho}[/tex]

    The same holds true for your other question.
     
  4. Apr 29, 2005 #3
    the problem is that the radius varies so i dont know how to solve it

    what i would really like to obtain is the variation of radius per turn


    besides aplying that formula i would get that the Fc of an inwards an outwards spiral (the same medium radius and the same v)would be the same while in reality you just have to spin a weight over your head in an inwards and outwards spiral to notice that the force or tension in the cable to be totally different
     
  5. Apr 29, 2005 #4
    may be im wrong now i think youve helped me a lot
     
  6. Apr 29, 2005 #5
    would it be then correct to say rho goes from 1 m to 100m when the Fc is equal to 100N and the speed is 100 m/s and ball weight 1 kg?

    now my question is how long or how much turn has it taken it to reach from a radius of 1 m to a radius of 100 m?

    i would apreciate very much someone answered to this last question my suposition is a quarter of turn which is yours
     
  7. Apr 29, 2005 #6
    i think i have the solution to the problem now:

    lest divide the trajectory of the ball in four quarters

    in the first quarter the ball goes from a radius of 1 m to a radius of 100 m, lets supose the tension to be 100 N (certainly the force will be lower than 10000N)

    in the second quarter the radius goes from 100 m to 1 m i think the tension will be 10000N( certainly the force will be bigger than 100N)

    in the 3rd and 4th quarter the tension will certainly be 10000N (being circular the trajectory) and then the first quarter repeats

    if we put wheels to the table we have 3 quarters of spin with a tension of 10000N and 1 quarter with 100N

    wouldnt that mean that the table would move away from the force of 100N?

    and besides if the table was frictionless what would stop the ball to spin and to acelerate the table?
     
  8. Apr 30, 2005 #7
    another thing has to be taken into account, in the first and second quarter the ball remains longer in the quarters because the trajectroy is longer being the speed of the ball the same, so the tension of 10000 and 100 N is aplied longer in the 1st and 2nd quarter tha in the rest

    this would mean that the aceleration of the table would not only be away from the 100N force of the first quarter but would also go towards the second quarter as well (being the tension in the first quarter just 100 N and in the second 10000 and the time the ball stays in 1st and 2nd quarter the same, the ball will always pull more in the second quarter than in the first)


    jesus this is a monologue
     
  9. Apr 30, 2005 #8

    Doc Al

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    Staff: Mentor

    Take a huge table on frictionless wheels. Get on it with your ball, cable, and anything else you might like. Spin and twirl the ball any way you want. If you start from rest, the center of mass of the system will stay put. The tension force is internal to the system and won't move the center of mass.
     
  10. May 1, 2005 #9
    i apreciate and respect your opinion

    but your reason is that a postulate says so

    its much older the 5th postulate that only one paralel line can go trough a given spot outside the straight line

    euclides said so in 300 Bc and it was undiscussed untill gauss in 1700 discovered it was wrong but didnt dare to publish it due to the trascendence of it specially with church ideas

    it would be lobachewsky in th 19th century who would change the 5th postulate saying that by an exterior point to a straight line infinite paralel lines can go

    this meant the separation of linear drawing and mathematics and a revolution

    think this mistaken postulated was kept for 22 centuries

    newtons principles are much younger

    so postulates can fail

    in a circular trajectory the tensions are of 10000N in each quarter the adition of it results 0

    in a linear trajectory there would be 10000N in each side which adition gives 0

    i have found an example in which the internal forces dont add 0 and it was not easy

    and logic deserves a chance in front of faith maybe my idea have not been thought of before

    the fact that nobody is able to answer how many revolutions does it take to go from a radius of 1 to a radius of 100m after you switch tension from 10000N to 100N proves this is not studied generally

    as nobody studies either in college why by moving your feet a playground swing rocks
     
  11. May 1, 2005 #10

    Doc Al

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    Staff: Mentor

    Give us a break.

    My answer is based upon the conservation of momentum, one of the most experimentally verified laws of physics. It, in turn, is a result of an even more fundamental translational symmetry (see Noether's theorem). It is hardly "a postulate" likely to be overturned just because of someone's confused misunderstanding of elementary mechanics.

    Sorry, but your idea about unbalanced tensions in the spiraling ball just won't work. Sure, you can stand there and twirl a ball on a string so that during part of the trajectory the tension pulls harder than another part. But the only thing that allows you to do that is the friction holding you in place. Try doing it without the friction, as a means of propulsion; it won't work.

    If you are serious about understanding how swings work (yes, there are some subtleties), there have been many articles published over the years. Go to the library; search through the Amer. Journal of Physics, starting in the late 60s.

    I'm moving this to TD. If you are not interested in discussing physics, please stop posting the same rant. It's not going to fly here.
     
  12. May 1, 2005 #11
    "But the only thing that allows you to do that is the friction holding you in place"


    i dont understand this i thought both the table and its wheels were frictionless and i still can hold any tension i want in the cable because the axe is tuck on the table


    all right according to conservation of momentum the ball can have any radius and still keep a speed of 100 m/s

    but what would happen according actual physics to the balls kinetik energy if the radius was to be 0?

    it should spin in the axe having transformated the linear kinetik energy into spinning kinetik energy, it must be this way unless you tell me that energy disapears

    so we are onto something new, linear movement can be transformed into spinning movement, linear to spin

    would it be very crazy to say the oposite? than spin can be transformed into linear speed?

    if this was true we could have one half of the table of linear speed=tension on the axe and the other half of spin= no tension on the axe

    so there would be 0 tension in one half and tension in the other

    besides according conservation of energy linear kinetik energy can be transformed into spin and therefore probably the oposite as well

    if this is true what happens seem so obvious that i wont say it
     
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