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Pulleys Jh

  1. Nov 12, 2015 #1
    1. The problem statement, all variables and given/known data
    https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-xfl1/v/t34.0-12/12231095_1486035821701632_2065647312_n.jpg?oh=dd0658f6afcfe91a852c72c3dab01560&oe=564836A2&__gda__=1447576857_f29a19be54068711687d57ac9593225d

    2. Relevant equations

    None

    3. The attempt at a solution
    None
     
  2. jcsd
  3. Nov 12, 2015 #2

    haruspex

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    You need to do more first. Please read the guidelines.
    What standard equations or theory do you expect to be relevant?
    What have you tried? What's blocking you?
     
  4. Nov 12, 2015 #3
    I just need to explain. I explained that the gravity center changed by this and that the pulley had more distance to go through in one side so there was no balance in the pulley making B statement true needing more force baceuse the lack of equilibrium. They put me a 4/10 saying that i made no effort but i have got another chance to improve. I don't know what else to put . Because i'm just in 7th grade i don't think i need an equation but just to explain. Any help is really welcome.

    What i wrote :
    b) because by changing the axis you are changing the center of gravity of the pulley that causes the need of more force on one side because there is not balance at all. One rope is having more weight than the other causing inequilibrium.
     
    Last edited: Nov 12, 2015
  5. Nov 12, 2015 #4

    haruspex

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    Gravity doesn't enter into it because you are supposed to ignore the mass of the pulley.
    It rather implies you should ignore friction too, but that would not be right. You should ignore friction at the axle, but assume enough friction between rope and pulley that the rope does not slip.
    You are on the right lines there. You can you put that more clearly?
    Not necessarily an equation, but some relevant theory of mechanics that you may have come across. Think levers
     
  6. Nov 12, 2015 #5
    Forget that , i was wrong.

    So , it would be b) but not because of the center of gravity but because of the rope friction not allowing the rope to move.

    What can i put? I could say that it does not have a mechanical advantage and that you need more force because of the friction.
     
  7. Nov 12, 2015 #6

    haruspex

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    No, no. The pulley is still free to turn on its axis, it's just that the axis is not in the centre. Don't worry about the friction - I was just pointing out that the rope does not slip on the pulley.
     
  8. Nov 13, 2015 #7
    so i was basically right at the beginning ?
     
  9. Nov 13, 2015 #8

    haruspex

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    The part I quoted in post #4 was getting there, but you weakened your original answer by mentioning several things that are not relevant too. Just concentrate on what it was you meant by the distances being different, and see if you can spell it out more clearly. You mentioned mechanical advantage....
     
  10. Nov 14, 2015 #9
    Sometimes it helps to take an extreme example.
    Suppose the axis were moved to the point where the left string contacts the pulley.
    Then there would be no way to keep the pulley from tipping - right?
     
  11. Nov 17, 2015 #10
    Thanks , this was used by my teacher to tell compound bows and how they work
     
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