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I How does pendulum go slack above the horizontal?

  1. Nov 4, 2016 #1
    So say a pendulum consisting of a non rigid string and mass bob is swung above the horizontal position. It's given an impulse at the bottom so that in the swing, there is nothing but tension and gravity acting. At above the horizontal, the gravity is pointing down while the tension can only point radially inwards. There is nothing causing the rope to have tension then at this point. Gravity is pulling down. So what is pulling radially outwards to oppose the rope? So why is there tension?


    at below the horizontal, there is tension because the weight is directly pulling on the rope. this makes sense.

    But not above the horizontal.
     
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  3. Nov 4, 2016 #2

    dextercioby

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    Do you know about pair forces? What are they and how do they influence the motion of the massive bob?
     
  4. Nov 4, 2016 #3

    russ_watters

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    Your post and title seem to contradict each other...But you haven't described the pendulum's motion so it isn't clear to me what your issue is.

    When the pendulum is swinging below the horizontal, there are two components that provide tension, not one: You have the gravity, but missed the centripetal acceleration of the weight.

    Above the horizontal, while still swinging up, the centripetal force is still there, pulling the weight in a circle. But when it slows down enough that the centripetal acceleration no longer exceeds the acceleration due to gravity, the string goes slack.
     
  5. Nov 4, 2016 #4
    Sure newtons third law. But that doesn't explain why its taut above the horizontal. If anything, pushing force on the rope from the bob should make it go slack very quickly.
     
  6. Nov 4, 2016 #5
    Yes the centripetal force is still there. That is why there must be tension. I understand that.

    But doesn't explain why there is tension when there is nothing pulling the rope.It's as if the rope knows to pull by itself.
     
  7. Nov 4, 2016 #6

    russ_watters

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    What do you mean there is nothing pulling on the rope? You're talking about a pendulum: it primarily consists of a large mass. That's what is pulling on the rope always!
     
  8. Nov 4, 2016 #7
    Oh wait, I think i got it. The bob is going up, pulling on the rope, and the rope pulls back.
     
  9. Nov 4, 2016 #8

    Dale

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    You are reasoning as though it were in static equilibrium. It is not. It is accelerating.
     
  10. Nov 4, 2016 #9
    Yeah I just got it. The motion of the bob is opposing the rope, causing the rope to be taut. There doesn't need to be a force component in the opposite direction of the rope. Just motion itself
     
  11. Nov 4, 2016 #10

    dextercioby

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    Above the horizontal, there's a net component of gravity increasing the pressure applied by the bob on the string, thus deforming it. You can see that when the bob is straight up, with the full gravity force of the bob pressing on the string and deforming it.
     
  12. Nov 4, 2016 #11

    A.T.

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    Just like your chair knows to push up on you.
     
  13. Nov 4, 2016 #12
    So that implies that the bob is pushing on the rope. So should not the rope push back?
     
  14. Nov 4, 2016 #13

    dextercioby

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    The rope pushes (acts on) back on the bob, it's the tension force for you put in Newton's second law.
     
  15. Nov 4, 2016 #14
    But then that would be a repulsive force.
     

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    Last edited: Nov 4, 2016
  16. Nov 4, 2016 #15

    dextercioby

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    What do you mean by repulsive ? That term makes sense to me only in electrostatics (the force between two charges of equal sign is called repulsive).
     
  17. Nov 4, 2016 #16
    pendulum.png



    In the pic, the tension is pulling here. But if the bob pushes on the string to the right to deform the string then the string must push to the left on the bob. But that isn't what happened in the pic
     
  18. Nov 4, 2016 #17

    russ_watters

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    The string tries to push back on the bob, it just isn't very good at it. So the bob falls.
     
  19. Nov 4, 2016 #18

    So the string is pushing and pulling at the same time. The pushing force is a reaction to the weight pressing down and the pulling force is a reaction to the bob trying move away
     
  20. Nov 4, 2016 #19

    russ_watters

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    No, at all times there is one resultant force on the string. It is either pushing or pulling.
     
  21. Nov 4, 2016 #20
    So when the mass is in orbit, the weight is not pressing down on the string. These moments would have the string pulling.

    But as it slows, the weight will matter since the bob would fall into the string, making it start to buckle, and the string will be pushing here not pulling. But this is where the string goes slack?
     
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