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I Force exerted by a rope wound around a disk

  1. Jun 28, 2018 #1
    f-d%3A8f4bdd88e2eba7947871ec3e3b57edd423e0b13f6633b10101af9c50%2BIMAGE_TINY%2BIMAGE_TINY.png The above picture shows the forces on a disk supported by a rope which is wound around it. The tension T is shown to be acting at the point where the rope loses contact with the disk. I think the reason for this is that the friction is assumed to be huge so that the tension in the rope also acts on the disk. But there is also the reaction from the rope and since the rope is wound around the entire circumference we may think that it cancels out but I think that the reaction from the rope at the bottom should be greater than at the top due to the weight of the disk. How to account for all these forces? And if it is true that the net force by the rope on the disk is just the tension T as shown then how can I prove it precisely?
  2. jcsd
  3. Jun 28, 2018 #2


    Staff: Mentor

    Accounting for those forces would indeed be very difficult. So I would take a different approach. Instead of considering the rope and the disk to be separate systems, I would consider them to be part of the same system. Then all of those forces between the disk and the rope become internal forces. The remaining external tension force is clearly acting along the line of the rope as shown.
  4. Jun 28, 2018 #3


    User Avatar
    Gold Member

    Dale gave a really good answer.
  5. Jun 28, 2018 #4
    Thank you very much. I completely missed that.
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