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Questions about tensile forces

  1. May 7, 2009 #1
    Hello again PF, I have some questions I have been wondering about.

    First of all, if you have a situation like this where a weight is hanging from a taut rope, which direction would the force of tension be in?

    http://img217.imageshack.us/img217/4074/tensionforce.jpg [Broken]

    The way I understand it, the tensile force only acts in the opposite direction of another force. The only forces I see here are that of the weight of the mass, and the tensile force of the rope which is directly suspending the block. What about the tensile force of the brown rope? It doesn't seem to make sense to have a tensile force in the up or down direction since it is going from side to side. But on the other hand, it seems there are no forces acting in the left/right direction to create a tensile force in those directions.

    And what about when the mass is heavier and causes the rope to bend slightly like in this picture?

    http://img528.imageshack.us/img528/931/tensionforce2.jpg [Broken]

    In what direction would the tensile force of the brown rope be and why?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 7, 2009 #2

    PhanthomJay

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    Whether the rope is taut or slack, tensile forces always pull away from the objects or points on which they act, and along the longitudinal axis of the rope for the (theortical) taut case, or along the tangent to the curve for the 2nd case. The left to right horizontal components of the tensile force in the brown rope cancel.
     
  4. May 7, 2009 #3
    The situation in your top figure cannot exist with a non-zero mass hanging in the middle; there must be some deflection.

    At the point of attachment, the forces in the rope must always have a vertical component. Typically "ropes" are considered "flexible" which is to say that that they have no bending stiffness at all (an assumption), so that at the point of attachment the FBD consists of three forces: the weight down, the tension to the left and up, and the tension to the right and up. The upward angle may be very small, but it cannot be zero.
     
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