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Understanding the concept of tension

  1. May 19, 2009 #1
    Hi everyone..
    I've run into a bit of a problem while studying the concept of tension.
    If a block which had a weight of 100N was to be hung from a rope which in turn was tied to say, the branch of a tall tree what would the tension in the rope be? According to all the text books it should be 100 N. This is where I get confused.
    According to newtons third law if the block exerts a force of 100N downwards then the branch of the tree must exert a force of 100 N upwards. This would mean that the rope would be pulled upwards with 100 N as well as downwards with 100 N. In that case why isn't the tension 200 N?
  2. jcsd
  3. May 19, 2009 #2
    Tension (and compression), requires two forces acting in oppositie directions. A rope being pulled with 100N and not supported at the other end will just move in the direction of the force and the tension in the rope will be zero. If it is restricted from moving the tension in the rope increases as you increase the pulling force so pulling with 100N will produce a tension of 100N in the rope.

    In your example, the tree is resisting the 100N force so that rope remains stationary.

    Just like if I am standing on the earth and the mass of my body under the acceleration due to gravity creates a downward force on the earth. The earth applies an equal and opposite force (otherwise I'd start moving in the direction of my weight (sinking into the earth)), but my legs are not compressed by a force that is double my weight.
  4. May 19, 2009 #3
    Thanks for your reply..
    yes.. the tree applies a force of 100 N but the object is also pulling down with 100 N. Wouldn't that be considered as two opposite forces?
  5. May 19, 2009 #4
    a tension is the force exerted by the rope on the tree: 100N
  6. May 19, 2009 #5
    The way I think of it, there are 4 forces acting:

    The weight of the block pulling down (100N)
    The tension of the string acting on the block to stop it falling (100N)
    The branch of the tree pulling up on the string to stop the rope falling (100N)
    The tension of the string pulling down on the tree to stop it floating away (100N)

    The tension in the string is pulling both ways, and it is only at either end of the string that they don't cancel each other out. (If you pinch the string in the middle while it is holding the block, you wont feel the string pull up or down, because there is no net force).

    On second thoughts this post might confuse you even more. It has confused me a little :)
  7. May 19, 2009 #6


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    Staff: Mentor


    Consider a 1N book sitting on a table: the book exerts a force of 1N on the table and the table exerts a force of 1N on the book. What is the total force between the book and table?

    What you are missing here is that forces always come in pairs.
    Last edited: May 19, 2009
  8. May 19, 2009 #7
    The total force would be zero (right?). 1N downwards + 1N upwards
    But that would mean that the tension of the rope is also zero. (100 N up and 100N down). ???
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