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Homework Help: Very basic tension question, i don't get the theory

  1. Mar 11, 2010 #1
    1. The problem statement, all variables and given/known data
    a)if an elevator is moving upwards, will the weight force be greater that the tension force in its cable?
    b) assume
    an elevator of mass 1500kg is accelerating upwards at 2.5ms^(-2) find T. is this greater than the weight force?

    2. Relevant equations


    3. The attempt at a solution

    so, I know the weight force, with the elevator in a free body diagram is
    f = ma
    f = 1500x-9.8
    and i know that if the elevator is acelerating upwards it must have an unbalanced force upwards. since there are only two forces acting, tension, and weight, then the tension must be greater than the weight to be unbalanced.

    but, when i try to calculate tension, using f = ma
    f = 1500x2.5
    this leaves me with a smaller force than the weight force. how does this make sense?
    can anyone, just explain the basic ides of tension. i interperate my book to say it acts in two directions, how can one force do that? obviously this question is designed to give people the problem im having! thanks
  2. jcsd
  3. Mar 11, 2010 #2


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    Gold Member

    You must remember that Newton's 2nd law is not f=ma; the law states that f_net = ma, where f_net is the resultant vector sum of all the forces acting on the object. For the elevator, there are 2 forces acting. One of them is the weight force acting down, which is equal to 1500(9.8) N. What's the other one? Then apply the law. Regarding the 2-directional nature of the force, you are apparently misreading the text, perhaps you are looking at a section of the rope in free body diagram, where one force T pulls on the rope to the right, and the other force T pulls on the rope to the left. If I apply a tension force of 100 N to an object, by attaching a rope to it and pulling away from it, then the object must apply an equal but opposite tension force through the rope on me, pulling away from me. That's just Newton's 3rd law, which applies to any force. Tension forces always pull away from the objects on which they act.
    Last edited: Mar 11, 2010
  4. Mar 11, 2010 #3
    ok, thanks. so the tension force a part of the net force. ok, i can see how that works.
  5. Mar 11, 2010 #4
    thats helped me with another question to, cheers
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