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Tension in a rope

  1. Oct 21, 2007 #1
    First post, so be nice :P

    1. The problem statement, all variables and given/known data
    A box with mas of 20kg is hanging from a rope on a building site. Calculate tension when:
    a) Stationary
    b)Moving upwards at consteant speed of 6m/s
    c)Moving upwards with accelleration of 6m/s

    2. Relevant equations

    3. The attempt at a solution
    I don't know as i have never seen 'tension' used in a question before but i am guessing that for a) you take acceleration as -9.8m/s2 (gravity) and work out force. for b) same as a and c) is -9.8+6 and then into equation.

    I am quite new to this (as you can tell) and got confused with tension. I am currently taking tension as the quivilent of force.Am i right? (P.S: I could use 9.8 as acceleration if i wanted positive answer).
    Last edited: Oct 21, 2007
  2. jcsd
  3. Oct 21, 2007 #2


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    Yes. The tension is the force pulling up on the box. The other force acting is the weight of the box, down. You must draw a free body diagram that identifies both forces, and solve for the unknown tension force using Newton's first law (Fnet = 0) or
    2nd law (Fnet = ma), as applicable. Please watch your directions and use of plus and minus signs. Don't take any shortcuts.
  4. Oct 21, 2007 #3
    so tension is the upward force not downward?
  5. Oct 21, 2007 #4


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    Yes, it acts upward on the box. Tension forces always pull away from the objects they act on. When you isolate the box in a free body diagram, the weight acts down on the box, toward the center of the earth, and the tension force pulls up on the box, toward the top. I understand your confusion; if you look at the hook at the top to which the rope is attached, the tension force, pulling away from the hook, acts down (per Newton 3). It is important to isolate the object in question with a good free body diagram.
  6. Oct 22, 2007 #5
    right thanks for the help! It has cleared up everything perfectly!
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