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Need explaination~ thanks~

  1. Aug 17, 2009 #1
    a) An elevator travels upward at a constant speed. The elevator is hung by a single cable. Friction and air resistance are negligible.

    ii) Is the tension in the cable greater than, less than, or equal to the weight of the elevator? Explain.
    iii) The elevator travels downward and is slowing down. Is the tension in the cable greater than, less than, or equal to the weight of the elevator ? Explain.


    ii) If moving upward i know that the tension is greater then W~ but the acceleration is 0...
    then is the tension = w~? (newton's second law state the acceleration depends on net
    force )

    iii) when the elevator is moving downward with slowing down, that means there consists
    deceleration .. so the the tension of cable is less then the w of elevator <<< is answer
    like this correct??
     
  2. jcsd
  3. Aug 17, 2009 #2

    dx

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    Tension is greater than weight. If the elevator is slowing down, in which direction is it accelerating? In which direction must the net force be so that the elevator accelerates in this direction?
     
  4. Aug 17, 2009 #3
    i) the accelaration = 0
    from equation
    T-W = ma
    so when a = 0
    T-W =0
    W = T
    the tension not equal to W ??

    ii)moving downward...
     
  5. Aug 17, 2009 #4

    kuruman

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    Moving downward, the net force (sum of all the forces) is still T - W. However, the other side of the equation is not zero any more. Taking up as positive and down as negative is ma (mass times magnitude of acceleration) positive or negative?
     
  6. Aug 17, 2009 #5
    this is for question (ii)...
    so the w is greater then tension??

    another one is the question (i)
    is the tension = weight of elevator??
    due to the acceleration is 0
     
  7. Aug 17, 2009 #6

    kuruman

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    Yes. When the acceleration is down, the net force (sum of all the forces) is in the same direction as the acceleration, therefore the weight is greater than the tension.

    Yes. When the acceleration is zero, the tension is equal to the hanging weight.
     
  8. Aug 17, 2009 #7


    thanks for explaning~
     
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