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Homework Help: Tension in the cord?

  1. Sep 25, 2007 #1
    Tension in the cord???

    An elevator starts from rest with a constant acceleration upward and moves 1m in 1.8s
    A passenger in the elevator is holding a 9.7kg bundle at the end of a vertical cord
    Acceleration of gravity is 9.8m/s squared
    What is the tension in the cord as the elevator accelerates? answer in units of N

    so i calculated the acceleration to be .55555m/s squared upward

    and the downward acceleration is 9.8m/s squared

    and the mass is 9.7 kg

    so where do i go from here
     
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  3. Sep 25, 2007 #2

    learningphysics

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    What are the two forces acting on the object hanging by the cord?

    you know the acceleration of this object is 0.5555m/s^2 upward.
     
  4. Sep 25, 2007 #3
    and gravity is 9.8m/s^2 downward
     
  5. Sep 25, 2007 #4
    so those are the 2 forces acting on the object
     
  6. Sep 25, 2007 #5

    learningphysics

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    gravity and???
     
  7. Sep 25, 2007 #6
    weight of the object
     
  8. Sep 25, 2007 #7
    so the tension on the rope is the difference between .5555 and 9.8*9.7kg????
     
  9. Sep 25, 2007 #8

    learningphysics

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    that's gravity... you have the weight of the object, and the tension in the cord... so what is the [tex]\Sigma{F} = ma[/tex] equation for the bundle?
     
  10. Sep 25, 2007 #9

    learningphysics

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    sum of all forces = ma

    What goes in the left side of the above equation? What goes in the right side?
     
  11. Sep 25, 2007 #10
    the bundle force is 95.06N??? and then what do i do with the acceleration?
     
  12. Sep 25, 2007 #11
    ok i think i got it...

    the upward is .555555* 9.7

    and the downward is 9.8*9.7

    ?
     
  13. Sep 25, 2007 #12

    learningphysics

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    That is not one of the individual forces acting on the cord...

    Remember, the vector sum of all the individual forces = ma

    There are two individual forces here. Tension and gravity. Let tension = T. So what is the vector sum of tension and gravity?
     
  14. Sep 25, 2007 #13
    the vector sum equals 0???

    so the gravity is 95.06 and the tension on the cord is -95.06
     
  15. Sep 25, 2007 #14

    learningphysics

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    no.

    just using T and mg... write out the vector sum arithmetically. don't worry about the ma for now... what is the vector sum of the tension and mg, written in terms of T and mg...
     
  16. Sep 25, 2007 #15
    vector sum = T + MG
     
  17. Sep 25, 2007 #16

    learningphysics

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    it should be T - mg. because tension acts upward... mg acts downwards...

    So T - mg = ma

    remember... all force problems are like this... the vector sum of all the individual forces = ma... you shouldn't think of ma as another force. All the individual forces add up to ma...
     
  18. Sep 25, 2007 #17
    ok so .... then T = MA + MG

    and in this problem T = (9.7)(.5555)+(9.7)(9.8)

    real fast problem that is multiple choice

    imagine a Y

    with the intersecting point being a box
    and the downward part bein 70N and the two upward ones bein 35N

    will the box experience acceleration?
    a - unable to determine without the angle
    b- Yes; upwards
    c-Yes;downwards
    d- no it is balanced

    i say the answer is D they are balanced
     
    Last edited: Sep 25, 2007
  19. Sep 25, 2007 #18
    after taht multiple choice one i have 3 left... mind helpin me?
     
  20. Sep 25, 2007 #19

    learningphysics

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    why do you say it is balanced? What is the net force in the vertical direction?
     
  21. Sep 25, 2007 #20

    learningphysics

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    sure. np. my computer keeps freezing, so I may be gone for short periods of time... but I'll do my best.
     
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