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Homework Help: Calculating tension of moving object

  1. Jun 8, 2015 #1
    So two masses M1 and M2, are attached to each end of a cable, and the cable was hung over a pulley.

    M1 is 5kg
    M2 is 3kg


    The system would be accelerating at 2.5ms-2 but how would you calculate the tension of the cable?

    (g=10, cable is massless, pulley is frictionless)
    Last edited: Jun 8, 2015
  2. jcsd
  3. Jun 8, 2015 #2
    I would guess it is 5*10 + 3*10 = 80N but the system is accelerating
  4. Jun 8, 2015 #3


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    Draw a free body diagram for one of the masses. You know how fast the mass is accelerating so you know the the net force.
  5. Jun 8, 2015 #4
    okay I did not realise tension on both ends can be different, but the question asks for the tension of the cable, would it be the total force acting on the calbe from both ends?
  6. Jun 8, 2015 #5


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    The tension would be different only if there is friction in the pulley.
    For this problem you should assume then that its the same at both ends.
    No the tension in the cable is in opposite directions at the two ends so the total
    tension would then be zero! That is why it is called tension in the cable it pulls
    in opposite directions at the two ends and keeps it under tension. The tension
    in the cable, Tc, and the force it exerts on a masspiece, Tm,
    is the same, but they are in opposite directions, that is they form an action-reaction
    pair according to Newton's 3rd law.
  7. Jun 8, 2015 #6
    The reason that you are having so much trouble with this problem is that you have not drawn free body diagrams of the two masses. Do you feel that you have advanced to the point where you no longer need to use free body diagrams? This problem kind of shows that you haven't.

  8. Jun 8, 2015 #7
    It seems i have misunderstood tension, thanks for the help, I'll remember to draw free body diagrams next times
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