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Having trouble with a simple problem

  1. Oct 16, 2009 #1
    Two insulated balls of mass 0.1 g hang from the same support point by massless insulating threads of length l (as shown in the diagram below.) A total positive charge of 5.70 x 10-07 C is added to the system. Half this charge is taken up by each ball, distributed uniformly, and the balls spread apart to a new equilibrium position.
    prob14a.gif
    Assuming that the balls hang essentially vertical before the charge is added, what is the tension in each thread before the charge is added?




    T=-m*g



    I believe that since the strings are mass less and said to be essentially vertical, then the only equation i should need is Tension=-mass*gravity to get the force of tension on each string. so I have T=-(.1 g)(-9.8 m/s2)=.98 g*m/s2=.98 N but when I enter it onto my course site it tells me I am incorrect. Am i being oblivious to something?
     
  2. jcsd
  3. Oct 16, 2009 #2
    I notice that you multiply 2 negatives in the last line, should not one of these be +ve?
     
  4. Oct 16, 2009 #3
    What is the total force acting per metre of wall when the applied pressure is as sho

    What is the total force acting per metre of wall when the applied pressure is as shown in the diagram? The maximum pressure is 65 kN/m2
     

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  5. Oct 16, 2009 #4
    In my problem i am orienting the gravity to go in a negative direction, thus force on the ball is F=(.1 g)(-9,8 m/s2)=-.98 N, however, the tension in the rope acting on the ball is in the opposite direction (if i am wrong, please explain), so the Tension will be a positive value.

    btw. I tried putting in -.98 N as well on the site and it told me i was incorrect. This is driving me crazy, it seems so simple!
     
  6. Oct 17, 2009 #5
    Are you including the Coulombic force?
     
  7. Oct 18, 2009 #6
    The balls are 0.1 g, not 0.1 kg
     
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