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Homework Help: What charge would have to be fixed at the origin?

  1. Nov 18, 2016 #1
    1. The problem statement, all variables and given/known data
    A ball of mass m is attached to a rope of length L which has one end at the origin; the ball moves in a horizontal circle at constant speed v0 on a frictionless plane; the velocity is always at 90◦ to the rope. The ball has a charge q, and moves in a constant magnetic field which is directed vertically B⃗ = B0kˆ. At the time illustrated, the ball is at location ⃗r = L cos θˆı + L sin θˆ, with velocity ⃗v = −v0 sin θˆı + v0 cos θˆ.

    1. Assume that q = 3.0[C], B0 = 1.0[T], v0 = 5.0[m/s], m = 4.0[kg], and L = 2.0[m], what charge would have to be fixed at the origin in order the tension in the rope became 0?

    2. Relevant equations
    T = mv0^2/L + qv0B0

    3. The attempt at a solution

    I set T to 0, plugged in the values given and tried to solve for q.
  2. jcsd
  3. Nov 18, 2016 #2


    Staff: Mentor

    Please explicitly show what you did and where you got stuck.
  4. Nov 18, 2016 #3
    0 = (4kg)(5m/s)2/2(2.0m) + q(5m/s)(1T)
    10 = q

    which obviously is not the answer. that was just a shot in the dark. I know q is given as 3.0C.. I just do not know how to solve for the charge at the origin.
  5. Nov 18, 2016 #4


    Staff: Mentor

    That's the charge on the ball. You are not being asked to solve for that. You are being asked to find what charge ##Q## (note the capital letter to show that it's a different charge we're talking about) would have to be placed at the origin for the tension in the rope to be zero. In other words, the charge ##Q##, when placed at the origin, should exert the same force on the ball as the tension in the rope does when the charge ##Q## is not there.
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