1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    PeterDonis

    User Avatar
    2016 Award

    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

    PeterDonis

    User Avatar
    2016 Award

    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted