Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Coulomb's Law question

  1. Dec 31, 2005 #1
    "Two identical small spheres of mass 2.0g are fastened to the ends of an insulating thread of length 0.60m. The spheres are suspended by a hook in the ceiling from the centre of the thread. The spheres are given identical electric charges and hand in static equilibrium, with an angle of 30.0 degrees between the string halves. Calculate the magnitude of the charge on each sphere."

    What I've done so far...
    The length of each part of string connected to each ball is 0.3m. The distance between the two masses is 0.076 approximately. That's all I can think of... I have the distance and the constant for Coulomb's Law. That still leaves me with two variables.... (electrical force and charge). Where do I go from here? TiA
  2. jcsd
  3. Dec 31, 2005 #2
    You need to do a free body diagram to identify the forces acting on the charge spheres.
  4. Dec 31, 2005 #3


    User Avatar
    Gold Member

    You know that the force between the balls is enough to keep them in equilibrium with gravity. Find out how much of the force is needed to cancel out the gravity. With that you can find the force and then the charge.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook