I am having a problem with the following question: Two identically charged ping pong balls each having a mass of 4.00 g are hung from fine threads, 0.75 m long. What is the charge on each ball so that the angle between the threads is 35 degrees? I don't know how to draw a free body diagram for this problem in such a way that the forces cancel out. Do I just take the vertical component of the gravity and have it cancel with the strictly horizontal electric force, and assume that the vertical component of the gravity is canceled out by the tension of the string? Do I find a way to have both forces acting perpendicularily to the string (and thus tangentially to an arc of motion)? Thanks in advance.