1. The problem statement, all variables and given/known data A small m=1.70×10-3 kg plastic ball is suspended by a l=20.0 cm long string in a uniform electric field, as shown below. If the ball is in equilibrium when the string makes a θ=13.1° angle with the vertical, what is the net charge on the ball? Take E=1.04×103i N/C. http://capa.physics.mcmaster.ca/figures/sb/Graph23/sb-pic2352.png [Broken] 2. Relevant equations Restoring force= mgsin theta?? E=F/q 3. The attempt at a solution I'm really lost on how to start this question, I first thought that since the pendulum is in equilibrium, it means that the horizontal forces would be equal, so these forces being the restoring force and the electric force exerted by the electric field... And then use the force to solve for q in q=F/E... but that wasn't right. Then I thought well since it's in equlibrium, maybe the potential energy of the pendulum is somehow suppose to play a role in it, but once I find the potential energy, how can i relate that to the equation to force and electric field to finally get the charge on the ball?