British Physics Olympiad Paper 2

Hey
Did anyone else get through to the next round of the british physics olympiad or actually understand how to do the questions because I'm stuck on the question about a pendulum in an electric field ( What is the time period of a pendulum of length l, and bob of mass m and charge Q in a constant electric field horizontal and perpedicular to the x-axis?) Thanx

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http://www.nrich.maths.org/discus/messages/27/7329.html?1072962682