1. The problem statement, all variables and given/known data http://img267.imageshack.us/img267/5830/80849533.jpg [Broken] Picture. The volume of that cylinder is V = 1l. In that cylinder we have 1g of hydrogen m=0,001 kg. The cylinder is separated in two equal parts with m1=0,005 kg and d=0.006m length piston. Imagine that we pushed that cylinder and piston in there began to swing. We need to calculate swings period dependence on temperature of the gases. 2. Relevant equations We need to find k in this formula. F= -k x ( it's the force, which trying to return piston back to his place) And place it in this formula T = 2 Pi sqrt(m1/k) 3. The attempt at a solution In my opinion that F is the difference of pressures in that cylinder. I would calculate both pressures with this formula: p V = m/M R T and then I would deprive. F would be : F = (delta)p S But when I calculate, my V and m depends on the x, and he disappears. In my head there are no more thoughts, so I'm asking you for help. P.S I tried to put symbols of Pi and delta, but instead of them in the post there were question-marks. Why?