For small amplitudes the oscillation period, T, of a pendulum only depends on the length, L, and on the gravitational acceleration, g, and is given by the following formula: 旼컴컴컴컴컴컴컴컴컴컴컴컴컴커 � T = 2 * pi * sqrt( L/g ) � 읕컴컴컴컴컴컴컴컴컴컴컴컴컴켸 where pi = 3.14159... The derivation of this formula requires some calculus and concepts such as moment of inertia, angular momentum etc. But you can get a good idea of how it works by just knowing that ** acceleration = force/mass **. The force that drives the motion is the tangential component of the weight. The radial component is canceled by the tension of the string. Notice that the tangential component always points towards the equilibrium position and thus accelerates the pendulum while moving towards the middle and decelerates it while moving away. Why doesn't the period depend on the mass? See what happens when you change the mass by pressing F2. Notice that the force changes in the same proportion as the mass. Therefore the acceleration = force/mass (yellow arrow) will be the same as before at any point. Since the acceleration is what causes the velocity to build up or to decrease; the velocity will also be the same as before at any point, the motion will not change and the period will remain the same.