A Foucault pendulum can be used to demonstrate that the Earth rotates . The time , t , in seconds , that it 2n 67 takes for one swing or period of the pendulum can be modeled by the equation where L is the length of the pendulum in meters and g is a constant of 9.81 m / . The first Foucault pendulum was constructed in 1851 and has a pendulum length of 67 m . Determine , to the nearest tenth of a second , the time it takes this pendulum to complete one swing . Another Foucault pendulum at the United Nations building takes 9.6 seconds to complete one swing . Determine , to the nearest tenth of a meter , the length of this pendulum