During the night when demand for electricity is low, a wind farm may be generating a large amount of power. explain how, by using another type of power station, this power could be stored and using when it is needed.

The period of a pendulum varies directly as the square root of the length of the pendulum and inversely as the square root of the acceleration due to gravity. find the period when the length is 144 cm and the acceleration due to gravity is 980 cm per second squared, if the period is 7pi seconds when the length is 289 cm and the acceleration due to gravity is 980 cm per second squared.