What is needed for work to be done?

A force must push on an object without moving it.

A force must pull on an object without moving it.

A force must cause an object to move along the same direction as the force.

A force must act on an object while ar equal force acts oppositely on the object.

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.