Work which is done to accelerate an object from rest to a velocity v is converted to kinetic energy this fact is called

The work-energy theorem

Explanation:

The change in the kinetic energy of the body is equal to the work done by the sum of all forces that caused displacement in the position of the body.

The applied force changes the velocity of the body with an acceleration. This force displaces the body from the initial position to the final position.

The work done on a body is the change in its Kinetic energy.

ma S = ½mv² - ½mu²

This relation between the work done and the kinetic energy of the system is called the work - energy Theorem.