A stationary object begins moving because an unbalanced force acts on it.
Acceleration is the result of an unbalanced force acting on the object.
If the forces become unbalanced, the object will accelerate and change speed, direction, or both. Net forces change an object's energy. Gravitational potential energy is the energy related to how high an object is above a reference surface. The higher the object is the more gravitational potential energy it has. The total energy in a closed system remains constant.