DEFINITION
For everyday situations close to the earths surface we consider the gravitational field to be constant, so that the force exerted on a body is equal to mass multiplied by a constant g, this constant has a value of about 9.8. Hence when a object is lifted a distance h vertically, it does so against a force of m g, and so by the definition, work is done on the object. This work is equal to the Force (m g) multiplied by the distance (h), hence