In physics, when a constant force F moves an object a distance d in the same direction as the force, the work done is defined as the product:W = FdIn many practical situations, the force is not constant, but varies as the object moves. Suppose the object is moving along the x-axis from a to b subject to a variable force of magnitude F(x) at point x, where F is a continuous function. So how much work was done? we divide the interval [a, b] into small pieces ?x in which the force would act in a constant way, that is: