So far we considered the work done by a constant force. In general the force does not remain constant during the process of doing work. For example, as a rocket moves away from the earth, work is done against the force of gravity, which varies as the inverse square of the distance from the earth’s center. Similarly, the force exerted by a string increases with the amount of stretch.Let’s suppose that there is a system such as that, here an object is being pushed along a non-uniform surface. The x-component Fx of the force F is effective in displacing the object on the rough surface. Suppose Fx acts over the space interval, x = a to x = b and varies along the path. When the object moves through the small distance Dx under the action of the approximately constant x-directed force Fx the small amount of work is done is DW = FxDx So there fore we notice that FxDx represents the area.
In general, the path of motion may not be as straight as discussed above. It could be a curve.
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