Application of Newton's second law of motion requires that we should have detailed information about the motion at each point of the trajectory. For this reason, we mostly restricted its application to cases, where trajectory was linear and acceleration was constant. We also limited our discussion of two dimensional motions to the cases of parabolic and circular motions only. As a matter of fact, the set of equations (like v = u + at) relating attributes of motion is developed only for constant acceleration.

On the other hand, the concept of energy is a property of state at a given instant or position and is described by the parameters of state at that point. Similarly, work involves displacement - a quantity that depends on the end points of a physical process (not on the intermediate points). Then, we have other concepts like work - kinetic energy theorem, potential energy and conservative force that we shall study in subsequent modules. These concepts allow us to make calculation based on end points without considering details of intermediate points, when force involved is conservative in nature. Incidently, important forces like force of gravitation and electromagnetic force are conservative in nature.

These concepts, therefore, gives a huge advantage to deal with motions, which are subjected to variable force system like pendulum, spring force and vertical circular motion. Also, concepts related to energy and work allow us to deal with motion on irregular trajectories, provided we have the requisite work and energy information at end points. We shall illustrate the working of these concepts in appropriate contexts.