13.1 Work and energy

Work is an important concept in analyzing physical process. We must know that we have actually made transition to a different analysis framework than that of force, which is based on Newton's laws of motion. What it means that our analysis will ,now, be governed by new set of laws - other than that of laws of motion. Before we discuss details of these laws, we need to first establish new concepts that consitute these laws. Work and energy are two important concepts that will be largely employed for building this new analysis technique.

We have already learned a bit about work. Therefore, we shall introduce the concept of energy first and then discuss relevance of work in the context of energy. Subsequently, we shall accomplish the important step to connect the concept of work with that of energy through a theorem known as "work - kinetic energy theorem" in a separate module.

Energy

Energy is one of two basic quantities (besides mass) that constitute this universe. Because of the basic nature and generality involved with energy, it is difficult to propose an explicit definition of energy, which is meaningful in all situations. We have two options either (i) we define energy a bit vaguely for all situations or (ii) we define energy explicitly in limited context. For all situations, we can say that energy is a quantity that measures the "state of matter".

There are wide verities or forms of energy. The meaning of energy in particular context is definitive. For example, kinetic energy, which is associated with the motion (speed) of a particle, has a mathematical expression to compute it precisely. Similarly, energy has concise and explicit meaning in electrical, thermal, chemical and such specific contexts.

We need to clarify here that our study of energy and related concepts presently deals with particle or particle like objects such that particles composing the object have same motion. We shall extend these concepts subsequently to group of particles and situation where particles composing object may have different motions (rotational motion).

For easy visualization, we (in mechanical context) relate energy with the capacity of a body to do work ("work" as defined in physics). This definition enables us to have the intuitive appreciation of the concept of energy. The capacity of doing work, here, does not denote that the particle will do the amount of work as indicated by the level of energy. For example, our hand has the capacity to do work. However, we may end up doing "no work" like when pushing a building with our hand.

On the other hand, thermal energy (non-mechanical context) of a body can not be completely realized as work. This is actually one of the laws of thermodynamics. Hence, we may appreciate the connection between energy and work, but should avoid defining energy in terms of the capacity to do work. We shall soon find that work is actually a form of energy, which is in "transit" between different types of energies.