These three terms are used frequently in mechanics, must understand their meaning. This is especially important because they are common words too in ordinary conversation, but with rather different shades of meaning.
A force is said to do Work on a body when it moves the body in the direction in which it is acting, and the amount of work done is measured by the product of the force and the distance moved in the direction of the force. Thus if a force of 10 Newton moves a body 2 m (along its line of action), it does 20 Newton meters (Nm) of work. A Newton meter, the unit of work is called a joule (J).
Notice that according to mechanics, you do no work at all if you push something without succeeding in moving it – no matter how long you push. Notice that you do no work if the body moves in the opposite direction or even at right angles to the direction in which you push. Someone else must be doing some pushing – and some work! Notice that brain-work does not count – unless perhaps some pen-pushing is required!
A body is said to have Energy if it has the ability to do work, and the amount of energy reckoned by the amount of work that it can do. The units of energy will therefore be the same as those of work. The amount of work that a body can do should be a same as the amount that has been done on it, that is to say, it should be able to give back what it has been given. Unfortunately it usually cannot do so, but that is beside point at the moment. We know that petrol can do work by driving a car or an aero plane , a man can do work by propelling a bicycle or even by walking, a chemical battery can derive an electric motor which can do on a train, an explosive can derive a shell at high speed from the muzzle of a gun.
All this means that energy can exist in many forms: heat, light, sound, electrical, chemical, magnetic, atomic – and, most useful of all, mechanical. A little thought will convince us how much of our time and energy is spent in converting, or trying to convert, other forms of energy into mechanical energy, the eventual form which enables us to get somewhere.
The human body is simply a form of engine – not a simple form of engine – in which the energy contained in food is converted into useful, or useless, work. Unfortunately there is a tendency for energy to slip back again, we might almost say deteriorate, into other forms, and our efforts to produce mechanical energy are not always very efficient.
Even mechanical energy can exist in more than one form; a weight that is high up can do work in descending, and it is said to possess potential energy or energy of position; a mass that is moving rapidly can do work in coming to rest, and it is therefore said to have kinetic energy or energy of motion; a spring that is wound up. A gas that is compressed, even an elastic material that is stretched, all can do work in regaining their original state, and all possess energy which is in a sense potential but which is given various names according to its application.