The terms ‘work’, ‘energy and ‘power’ are used frequently in everyday life. A student studying for examination, mother cooking in the kitchen, a mechanic doing repair of a washing machine, are all said to be working. In Physics, ‘work’ covers a definite and precise meaning. The duration for which somebody works is called capacity. If a person puts in 13-14 hours a day is said to be having high stamina. Therefore, ‘Energy’ is capacity to do work. The word ‘power’ is used in everyday life with different shades of meaning. In boxing we talk of ‘powerful’ punches. These are delivered at a great speed. This shade of meaning is close to the meaning of ‘power’ used in physics. Let us find out the loose correlation between the physical definitions and physiological pictures these terms generate in our minds. This chapter encapsulates the understanding of these physical quantities. Prior to that, we have to develop a mathematical prerequisite, namely the scalar product of two vectors.