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CHAPTER SIX
WORK, ENERGY AND POWER
6.1 INTRODUCTION
The terms ‘work’, ‘energy’ and ‘power’ are frequently used
in everyday language. A farmer ploughing the field, a
construction worker carrying bricks, a student studying for
6.1 Introduction
a competitive examination, an artist painting a beautiful
6.2 Notions of work and kinetic
landscape, all are said to be working. In physics, however,
energy : The work-energy
theorem the word ‘Work’ covers a definite and precise meaning.
Somebody who has the capacity to work for 14-16 hours a
6.3 Work
day is said to have a large stamina or energy. We admire a
6.4 Kinetic energy
long distance runner for her stamina or energy. Energy is
6.5 Work done by a variable
thus our capacity to do work. In Physics too, the term ‘energy’
force
is related to work in this sense, but as said above the term
6.6 The work-energy theorem for
‘work’ itself is defined much more precisely. The word ‘power’
a variable force
is used in everyday life with different shades of meaning. In
6.7 The concept of potential
karate or boxing we talk of ‘powerful’ punches. These are
energy
delivered at a great speed. This shade of meaning is close to
6.8 The conservation of the meaning of the word ‘power’ used in physics. We shall
mechanical energy
find that there is at best a loose correlation between the
6.9 The potential energy of a physical definitions and the physiological pictures these
spring
terms generate in our minds. The aim of this chapter is to
6.10 Various forms of energy : the develop an understanding of these three physical quantities.
law of conservation of energy
Before we proceed to this task, we need to develop a
6.11 Power mathematical prerequisite, namely the scalar product of two
6.12 Collisions vectors.
Summary 6.1.1 The Scalar Product
Points to ponder We have learnt about vectors and their use in Chapter 4.
Exercises Physical quantities like displacement, velocity, acceleration,
Additional exercises
force etc. are vectors. We have also learnt how vectors are
Appendix 6.1
added or subtracted. We now need to know how vectors are
multiplied. There are two ways of multiplying vectors which
we shall come across : one way known as the scalar product
gives a scalar from two vectors and the other known as the
vector product produces a new vector from two vectors. We
shall look at the vector product in Chapter 7. Here we take
up the scalar product of two vectors. The scalar product or
dot product of any two vectors A and B, denoted as A . B (read
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