NCERT Book Class 11 Physics Chapter 5 Work, Energy and Power

CHAPTER FIVE

WORK, ENERGY AND POWER

5.1 INTRODUCTION
The terms ‘work’, ‘energy’ and ‘power’ are frequently used
in everyday language. A farmer ploughing the field, a
5.1 Introduction construction worker carrying bricks, a student studying for
a competitive examination, an artist painting a beautiful
5.2 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
5.3 Work
day is said to have a large stamina or energy. We admire a
5.4 Kinetic energy
long distance runner for her stamina or energy. Energy is
5.5 Work done by a variable force thus our capacity to do work. In Physics too, the term ‘energy’
5.6 The work-energy theorem for is related to work in this sense, but as said above the term
a variable force ‘work’ itself is defined much more precisely. The word ‘power’
5.7 The concept of potential is used in everyday life with different shades of meaning. In
energy karate or boxing we talk of ‘powerful’ punches. These are
5.8 The conservation of delivered at a great speed. This shade of meaning is close to
mechanical energy the meaning of the word ‘power’ used in physics. We shall
5.9 The potential energy of a find that there is at best a loose correlation between the
spring physical definitions and the physiological pictures these
5.10 Power terms generate in our minds. The aim of this chapter is to
5.11 Collisions develop an understanding of these three physical quantities.
Summary
Before we proceed to this task, we need to develop a
Points to ponder
mathematical prerequisite, namely the scalar product of two
vectors.
Exercises
5.1.1 The Scalar Product
We have learnt about vectors and their use in Chapter 3.
Physical quantities like displacement, velocity, acceleration,
force etc. are vectors. We have also learnt how vectors are
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 6. 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