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SAI INTERNATIONAL SCHOOL
PHYSICS

CLASS IX
Chapter:WORK, ENERGY AND POWER

LESSON NOTES

Introduction

 In our everyday life we use terms like work and energy.
 Term work is generally used in context to any kind of activity requiring
physical or mental effort.
 When we push or pull a heavy load or lift it above the floor then we are doing
work, but a man carrying heavy load and standing still is not doing any work
according to scientific definition of work.
 Another term we often use is energy. Energy is usually associated with work
done in the sense that a person feeling very energetic is capable of doing lot
of work.
 This way energy is defined to be as capacity of doing work.
 There are many forms of energy like chemical energy, mechanical energy,
electrical energy, heat energy etc. These forms of energies can be used in
number of ways.
 One form of energy can be converted into another form of energy.
 In this chapter we will study about work, relation between work and energy,
conservation of energy etc.

Work

 We already know that work is said to be done when a force produces motion.
 So we can formally define work done as

Work done by force acting on an object is equal to the product of force
and the displacement of the object in the direction of the force.
 Work done is defined in such a way that it involves both force applied on the
body and the displacement of the body.
 Consider the figure given below where a block is placed on a frictionless
horizontal floor. This block is acted upon by a constant force F. Action of this
force is to move the body through a distance d in a straight line in the direction