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Class 9 Chapter 11 – Work & Energy
 Doing anything is termed as ‘Work’.
 For work to be done a force should act on an object, and the object must be displaced.
 We define work to be equal to the product of the force and displacement.
 Work done by force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force
 Work has only magnitude and no direction.
 The unit of work is newton metre (N m) or joule (J).
 1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.
 Work done is negative when the force acts opposite to the direction of displacement.
 Work done is positive when the force is in the direction of displacement.
ENERGY
 The Sun is the biggest natural source of energy to us. Many of our energy sources are derived from the Sun.
 An object that possesses energy can exert a force on another object.
 The energy possessed by an object is thus measured in terms of its capacity of doing work.
 The energy of an object is its capacity for doing work.
 The unit of energy is, therefore, the same as that of work, that is, joule (J).
 1 J is the energy required to do 1 joule of work.
 Kilo joule (kJ) is the larger unit for energy.
 1 KJ equals 1000 J. 1000 J = 1 kilo joule(KJ)
 There are different forms of energy. They are heat energy, light energy, electrical energy, chemical energy (potential energy+kinetic energy) etc.
FORMS OF ENERGY
 Mechanical energy (potential energy + kinetic energy)
 Heat energy
 Chemical energy
 Electrical energy
 Light energy.
KINETIC ENERGY
 The kinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.
 Kinetic energy is the energy possessed by an object due to its motion. The kinetic energy of an object increases with its speed.
 The work done is equal to the change in the kinetic energy of an object
POTENTIAL ENERGY
 Potential energy is energy that is stored within a system
 The force is often called a restoring force.
 PE = W = F x d = m a h
 Familiar examples of potential energy:
 A woundup spring
 A stretched elastic band
 An object as some height above the ground
 The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.
 The potential energy of an object at a height depends on the ground level or the zero level you choose.
 An object in a given position can have a certain potential energy with respect to one level and a different value of potential energy with respect to another level.
LAW OF CONSERVATION OF ENERGY
 According to this law, energy can only be converted from one form to another; it can neither be created nor destroyed.
 The total energy before and after the transformation remains the same.
 The law of conservation of energy is valid in all situations and for all kinds of transformations.
 During the free fall of the object, the decrease in potential energy, at any point in its path, appears as an equal amount of increase in kinetic energy.
POWER
 Power measures the speed of work done, that is, how fast or slow work is done.
 Power is defined as the rate of doing work or the rate of transfer of energy.
 The unit of power is watt [in honour of James Watt (1736 – 1819)].
Course Curriculum

Lessons
 Chapter 1 – Matter in our surroundings
 Chapter 2 – Is matter around us pure
 Class 9 chapter 3 – Atoms & Molecules
 Class 9 Chapter 4 – Structure of the Atom
 Class 9 Chapter 5 – The fundamental unit of life
 Class 9 Chapter 6 – Tissues
 Class 9 Chapter 7 – Diversity in living organisms
 Class 9 Chapter 8 – Motion
 Class 9 Chapter 9 – Force and laws of motion
 Class 9 Chapter 10 – Gravitation
 Class 9 Chapter 11 – Work & Energy
 Class 9 Chapter 12 – Sound
 Class 9 Chapter 13 – Why do we fall ill
 Class 9 Chapter 14 – Natural Resources

Question Bank
 Class 9 Question Bank Chapter 1 – Matter in our surrounding
 Class 9 Question Bank Chapter 2 – Is matter around us pure
 Class 9 Question Bank Chapter 3 – Atoms and molecules
 Class 9 Question Bank Chapter 4 – Structure of the Atom
 Class 9 Question Bank Chapter 5 – The fundamental unit of life
 Class 9 Question Bank Chapter 6 – Tissues
 Class 9 Question Bank Chapter 7 – Diversity in living Organism
 Class 9 Question Bank Chapter 8 – Motion
 Class 9 Question Bank Chapter 9 – Force and laws of Motion
 Class 9 Question Bank Chapter 10 – Gravitation
 Class 9 Question Bank Chapter 11 – Work and energy
 Class 9 Question Bank Chapter 12 – Sound
 Class 9 Question Bank Chapter 13 – Why do we fall ill
 Class 9 Question Bank Chapter 14 – Natural Resources