Chapter 11 Work and Energy Class 9 Notes | EduRev

Science - Short Notes Class IX

Class 9 : Chapter 11 Work and Energy Class 9 Notes | EduRev

 Page 1


Chapter 11 Work and Energy 
Concept of work in science: 
? Work is said to be don only when an object has been displaced from its original position. 
? Work is said to be done when the object travels some distance 
 
Therefore, 
? A person sanding at his place with a work is not doing any work according to science. 
? A student preparing for her exams while sitting on a chain is not doing any work 
 
Two necessary conditions for work to be done: 
? Some force is applied on the object 
? It is displaced from its original position 
 
Work done by a constant force 
Constant force – a force having same magnitude and direction during a given interval of time. 
If a constant force ‘F’ displaces an object by a distance ‘s’ in the direction of force; the work done is equals to 
the product of force and displacement. 
That is, 
  W = F*s 
Where, 
W = work done 
F = force applied in the direction of displacement 
s = displacement or distance covered by the object. 
Since, unit of force is Newton (N) and unit of displacement is meter (m), the unit of work is Newton*meter 
(Nm) or joule (J) 
? 1Nm or 1 joule (J) of work is the amount of work done when 1N (Newton) of force displaces and object 
by 1m (meter). 
 
Negative and positive work 
? If the displacement occurs in the direction of the force; the work done is said to be positive. 
? If the displacement occurs in the direction opposite to that of force, the work done is said to be negative.  
If a man pulls a moving car from behind but it keeps on moving forward. The work done by the man is 
said to be negative. 
In the above example itself, the work done by the engine will be positive, since the engine is applying 
the force forward and the car is moving forward. 
 
Page 2


Chapter 11 Work and Energy 
Concept of work in science: 
? Work is said to be don only when an object has been displaced from its original position. 
? Work is said to be done when the object travels some distance 
 
Therefore, 
? A person sanding at his place with a work is not doing any work according to science. 
? A student preparing for her exams while sitting on a chain is not doing any work 
 
Two necessary conditions for work to be done: 
? Some force is applied on the object 
? It is displaced from its original position 
 
Work done by a constant force 
Constant force – a force having same magnitude and direction during a given interval of time. 
If a constant force ‘F’ displaces an object by a distance ‘s’ in the direction of force; the work done is equals to 
the product of force and displacement. 
That is, 
  W = F*s 
Where, 
W = work done 
F = force applied in the direction of displacement 
s = displacement or distance covered by the object. 
Since, unit of force is Newton (N) and unit of displacement is meter (m), the unit of work is Newton*meter 
(Nm) or joule (J) 
? 1Nm or 1 joule (J) of work is the amount of work done when 1N (Newton) of force displaces and object 
by 1m (meter). 
 
Negative and positive work 
? If the displacement occurs in the direction of the force; the work done is said to be positive. 
? If the displacement occurs in the direction opposite to that of force, the work done is said to be negative.  
If a man pulls a moving car from behind but it keeps on moving forward. The work done by the man is 
said to be negative. 
In the above example itself, the work done by the engine will be positive, since the engine is applying 
the force forward and the car is moving forward. 
 
Energy 
? Ability to do work is called energy. 
? An object that has energy can exert some force on another object.  
? The energy required to do a particular work is same as the amount of work done. 
 Units of energy is Joule (J) 
 1 Joule of energy is required to do 1Nm or 1 joule of work. 
 
Types of Energy 
 
1. Kinetic energy 
A type of energy possessed by a moving object is called kinetic energy. 
 
W = Fs      (a) 
 
According to third equation of motion, 
 
2as = v
2
 – u
2
 
s = v
2
 – u
2
     (b) 
         2a 
Putting the value of ‘s’ in equation (a) 
 
W = F v
2
 – u
2
     (c)   
      2a 
 
According to second law of motion 
F = ma 
 
Putting the value of ‘F’ in equation (c) 
W = ma* (v
2
 – u
2
)       
      2a 
     = m*(v
2
 – u
2
) 
        2 
If the object stated from stationary initial velocity or ‘u’ = 0 
 
W = ½ mv
2
 
 
And energy is same as the amount of work done by an object 
So,  
Kinetic energy or Ek = ½ mv
2
 
Where m = mass of the object 
v = velocity of the object. 
 
 
Page 3


Chapter 11 Work and Energy 
Concept of work in science: 
? Work is said to be don only when an object has been displaced from its original position. 
? Work is said to be done when the object travels some distance 
 
Therefore, 
? A person sanding at his place with a work is not doing any work according to science. 
? A student preparing for her exams while sitting on a chain is not doing any work 
 
Two necessary conditions for work to be done: 
? Some force is applied on the object 
? It is displaced from its original position 
 
Work done by a constant force 
Constant force – a force having same magnitude and direction during a given interval of time. 
If a constant force ‘F’ displaces an object by a distance ‘s’ in the direction of force; the work done is equals to 
the product of force and displacement. 
That is, 
  W = F*s 
Where, 
W = work done 
F = force applied in the direction of displacement 
s = displacement or distance covered by the object. 
Since, unit of force is Newton (N) and unit of displacement is meter (m), the unit of work is Newton*meter 
(Nm) or joule (J) 
? 1Nm or 1 joule (J) of work is the amount of work done when 1N (Newton) of force displaces and object 
by 1m (meter). 
 
Negative and positive work 
? If the displacement occurs in the direction of the force; the work done is said to be positive. 
? If the displacement occurs in the direction opposite to that of force, the work done is said to be negative.  
If a man pulls a moving car from behind but it keeps on moving forward. The work done by the man is 
said to be negative. 
In the above example itself, the work done by the engine will be positive, since the engine is applying 
the force forward and the car is moving forward. 
 
Energy 
? Ability to do work is called energy. 
? An object that has energy can exert some force on another object.  
? The energy required to do a particular work is same as the amount of work done. 
 Units of energy is Joule (J) 
 1 Joule of energy is required to do 1Nm or 1 joule of work. 
 
Types of Energy 
 
1. Kinetic energy 
A type of energy possessed by a moving object is called kinetic energy. 
 
W = Fs      (a) 
 
According to third equation of motion, 
 
2as = v
2
 – u
2
 
s = v
2
 – u
2
     (b) 
         2a 
Putting the value of ‘s’ in equation (a) 
 
W = F v
2
 – u
2
     (c)   
      2a 
 
According to second law of motion 
F = ma 
 
Putting the value of ‘F’ in equation (c) 
W = ma* (v
2
 – u
2
)       
      2a 
     = m*(v
2
 – u
2
) 
        2 
If the object stated from stationary initial velocity or ‘u’ = 0 
 
W = ½ mv
2
 
 
And energy is same as the amount of work done by an object 
So,  
Kinetic energy or Ek = ½ mv
2
 
Where m = mass of the object 
v = velocity of the object. 
 
 
2. Potential Energy 
A form of stored energy that can change the velocity of an object when released. 
For e.g.  
? A stretched bow with an arrow 
? A stretched or compressed spring 
? A ball held at a certain height. 
In all the above three examples when the bow, spring or ball is released all of them will increase 
their speed and start moving. 
Hence, the energy that was stored in the above cases was potential energy. 
 
 
Potential energy of an object kept at a height: 
 
Let an object of mass ‘m’ be held at a height ‘h’  
 
Work that can be done by the object = Force * displacement 
     = Fs 
     = m*a*s (F = ma second law of motion) 
      
But here acceleration will be due to gravity so ‘a’ = ‘g’ (g = acceleration due to gravity) 
And ‘s’ is equal to height of the object or ‘h’ 
 
So, Potential work or W P = m*g*h or mgh 
 Since, same amount of energy is required to do a work 
    EP = mgh 
 
Potential energy does not depend on the path followed by the object while moving up or down. It only depends 
on the perpendicular (straight line) height of the object from the ground. 
 
Potential energy can be converted into kinetic energy once the object is allowed to fall from the height ‘h’ 
One form of energy can be converted into another. 
For e.g. Plants convert solar energy into chemical energy. 
 Solar cells convert solar energy into electric energy. 
 
Law of conservation of Energy 
The total energy can neither be created or destroyed. It can only be converted from one form to another. 
? An object held at a height is left and starts falling freely. 
? When it was held at height ‘h’ it had maximum amount of kinetic energy and zero kinetic energy, since 
it was not moving at all. 
? When it was left it stated falling freely. 
Page 4


Chapter 11 Work and Energy 
Concept of work in science: 
? Work is said to be don only when an object has been displaced from its original position. 
? Work is said to be done when the object travels some distance 
 
Therefore, 
? A person sanding at his place with a work is not doing any work according to science. 
? A student preparing for her exams while sitting on a chain is not doing any work 
 
Two necessary conditions for work to be done: 
? Some force is applied on the object 
? It is displaced from its original position 
 
Work done by a constant force 
Constant force – a force having same magnitude and direction during a given interval of time. 
If a constant force ‘F’ displaces an object by a distance ‘s’ in the direction of force; the work done is equals to 
the product of force and displacement. 
That is, 
  W = F*s 
Where, 
W = work done 
F = force applied in the direction of displacement 
s = displacement or distance covered by the object. 
Since, unit of force is Newton (N) and unit of displacement is meter (m), the unit of work is Newton*meter 
(Nm) or joule (J) 
? 1Nm or 1 joule (J) of work is the amount of work done when 1N (Newton) of force displaces and object 
by 1m (meter). 
 
Negative and positive work 
? If the displacement occurs in the direction of the force; the work done is said to be positive. 
? If the displacement occurs in the direction opposite to that of force, the work done is said to be negative.  
If a man pulls a moving car from behind but it keeps on moving forward. The work done by the man is 
said to be negative. 
In the above example itself, the work done by the engine will be positive, since the engine is applying 
the force forward and the car is moving forward. 
 
Energy 
? Ability to do work is called energy. 
? An object that has energy can exert some force on another object.  
? The energy required to do a particular work is same as the amount of work done. 
 Units of energy is Joule (J) 
 1 Joule of energy is required to do 1Nm or 1 joule of work. 
 
Types of Energy 
 
1. Kinetic energy 
A type of energy possessed by a moving object is called kinetic energy. 
 
W = Fs      (a) 
 
According to third equation of motion, 
 
2as = v
2
 – u
2
 
s = v
2
 – u
2
     (b) 
         2a 
Putting the value of ‘s’ in equation (a) 
 
W = F v
2
 – u
2
     (c)   
      2a 
 
According to second law of motion 
F = ma 
 
Putting the value of ‘F’ in equation (c) 
W = ma* (v
2
 – u
2
)       
      2a 
     = m*(v
2
 – u
2
) 
        2 
If the object stated from stationary initial velocity or ‘u’ = 0 
 
W = ½ mv
2
 
 
And energy is same as the amount of work done by an object 
So,  
Kinetic energy or Ek = ½ mv
2
 
Where m = mass of the object 
v = velocity of the object. 
 
 
2. Potential Energy 
A form of stored energy that can change the velocity of an object when released. 
For e.g.  
? A stretched bow with an arrow 
? A stretched or compressed spring 
? A ball held at a certain height. 
In all the above three examples when the bow, spring or ball is released all of them will increase 
their speed and start moving. 
Hence, the energy that was stored in the above cases was potential energy. 
 
 
Potential energy of an object kept at a height: 
 
Let an object of mass ‘m’ be held at a height ‘h’  
 
Work that can be done by the object = Force * displacement 
     = Fs 
     = m*a*s (F = ma second law of motion) 
      
But here acceleration will be due to gravity so ‘a’ = ‘g’ (g = acceleration due to gravity) 
And ‘s’ is equal to height of the object or ‘h’ 
 
So, Potential work or W P = m*g*h or mgh 
 Since, same amount of energy is required to do a work 
    EP = mgh 
 
Potential energy does not depend on the path followed by the object while moving up or down. It only depends 
on the perpendicular (straight line) height of the object from the ground. 
 
Potential energy can be converted into kinetic energy once the object is allowed to fall from the height ‘h’ 
One form of energy can be converted into another. 
For e.g. Plants convert solar energy into chemical energy. 
 Solar cells convert solar energy into electric energy. 
 
Law of conservation of Energy 
The total energy can neither be created or destroyed. It can only be converted from one form to another. 
? An object held at a height is left and starts falling freely. 
? When it was held at height ‘h’ it had maximum amount of kinetic energy and zero kinetic energy, since 
it was not moving at all. 
? When it was left it stated falling freely. 
? The height of the object kept on decreasing and as the height decreased; its potential energy (mgh) also 
decreased 
? However, the velocity of the object kept on increasing because of acceleration due to gravity; its kinetic 
energy (½ mv
2
) kept on increasing. 
? In reality the potential energy was being converted into kinetic energy as the object fall towards the 
ground. 
In other words, 
 
Kinetic energy + potential energy = constant 
 
 ½ mv
2
 + mgh = constant 
Power 
The rate of doing work is called power 
Or 
 The amount of work done per unit time is called power. 
P = W/t 
Where, 
P = Power 
W = work done 
t = time 
SI unit of Joule/sec (Js
-1
) or Watt (W) 
 
Commercial units of power 
Since, watt (W) is a very small unit of power the commercial units are expressed in kilowatt hour or kW*h 
1kiloWatt or 1kW = 1000W (Watt) 
 
1W means 1J of energy consumed per second. 
1000W or 1kW means 1000 J of energy consumed per second 
1kW*h or 1kWh means 1000 J of energy consumed per second for one hour 
  1 hour = 3600 seconds 
So 1kWh = 1000*3600 J of energy 
   = 3.6*10
6
J of energy. 
The commercial unit used in household is kWh or kilowatt hour, which is commonly known as units. 
 
*For numerical refer to NCERT book 
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