NCERT Notes, Chapter 11: Work & Energy, Class 9 (Science) PDF Download

 Page 1


117 
 
CHAPTER 11 “Work & Energy” 
                              KEY CONCEPTS [ *rating as per the significance of concept] 
1 Work Done By A Constant Force *** 
2 Energy And Its Forms(Kinetic Energy & 
Potential Energy) 
 Are Various Energy Forms Inter convertible? 
***** 
3 Potential Energy Of An Object At A Height ***** 
4 Law Of Conservation Of Energy **** 
5 Rate Of Doing Work & Commercial Unit Of 
Energy 
**** 
 
1.Work Done By A Constant Force 
? Work is a scalar quantity equal to the product of the displacement x and the 
component of the force F
x
 in the direction of the displacement.. 
? Work is defined as a force acting upon an object to cause a displacement  
? Mathematically, work can be expressed by the following equation. 
?  W= F x d cos q ( cos 0
0
 = 1)
 
 
? where F = force, d = displacement, and the angle (theta) is defined as the angle 
between the force and the displacement vector 
? Three things are necessary for the performance of work:  
? There must be an applied force F. 
? There must be a displacement x. 
? The force must have a component along the displacement 
?  
Negative Work 
Page 2


117 
 
CHAPTER 11 “Work & Energy” 
                              KEY CONCEPTS [ *rating as per the significance of concept] 
1 Work Done By A Constant Force *** 
2 Energy And Its Forms(Kinetic Energy & 
Potential Energy) 
 Are Various Energy Forms Inter convertible? 
***** 
3 Potential Energy Of An Object At A Height ***** 
4 Law Of Conservation Of Energy **** 
5 Rate Of Doing Work & Commercial Unit Of 
Energy 
**** 
 
1.Work Done By A Constant Force 
? Work is a scalar quantity equal to the product of the displacement x and the 
component of the force F
x
 in the direction of the displacement.. 
? Work is defined as a force acting upon an object to cause a displacement  
? Mathematically, work can be expressed by the following equation. 
?  W= F x d cos q ( cos 0
0
 = 1)
 
 
? where F = force, d = displacement, and the angle (theta) is defined as the angle 
between the force and the displacement vector 
? Three things are necessary for the performance of work:  
? There must be an applied force F. 
? There must be a displacement x. 
? The force must have a component along the displacement 
?  
Negative Work 
118 
 
 
 
     The friction force f opposes the displacement 
Positive Work
 
 
Force F contributes to displacement x. 
Test Yourself: 
1.Calculate Work when F= 40 N and x = 4 m.  
2.Calculate Work when F = -10 N and x = 4 m. 
3. A lawn mower is pushed a horizontal distance of 20 m by a force of 200 N directed at an 
angle of 30
0
 with the ground. What is the work of this force? 
4. A student lifts a 50 pound (lb) ball 4 feet (ft) in 5 seconds (s). How many joules of work has 
the student completed? 
2.Energy And Its Forms  
 
 
 
James Joule 
The metric system unit of energy is the joule (J), after James Joule. 
• Mechanical energy is the energy which is possessed by an object due to its motion or 
its stored energy of position 
Forms of Energy 
•  Kinetic energy : is the energy of motion 
Energy which a body possesses because of its motion, which occurs anywhere 
from an atomic level to that of a whole organism 
Page 3


117 
 
CHAPTER 11 “Work & Energy” 
                              KEY CONCEPTS [ *rating as per the significance of concept] 
1 Work Done By A Constant Force *** 
2 Energy And Its Forms(Kinetic Energy & 
Potential Energy) 
 Are Various Energy Forms Inter convertible? 
***** 
3 Potential Energy Of An Object At A Height ***** 
4 Law Of Conservation Of Energy **** 
5 Rate Of Doing Work & Commercial Unit Of 
Energy 
**** 
 
1.Work Done By A Constant Force 
? Work is a scalar quantity equal to the product of the displacement x and the 
component of the force F
x
 in the direction of the displacement.. 
? Work is defined as a force acting upon an object to cause a displacement  
? Mathematically, work can be expressed by the following equation. 
?  W= F x d cos q ( cos 0
0
 = 1)
 
 
? where F = force, d = displacement, and the angle (theta) is defined as the angle 
between the force and the displacement vector 
? Three things are necessary for the performance of work:  
? There must be an applied force F. 
? There must be a displacement x. 
? The force must have a component along the displacement 
?  
Negative Work 
118 
 
 
 
     The friction force f opposes the displacement 
Positive Work
 
 
Force F contributes to displacement x. 
Test Yourself: 
1.Calculate Work when F= 40 N and x = 4 m.  
2.Calculate Work when F = -10 N and x = 4 m. 
3. A lawn mower is pushed a horizontal distance of 20 m by a force of 200 N directed at an 
angle of 30
0
 with the ground. What is the work of this force? 
4. A student lifts a 50 pound (lb) ball 4 feet (ft) in 5 seconds (s). How many joules of work has 
the student completed? 
2.Energy And Its Forms  
 
 
 
James Joule 
The metric system unit of energy is the joule (J), after James Joule. 
• Mechanical energy is the energy which is possessed by an object due to its motion or 
its stored energy of position 
Forms of Energy 
•  Kinetic energy : is the energy of motion 
Energy which a body possesses because of its motion, which occurs anywhere 
from an atomic level to that of a whole organism 
119 
 
              
Examples of Kinetic Energy: This is not an all-inclusive list. 
? Electrical – The movement of atoms 
? Electromagnetic or Radiant – The movement of waves  
? Thermal or Heat – The movement of molecules  
? Motion – The movement of objects  
? Sound – The movement through waves  
Engineers generally refer to thermal/heat energy as “internal energy” and use 
“kinetic energy” strictly in reference to motion. 
 Potential Energy (Stored energy or gravitational energy)  
? The capacity to do work by virtue of position or configuration  
? an object can store energy as the result of its position or elastic source  
? Potential Energy is maximum at the maximum HEIGHT 
            
 
Energy transformation involves the conversion of one form of energy into another form.  
Examples of energy transformation include: 
Page 4


117 
 
CHAPTER 11 “Work & Energy” 
                              KEY CONCEPTS [ *rating as per the significance of concept] 
1 Work Done By A Constant Force *** 
2 Energy And Its Forms(Kinetic Energy & 
Potential Energy) 
 Are Various Energy Forms Inter convertible? 
***** 
3 Potential Energy Of An Object At A Height ***** 
4 Law Of Conservation Of Energy **** 
5 Rate Of Doing Work & Commercial Unit Of 
Energy 
**** 
 
1.Work Done By A Constant Force 
? Work is a scalar quantity equal to the product of the displacement x and the 
component of the force F
x
 in the direction of the displacement.. 
? Work is defined as a force acting upon an object to cause a displacement  
? Mathematically, work can be expressed by the following equation. 
?  W= F x d cos q ( cos 0
0
 = 1)
 
 
? where F = force, d = displacement, and the angle (theta) is defined as the angle 
between the force and the displacement vector 
? Three things are necessary for the performance of work:  
? There must be an applied force F. 
? There must be a displacement x. 
? The force must have a component along the displacement 
?  
Negative Work 
118 
 
 
 
     The friction force f opposes the displacement 
Positive Work
 
 
Force F contributes to displacement x. 
Test Yourself: 
1.Calculate Work when F= 40 N and x = 4 m.  
2.Calculate Work when F = -10 N and x = 4 m. 
3. A lawn mower is pushed a horizontal distance of 20 m by a force of 200 N directed at an 
angle of 30
0
 with the ground. What is the work of this force? 
4. A student lifts a 50 pound (lb) ball 4 feet (ft) in 5 seconds (s). How many joules of work has 
the student completed? 
2.Energy And Its Forms  
 
 
 
James Joule 
The metric system unit of energy is the joule (J), after James Joule. 
• Mechanical energy is the energy which is possessed by an object due to its motion or 
its stored energy of position 
Forms of Energy 
•  Kinetic energy : is the energy of motion 
Energy which a body possesses because of its motion, which occurs anywhere 
from an atomic level to that of a whole organism 
119 
 
              
Examples of Kinetic Energy: This is not an all-inclusive list. 
? Electrical – The movement of atoms 
? Electromagnetic or Radiant – The movement of waves  
? Thermal or Heat – The movement of molecules  
? Motion – The movement of objects  
? Sound – The movement through waves  
Engineers generally refer to thermal/heat energy as “internal energy” and use 
“kinetic energy” strictly in reference to motion. 
 Potential Energy (Stored energy or gravitational energy)  
? The capacity to do work by virtue of position or configuration  
? an object can store energy as the result of its position or elastic source  
? Potential Energy is maximum at the maximum HEIGHT 
            
 
Energy transformation involves the conversion of one form of energy into another form.  
Examples of energy transformation include: 
120 
 
• Chemical – Food is consumed and converted into motion for playing sports or taking 
a test. 
• Radiant – Sunlight is consumed by plants and converted into energy for growth. 
• Electrical – Energy transferred to an oven is converted to thermal energy for heating 
our food. 
Now you know the basic forms of energy. The next question is “What are the energy 
sources?”  
There are renewable and nonrenewable sources of energy. A renewable energy source 
is a form of energy that is constantly and rapidly replenished by natural processes. 
Examples of renewable energy sources include: 
• Biomass – The use of a living or once living organism as fuel 
• Hydropower – The energy produced from the movement of water 
• Geothermal – The use of heat from within the Earth or from the atmosphere 
near oceans to warm houses or other buildings 
• Wind – The use of wind to generate electricity 
Solar – The use of the sun as a source of heat; for instance, to heat a room within a 
house, etc. 
Energy Conversion 
Examples 
Fossil fuels Chemical ? Heat ? Mechanical ? Electrical 
Solar cells   Sunlight ? Electrical 
Wind turbines Kinetic ? Mechanical ? Electrical 
Hydroelectric Gravitational potential ? Mechanical ? Electrical 
Nuclear  Nuclear ? Heat ? Mechanical ? Electrical 
Vehicle System Conversion  
 
Mechanical                     Heat 
Page 5


117 
 
CHAPTER 11 “Work & Energy” 
                              KEY CONCEPTS [ *rating as per the significance of concept] 
1 Work Done By A Constant Force *** 
2 Energy And Its Forms(Kinetic Energy & 
Potential Energy) 
 Are Various Energy Forms Inter convertible? 
***** 
3 Potential Energy Of An Object At A Height ***** 
4 Law Of Conservation Of Energy **** 
5 Rate Of Doing Work & Commercial Unit Of 
Energy 
**** 
 
1.Work Done By A Constant Force 
? Work is a scalar quantity equal to the product of the displacement x and the 
component of the force F
x
 in the direction of the displacement.. 
? Work is defined as a force acting upon an object to cause a displacement  
? Mathematically, work can be expressed by the following equation. 
?  W= F x d cos q ( cos 0
0
 = 1)
 
 
? where F = force, d = displacement, and the angle (theta) is defined as the angle 
between the force and the displacement vector 
? Three things are necessary for the performance of work:  
? There must be an applied force F. 
? There must be a displacement x. 
? The force must have a component along the displacement 
?  
Negative Work 
118 
 
 
 
     The friction force f opposes the displacement 
Positive Work
 
 
Force F contributes to displacement x. 
Test Yourself: 
1.Calculate Work when F= 40 N and x = 4 m.  
2.Calculate Work when F = -10 N and x = 4 m. 
3. A lawn mower is pushed a horizontal distance of 20 m by a force of 200 N directed at an 
angle of 30
0
 with the ground. What is the work of this force? 
4. A student lifts a 50 pound (lb) ball 4 feet (ft) in 5 seconds (s). How many joules of work has 
the student completed? 
2.Energy And Its Forms  
 
 
 
James Joule 
The metric system unit of energy is the joule (J), after James Joule. 
• Mechanical energy is the energy which is possessed by an object due to its motion or 
its stored energy of position 
Forms of Energy 
•  Kinetic energy : is the energy of motion 
Energy which a body possesses because of its motion, which occurs anywhere 
from an atomic level to that of a whole organism 
119 
 
              
Examples of Kinetic Energy: This is not an all-inclusive list. 
? Electrical – The movement of atoms 
? Electromagnetic or Radiant – The movement of waves  
? Thermal or Heat – The movement of molecules  
? Motion – The movement of objects  
? Sound – The movement through waves  
Engineers generally refer to thermal/heat energy as “internal energy” and use 
“kinetic energy” strictly in reference to motion. 
 Potential Energy (Stored energy or gravitational energy)  
? The capacity to do work by virtue of position or configuration  
? an object can store energy as the result of its position or elastic source  
? Potential Energy is maximum at the maximum HEIGHT 
            
 
Energy transformation involves the conversion of one form of energy into another form.  
Examples of energy transformation include: 
120 
 
• Chemical – Food is consumed and converted into motion for playing sports or taking 
a test. 
• Radiant – Sunlight is consumed by plants and converted into energy for growth. 
• Electrical – Energy transferred to an oven is converted to thermal energy for heating 
our food. 
Now you know the basic forms of energy. The next question is “What are the energy 
sources?”  
There are renewable and nonrenewable sources of energy. A renewable energy source 
is a form of energy that is constantly and rapidly replenished by natural processes. 
Examples of renewable energy sources include: 
• Biomass – The use of a living or once living organism as fuel 
• Hydropower – The energy produced from the movement of water 
• Geothermal – The use of heat from within the Earth or from the atmosphere 
near oceans to warm houses or other buildings 
• Wind – The use of wind to generate electricity 
Solar – The use of the sun as a source of heat; for instance, to heat a room within a 
house, etc. 
Energy Conversion 
Examples 
Fossil fuels Chemical ? Heat ? Mechanical ? Electrical 
Solar cells   Sunlight ? Electrical 
Wind turbines Kinetic ? Mechanical ? Electrical 
Hydroelectric Gravitational potential ? Mechanical ? Electrical 
Nuclear  Nuclear ? Heat ? Mechanical ? Electrical 
Vehicle System Conversion  
 
Mechanical                     Heat 
121 
 
Test Yourself 
1. How much potential energy is lost by a 5Kg object to kinetic energy due a decrease in 
height of 4.5 m. 
3. Potential energy of an object at a height 
An object increases its energy when raised through a height. 
The potential energy of an object at a height depends on the ground level or the zero level
 
4. Law Of Conservation Of Energy  
The principle of Conservation of Mechanical Energy 
The total mechanical energy (E=KE+PE) of an object remains constant as the object moves, 
provided that the net work done by external non-conservative forces is zero, W
nc
=0J 
Total mechanical energy: the sum of kinetic energy and gravitational potential energy 
 E = KE + PE  
W
nc 
= (KE
f 
- KE
0
) + (PE
f 
- PE
0
) 
W
nc 
= (KE
f 
+ PE
f
)
 
- (KE
0
 + PE
0
) 
W
nc
 = E
f
 - E
0
  
E
f
 = KE
f 
+ PE
f
)
 
 E
0
 = KE
0
 + PE
0