Short Notes - Work & Energy - Class 9 PDF Download

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Class IX: Science 
Chapter 11: Work and Energy 
Chapter notes 
Key Learning: 
1. Work is done when force acting on a body produces displacement in it. 
2. Work done = Force ? displacement in the direction of force 
3. Work is a scalar quantity. 
4. The SI unit of work is joule (j). 
5. Work done is positive if the angle between force and displacement is 
acute. 
6. Work done is negative if the angle between force and displacement is 
obtuse. 
7. Work done on an object by a force would be zero if the displacement 
of the object is zero. 
8. When a body moves along a circular path, the force acts along the 
radius of the circular path and the motion of the body is along the 
tangential direction. Therefore, the angle between the direction of 
motion and the force is 90°. Hence, no work is done on a body when it 
moves in a circular path. 
9. An object having a capability to do work is said to possess energy. 
10. The energy possessed by a body by virtue of its motion is called 
kinetic energy. 
11. The energy possessed by a body by virtue of its position or change in 
configuration is called potential energy. 
12. Power is defined as the rate at which work is done. 
13. The change of one form of energy into another is called transformation 
of energy.  
14. Law of conservation of energy states that energy can neither be 
created nor be destroyed but can be transformed from one form to 
another. 
15. Energy exists in nature in several forms such as kinetic energy, 
potential energy, heat energy, chemical energy etc. The sum of the 
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 1 
Class IX: Science 
Chapter 11: Work and Energy 
Chapter notes 
Key Learning: 
1. Work is done when force acting on a body produces displacement in it. 
2. Work done = Force ? displacement in the direction of force 
3. Work is a scalar quantity. 
4. The SI unit of work is joule (j). 
5. Work done is positive if the angle between force and displacement is 
acute. 
6. Work done is negative if the angle between force and displacement is 
obtuse. 
7. Work done on an object by a force would be zero if the displacement 
of the object is zero. 
8. When a body moves along a circular path, the force acts along the 
radius of the circular path and the motion of the body is along the 
tangential direction. Therefore, the angle between the direction of 
motion and the force is 90°. Hence, no work is done on a body when it 
moves in a circular path. 
9. An object having a capability to do work is said to possess energy. 
10. The energy possessed by a body by virtue of its motion is called 
kinetic energy. 
11. The energy possessed by a body by virtue of its position or change in 
configuration is called potential energy. 
12. Power is defined as the rate at which work is done. 
13. The change of one form of energy into another is called transformation 
of energy.  
14. Law of conservation of energy states that energy can neither be 
created nor be destroyed but can be transformed from one form to 
another. 
15. Energy exists in nature in several forms such as kinetic energy, 
potential energy, heat energy, chemical energy etc. The sum of the 
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kinetic and potential energies of an object is called its mechanical 
energy. 
16. The energy used in one hour at the rate of 1kW is called 1 kW h. 
 
Top Formulae: 
1. The amount of work done, W = FS cos ? 
2. If  ? = 90°, W  = FS cos 90° 
    = 0, as cos 90° = 0 
3. Kinetic energy of an object of mass m and moving with velocity v 
K. E. = 
2
1
2
mv 
4. Gravitational potential energy of an object of mass m at height h 
    P. E. = m g h 
5. Law of conservation of energy 
    
2
1
2
mv + m g h = constant 
6. 
Energy spent (E)
Power (P)
Time taken (t)
? 
7. 1 watt = 1 joule/second or 1 W = 1 J / s 
8. 1 kilowatt = 1000 watts 
9. 1 kW = 1000 W 
10. 1 kW = 1000 J / s 
11. 1 KWh = 3.6 x 10
6
 J 
12. 1 hp  = 746 W = 0.746 KW