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# Chapter Notes - Work and Energy Class 9 Notes | EduRev

## Class 9 : Chapter Notes - Work and Energy Class 9 Notes | EduRev

The document Chapter Notes - Work and Energy Class 9 Notes | EduRev is a part of the Class 9 Course Science Class 9.
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1. In our day-to-day life, any useful physical or mental labour is taken as work. But in science, this concept is not correct.
2. In mechanics, work is said to be done when under the influence of an applied force an object moves through a certain distance. Thus, there are two essential conditions for work to be done. These are;
(i) application of some force on the object,
(ii) displacement of the object in the direction of a force.
3. Work done on an object is defined as the magnitude of the force acting on the object multiplied by the distance moved by the object in the direction of the applied force.
∴ Work (W) = Constant force applied (F) x Displacement along the direction of force (s).
4. SI unit of work is called joule (J or Nm). Work is said to be 1 joule if under the influence of a force of 1 N the object moves through a distance of 1 m along the direction of applied force.
5. Work has only a magnitude and no sense of direction i.e., it is a scalar.
6. Work is considered positive if the displacement of the object is along the direction of force applied. On the other hand, work is taken as negative if the displacement of the object is in a direction opposite to the direction of force applied.
As an example, work done by a man is taken positive when he moves from ground floor to second floor of his house. But work done by the same man is negative when he is descending from second floor of house to ground floor.
7. If displacement of an object is in a direction perpendicular to the application of force, work done is zero inspite of the fact that force is acting and there is some displacement too.
8. An object having the capability to do work is said to possess energy. Hence, the energy of an object is defined as its capacity of doing work. Energy of an object is measured by the total amount of work done by the object.
9. Unit of energy is same as the unit of work. So, SI unit of energy will be joule (J). Energy too has magnitude only.
10. Energy has many forms. Some important forms of energy are mechanical energy, heat energy, electrical energy, light energy, chemical energy, nuclear energy etc.
11. Mechanical energy is of two kinds, namely, (i) kinetic energy, and (ii) potential energy,
12. Kinetic energy of an object is the energy possessed by it by virtue of its state of motion. A speeding vehicle, a rolling stone, a flying aircraft, flowing water, blowing wind, a running athlete possess kinetic energy.
13. For an object of mass m and having a speed v, the kinetic energy is given by 14. Potential energy possessed by an object is the energy present in it by virtue of its position or configuration (i.e., size and shape) or change thereof.
Water stored in a dam, a stretched or compressed spring, stretched bow and arrow, an object situated at a height possess potential energy.
15. The simplest form of potential energy is the “gravitational potential energy”. 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 acquired by an object due to change in its configuration is known as the ‘elastic potential energy’.
16. If an object of mass m is situated at a height h above the ground, then its gravitational potential energy is given by
Ep - mgh.
17. Change in potential energy between two given points (or the work done for taking an object from one point to another point) simply depends on the difference in vertical heights of the initial and final positions of the object and does not depend on the path along which the object is moved.
18. Energy can be changed from one form to another. But the total energy remains unchanged. Energy lost in one form exactly reappears in some other form so that total energy remains unchanged.
19. According to the law of conservation (transformation) of energy, we can neither create nor destroy energy. Energy may only be transformed from one form to another such that total energy before and after the transformation remains exactly the same.
20. The law of conservation of energy is valid in all situations and for all sorts of transformations. Thus, like conservation law of momentum, the conservation law of energy too is a fundamental law of nature which never fails.
21. For a freely falling object, there is a continual transformation of gravitational potential energy into kinetic energy such that their sum remains constant at all points during the fall.
22. The rate of doing work or the rate of transfer of energy is known as the power. 23. Power means speed with which a machine does work or the speed at which a machine supplies energy.
24. SI unit of power is a watt. Power is said to be 1 watt (1 W) if rate of doing work is 1 Js-1,
25. Some other common units of power are:
1 kilowatt = 1 kW = 1000 W = 1000 J s-1
1 megawatt = 1 MW - 106 W - 106 J s-1
1 horse power = 1 hp = 746 watt.
26. If power of a person/machine varies with time, then his average power may be obtained by dividing the total energy consumed (or total work done) by the total time taken. 27. For electrical energy used in households and commercial/industrial establishments we generally use a bigger unit of energy, which is known as 1 kilowatt hour (1 kW h). One kW h is the total energy consumed in 1 hour at the rate of 1 kW (or 1000 W or 1000 J s-1).
1 kWh = 3.6 x 106 J

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## Science Class 9

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