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Science Class 9 Notes for Chapter 11 – Science Notes for Class 9 CBSE

### Text notes

## Science Class 9 Notes – Work and Energy

1. Work : In physics work is defined if force applied on object displaces the object in direc¬tion of force. Hei-e all three terms force, dis¬placement and direction of force are important W= Force x displacement (force in direction of displacement)

Unit of work

1 Joule = 1 Newton. 1 metre

1 J= 1 Nm

When a force of 1 Newton moves a body through a distance of 1 metre in its own direction the work done is 1 Joule.

Other units of work

1 joule = 1 N x lm = 10^{5} dyne x 10^{2}cm = 10^{7} erg

2. Work done by a force applied at an angle

W = component of force in the direction of displacement *, magnitude of displacement W = Fcos0 S

Work done by a force can be positive or negative according as the value of cos 0 is positive or negative.

(therefore, F and s, being magnitudes, are always positive)

W = +ve for 0 = acute angle W = —ve for 0 = obtuse angle

work done by the force does not depend on the time taken in the displacement of point of action.

**3. Energy :** The energy may be defined as the capacity of a body to do work. –

The SI unit of energy is joule (J).

or 1 kJ = 1000 J

**Forms of energy :** The various forms include potential energy, kinetic energy, heat energy, chemical energy, and light energy.

**4. Kinetic Energy :** Energy possessed by a body by virtue of its state of motion is called Kinetic energy. Kinetic energy is always positive and is a scalar. The fact, that moving bodies carry energy with them is proved by some, of the several happenings in day to day life.

Kinetic Energy, K = 1/mv^{2}, when m is the mass and v is the velocity of body.

**5. Potential energy :** Potential energy is energy. due to position. If .a body is in a position such that if it were released it would begin to move, it has potential energy. There are two common forms of potential energy, gravitational and elastic.

**(i) Gravitational Potential Energy :** When an object is allowed to fall from one level to a lower level it gains speed due. to gravitational pull, i.e. it gains kinetic energy. Therefore, in possessing height, a body has the ability to convert its height into kinetic energy, i.e. it” possesses potential energy. If a mass m is at a height h above a lower level the P.E. possessed by the mass is (mg) (h).

**(ii) Elastic Potential energy :** Same work has to be done to change the shape of a body. This work gets stored in the deformed body in the form of elastic potential energy. Elastic potential energy is never negative whether due to extension or to compression.

**6. Law of Conservation of Energy :** According to this law, energy can only be converted from one form to another: it can neither be created or de-stroyed. 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.

**7. Power :** The time rate of doing work is defined as power (P). More quickly work is done; power will be more.

Power= work/time

**8. Unit of power :** The unit of power is the joule per second and this is called the Watt (W). When large amounts of power are involved, a more convenient unit is the kilowatt (kW) where 1 kW = 1000W.

1 Megawatt = 10^{6} watt

1 horse power = 746 watt

The unit kilowatt-hour means one kilowatt of power supplied for one hour. It is, therefore, the unit of energy.

1 KWh = (1000 J/s) x 60 x 60s = 3.6 x 10^{6}J

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