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Class 9 Work & Energy
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Introduction

Content

• Work
• Energy
• Law of conservation of energy
• Power

Work and Energy

• A terms work and energy are frequently used in everyday language. A woman carrying water from a well to her house is said to be working. In a drought affected region she may be required to carry it over large distances. If she can do so, she is said to have a large stamina or energy. Energy is thus the capacity to do work. The term Power is usually associated with speed.

Figure 1 Work done

––

• The displacement of the body due to a force is known as work done by that forceIf a constant force, F act on an object, displaced through a distance, S in the direction of the force, then work done W is given as

W = F x S .....(1)

Work has only magnitude and no direction, the unit of work is Newton meter (NM) or Joule (J).
• The work done by a force can be positive or negative, It is depends on the direction of force.
• If the direction of force on an object is direction of displacement, then work done by that force is taken positive.
• If the direction of force is opposite to direction of displacement, then work done by that force is taken negative.

Note:

The work done by the force is said to be zero, if the displacement of object is zero

• Energy possessed by an object is measured in terms of its capacity to doing work. The unit of energy is the same as the work i.e. (Joule)

The larger unit of energy is Kilo joule (KJ)

1KJ = 1000 J

• The various forms of energy are :-

e.g. Mechanical energy ( Kinetic energy + Potential energy), heat energy, Chemical energy, Electrical energy and light energy.

• Kinetic energy is energy possessed by an object due to its motion. The kinetic energy of an object increases with its speed, and given as

E

k

+1/2 mv

2

......(ii)

Hence, m is mass of an object moving with velocity v.

• Also the work done on the object due to change in its velocity, given as

W = 1/2 m (v

2

- u

2

)

Here, u and v are final and initial velocity of an object.

• Potential energy possessed by the object is the energy present in it by virtue of its position or configuration, and given as

E

P

= mgh

Where h is height of object from ground.

• An object increases its energy when raised through a height. This is because work is done on it against gravity present in such an object is the gravitational potential energy.

The gravitational potential energy of an object at a point above the ground is defined as work done in raising it from the ground to that point against gravity, as shown in fig

Figure 2 An object above the ground

• Work done on object against gravity is given as

W = mgh

Note:

The work done by gravity depends on the difference in vertical heights of the initial and final position of the object and not on the path along which the object is moved.

• Law of conservation of energy states that energy can only be converted from one form to another, It can neither be created or destroyed. The total energy before and after the transformation remains the same. The law of conservation of energy is valid in all the situations and for all kinds of transformation.
• Power is defined as the rate of doing work or the rate of transfer of energy. If an agent does a work w in time t , then power is given by

P = w/t

The unit of power is watt (W)

1 watt = 1 joule /second

Larger unit of power is Kilowatt (KW)

1 KW = 1000 Watt

To obtain average power by dividing the total energy consumed by the total time

• Commercial unit of energy called Kilowatt hour (KWH)

1 KWH = 3.6 x 10

6

J

Note:

The electrical energy used during a month is expressed in terms of ‘units’

1 Unit = 1 Kilowatt hour

Points to remember

• The work done by a force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force
• The object which does the work loses energy and the object on which the work is done gains energy
• An object moving faster can do more work than an identical object moving relatively slow.
• The sum of kinetic energy and potential energy of an object is its total mechanical energy.
• The energy used in one hour at the rate of 1 KW is called 1 KWH