Work and Energy Class 9 full notes

 Work and Energy 

Work and Energy

Work : Work is defined as the product of Force and Displacement. 

Let us consider, we applied a force of F newton to an object in a particular direction such that the Displacement occurring in the position of the object is S, then the work done by the force is W, given by

                          W = F × S

The SI unit of work is N-m or joule (J). 

Work is a scalar quantity. It means it has only magnitude and not direction. 

Positive Work done : The work done by a force is positive if the displacement occurring in the body is in the direction of force. 

Ex. We are pushing an object toward the east and the body starts displacing in the same direction (east). 

Negative Work done : The work done by a force is negative if the displacement occurring in the body is in the opposite direction with respect to the force, i.e. in a direction antiparallel to the direction of force. 

Ex. If we lift  an object upto a height h against gravity, then the work done by us is positive while the work done by the gravitational force is negative. 

Zero Work done : The work done by a force is said to be zero if the displacement of the body is zero or the displacement of the body is in a direction perpendicular to the direction of force. 

Energy : 

Energy is defined as the capacity to do work. Whenever we work we put some effort into it. We provide our energy to do work. So we measure the energy in terms of work done.

The si unit of Energy is the same as the unit of work done, i.e. joule (J).

There are different forms of energy. For example, kinetic energy, potential energy, mechanical energy, electrical energy, nuclear energy, chemical energy, etc.

Kinetic energy : 

Energy of a body due to.its velocity is known as kinetic energy. 

If a body of mass m is moving with velocity v, then its kinetic energy is measured as 

                   K.E. =  ½ mv2

Potential Energy:

The energy of a body due to its position is Known as its potential energy.

If we do some work in a body then this work is conserved in the body in the form of its potential energy.

For example, if we lift a body mass m upto a height of h , then the work done on the body is given by:

          Work done  = force × displacement

                       W   = mg × h

And hence the potential energy of the body is given by

                P.E.  = mgh

Law of Conservation of Energy:

The Law of Conservation of energy states that the energy can neither be created nor be destroyed, it can only be changed from one form of energy to another forms.

For Example : If you pull the string of a bow with an arrow, it gains some potential energy and when you leave the string the potential energy of the string converts into the kinetic energy of the arrow and the arrow starts moving in a particular direction. This shows that the energy provided by you in the string converts into the kinetic energy of the arrow.

Similarly, you can observe different examples in your daily life as the electrical energy converts into light energy through a light bulb, etc.

Power :

Power is defined as the rate of doing work. 

        Power = Work done / time 

      

                   = W / t

The SI unit of power is watt ( W ).

Other Unit of Power :

             kilowatt   :  1kW = 1000 W

      

       horsepower  :   1hp =  746 W

Commercial Unit of Energy:

In our daily life we use a term ' Unit ' for measuring electricity bill. This 'Unit' refers to the energy consumed in your house.

The measurement of this energy is given in terms of kilowatt hour ( kWh), such that ,

1 Unit = 1 kWh

                         = 1000 W × 3600 s

                         = 3.6 × 10^6  Joules

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