## LAW OF CONSERVATION OF ENERGY

**LAWOF CONSERVATION OF ENERGY(9th physics) **

Energy can neither be created nor

destroyed, but it

is transformed from one form to another. Alternatively,

whenever energy gets transformed, the total energy

remains unchanged.**Proof � Freely falling body**

It may be shown that in the absence of external�frictional force the total

mechanical�energy of a body remains constant.

Let a body of mass m falls from a point A,�which is at a height h from the

ground as shown in fig.

**At A,**

Kinetic energy kE = 0

Potential energy Ep = mgh

Total energy E = Ep + Ek�= mgh + 0= mgh

During the fall, the body is at a

position B. The�body has moved a distance x from A.

**At B,**

velocity�v^{2} = u^{2} + 2as

applying,�v^{2}�=

0 + 2ax = 2ax

Kinetic energy Ek =�1/2 mv^{2}�=

1/2�m x 2gx�= mgx

Potential energy Ep = mg (h � x)

Total energy E = Ep + Ek =�mg (h-x) + mgx�= mgh � mgx + mgx= mgh

If the body reaches the position C.**At C,**

Potential energy Ep = 0

Velocity of the body C is

v^{2} = u^{2} + 2as

u = 0, a = g, s = h

applying v^{2} = 0 + 2gh = 2gh

kinetic energy Ek =1/2�mv^{2}=1/2�m x 2gh= mgh

Total energy at C

� � � � � � � � � � � E

= Ep + Ek

� � � � � � � � � � �E =

0 + mgh

� � � � � � � � � � �E =

mgh*Thus we have seen that sum of�potential and kinetic energy offreely�falling body at all points remains same.�Under the force ofgravity, the�mechanical energy of a body remains�constant.*

__RATE OF DOING WORK (OR) POWER__

*Power is defined as the rate of doing work or work done per unit time.*

Power �=work done/time taken

� � � � P

=�w/t

UNIT OF POWER

The unit of power is J/S

known as�watt, its symbol is W.

1 watt =1 joule/1 second

1 W = 1 J/ S

Commercial unit of energy is kilo

watt�hour

We pay electricity bill in terms of

unit�or kWh. It is a commercial unit of electric�energy consumed by

the user.

Watt hour = power in watt x time in

hour.

Example : How much energy will

be�used when a hundred watt bulb is used�for 10 hour?

Energy = 100 watt x 10 hour�=

1000 w h = 1kw h

I kwh is known as 1 unit.

One kilowatt hour means

thousand�watt of power is consumed in one hour.

1 kWh �= 1 kW x 1 h

� � � � �

� = 1000 W x 60 x 60 s

� � � � �

� = 1000 Js-1 x 3600 s

� � � � �

� = 3.6 x 106 J

1 unit = 1 kilowatt hour = 3.6x10^{6�}

J