20 October 2011



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 v2 = u2 + 2as
applying, v2 = 0 + 2ax = 2ax
Kinetic energy Ek = 1/2 mv2 = 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
v2 = u2 + 2as
u = 0, a = g, s = h
applying v2 = 0 + 2gh = 2gh

kinetic energy Ek =1/2 mv2=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 of freely falling body at all points remains same. Under the force of gravity, the mechanical energy of a body remains constant.

Power is defined as the rate of doing work or work done per unit time.
Power  =work done/time taken
        P = w/t
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 J

Solved CBSE Test papers:

Class 9 Work, Energy ,Power_ Solved Questions - 01

Class 9 Work, Energy ,Power_ Solved Questions - 02

Class 9 Work, Energy ,Power_ Solved Questions - 03

Class 9 Work, Energy ,Power_ Solved Questions - 04

Class 9 Work, Energy ,Power_ Solved Questions - 05

Class 9 Work, Energy ,Power_ Solved Questions - 06          Download Files

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