WORK AND ENERGY, CBSE CLASS IX, PHYSICS NOTES PART II
CBSE CLASS IX, WORK AND ENERGY, SCIENCE (PHYSICS) NOTES-(PART II)
WORK AND ENERGY
According to the CBSE Syllabus 2025-26
CBSE Class 9 Science Chapter 11 Work and Energy Notes
In Class 9 Science Chapter 11 Work and Energy.
Remaining Topics in the Chapter
5. Forms of Energy
a. Mechanical Energy.
b. Kinetic Energy.
6. Potential Energy.
7. Transformation Energy.
8. Law of conservation of Energy.
9. Free Fall of a Body (Energy Conservation).
10. Power.
a. Unit of Power.
11. Commercial unit of Power.
MECHANICAL ENERGY
The sum of kinetic energy & potential energy of a body is called mechanical energy. The energy possessed by a body on
account of its motion or position is called mechanical energy.
KINETIC ENERGY
Objects in motion possess energy and can do work. This
energy is called Kinetic Energy.
FORMULA FOR KINETIC ENERGY
If
an object of mass ‘m’ moving with uniform velocity ‘u’, it is displaced through
a distance ‘s’. Constant force ‘f’ acts on it in the direction of displacement.
Its velocity changes from ‘u’ to ‘v’.
Then the acceleration is ‘a'.
Work
done, W = F × s ....(i)
F =
ma ....(ii)
According
to the third equation of motion, the relationship between u, v, s, and a is as follows:
v2 -
u2 = 2as
⇒ s = (v2 - u2)/2a
.... (iii)
Now
putting the value of f and s from (ii) and (iii) in equation (i),
If
u = 0 (when the body starts moving from rest)
W
= ½mv2
E
K = ½mv2
When two identical bodies are in motion, the body with a
higher velocity has more kinetic energy.
• Examples of kinetic energy
1. A moving cricket ball.
2. Running water.
3. A moving bullet.
4. Flowing wind.
5. A moving car.
6. A running athlete.
7. A rolling stone.
WORK-ENERGY THEOREM
The
work-energy theorem states that the net work done by a moving body can be
calculated by finding the change in kinetic energy.
⇒
W net = KE final − KE initial
⇒Wnet= 1/ 2 m[v2−u2]
Factors Affecting Kinetic Energy
a.
Mass
b.
Velocity
c.
Momentum
POTENTIAL ENERGY
The energy of a body due to its
position or change in shape is known as potential energy.
For example, stretching a rubber string. The energy that is
possessed by a body by virtue of its configuration or change in position is known
as Potential Energy.
Three types of potential energy
1. GRAVITATION
Energy potential that comes from an object’s
height and weight.
2. CHEMICAL
Energy potential that comes from the atoms it
contains and the chemical reaction that takes place within the object.
3. ELASTIC
Energy potential of an object being compressed
or stretched.
FACTORS
AFFECTING POTENTIAL ENERGY
(i)
Potential Energy:
P.
E. ∝ m
The greater the mass of the body, the greater is the potential energy and vice versa.
(ii)
Height above the ground:
P.
E. ∝ h (Not dependent on the path it follows)
The greater the height above the ground, the greater is the P.E. and vice versa.
(iii) Change in shape:
The greater the stretching, twisting, or bending, the greater is the
potential energy.
Potential Energy of an Object on a Height
If
a body of mass ‘m’ is raised to a height ‘h’ above the surface of the earth,
the gravitational pull of the earth (m × g) acts in the downward direction. To lift the body, we have to
do work against the force of gravity.
Thus,
Work done (W) = Force × Displacement
W = m × g × h = mgh
ΔPE=mg(h final−h initial)
This work is stored in the body as potential energy (gravitational potential
energy).
Thus,
Potential energy, Ep = m × g × h
where,
g
= acceleration due to gravity.
• Examples:
(i) Water kept in the dam:
It can rotate the turbine to generate electricity due to its
position above the ground.
(ii) Wound up the spring of a toy car :
It possesses potential energy, which is released
during the unwinding of the spring. Therefore, the toy car moves.
(iii) Bent string of bow :
Potential energy due to a change in its shape (deformation) is released in the form of kinetic energy while shooting an arrow.
Transformation of Energy
The change of one form of energy to another form of energy is
known as the transformation of energy.
Example:
(i)
A stone on a certain height has entire potential energy. But when it starts
moving downward, the potential energy of the stone goes on decreasing as height goes on
decreasing, but its kinetic energy goes on increasing as the velocity of the stone goes
on increasing.
→
At the time the stone reaches the ground, potential energy becomes zero, and kinetic
energy is maximum. Thus, its entire potential energy is transformed into
kinetic energy.
(ii)
At a hydroelectric power house, the potential energy of water is transformed into
kinetic energy and then into electrical energy.
(iii)
At the thermal power house, chemical energy of coal is changed into heat energy,
which is further converted into kinetic energy and electrical energy.
(iv)
Plants use solar energy to make chemical energy in food by the process of
photosynthesis.
LAW OF CONSERVATION OF ENERGY
The law of conservation of energy states that energy can neither
be created nor destroyed, but can be transferred from one form to another. The
total energy before and after the transformation remains constant.
Total energy = KE + PE
where,
1/2 mv2 + mgh = constant
For example, consider a ball falling freely from a height. At height h,
it has only
PE = mgh.
By
the time it is about to hit the ground, it has a velocity and therefore
has KE = 1/2 mv2. Therefore, energy gets transferred
from PE to KE, while the total energy remains the same.
CONSERVATION OF ENERGY DURING FREE FALL OF A BODY
A ball of mass m at a height ‘h’ has potential energy = mgh.
As the ball falls downwards, the height ‘h’ decreases, so the potential energy also
decreases.
Kinetic energy at ‘h’ is zero, but it is increasing during the fall of the ball.
The sum of potential energy & kinetic energy of the ball remains the same
at every point during its fall.
½mv2 + mgh = Constant
Kinetic energy + Potential energy = Constant
POWER
The
rate of doing work or the rate of transfer of energy is called power. It is
denoted by P
Power
(P) = Work done /Time Taken
P
= W/t
where,
P
= Power
W
= Work done
t
= Time taken
UNIT OF POWER
SI
unit of Power is the Watt (W) = 1 Joule/second.
1
Watt (W) = 1Joule/1 second = 1J/1s
Power
is one Watt when one Joule of work is done in one second.
•
Average Power = Total work done or total energy used/Total time taken
COMMERCIAL UNIT OF POWER
The commercial unit of power is kWh, i.e. energy used in 1
hour at 1000 Joules/second.
1kWh=3.6×106 J
BIGGER UNIT OF POWER
A bigger unit of power is called a Kilowatt or KW.
1
Kilowatt (KW) = 1000 Watt = 1000 W or 1000 J/s
COMMERCIAL UNIT OF ENERGY
Joule is a very small unit of energy, and it is inconvenient to use it where a large quantity of energy is involved.
For commercial purposes, a bigger unit of energy is a kilowatt hour
(kWh).
1 KWh: 1 KWh is the amount of energy consumed when an electric
appliance having a power rating of 1 Kilowatt is used for 1 hour.
RELATION BETWEEN KILOWATT-HOUR AND JOULE
1
Kilowatt hour is the amount of energy consumed at the rate of 1 Kilowatt for 1
hour.
1
Kilowatt hour = 1 Kilowatt for 1 hour
=
1000 Watt for 1 hour
=
1000 Watt × 3600 seconds (60 × 60 seconds = 1 hour)
=
36,00,000 Joules
∴ 1 KWh = 3.6 × 106 J = 1
unit
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