easy notes of chapter work and energy
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CBSE Class 9 Science Syllabus 2017-2018
Main topics covered in this part of CBSE Class 9 Science, Work and Energy: Chapter Notes, are:
Work
Energy and its forms
Kinetic Energy and its expression
Potential Energy and its expression
Conservation of Energy
Power
Unit of Energy
Key notes for Chapter - Work and Energy, are:
Work (W)
Work is defined as a force acting upon an object to cause a displacement
It is expressed as the product of force and displacement in the direction of force.
W=F x s
Here, W= work done on an object
F = Force on the object
s = Displacement of the object
The unit of Work is Newton metre (Nm) or joule (J).
1 Joule is defined as the amount of work done by force of 1 N when displacement is 1 m.
Sign Conventions for Work Done
when both the force and the displacement are in the same direction, positive work is done.
W = F x s
when force acts in a direction opposite to the direction of displacement, the work done is negative.
W= − F x s
Angle between force and displacement is 180o.
If force and displacement are inclined at an angle less than 180o, then work done is given as:
W= Fs cosθ
If force and displacement act at an angle of 90° then work done is zero.
Necessary Conditions for Work to be done
Two conditions need to be satisfied for work to be done:
Force should act on the object.
Object must be displaced.
Energy
The capacity of a body to do work is called the energy of the body.
Unit of energy = Joules
1KJ = 1000 J
Forms of Energy
The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.
Kinetic Energy
It is the energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.
Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity
Derivation
Let an object of mass m, starts from rest and attains a uniform velocity v, after a force F is applied on it. Let during this period the object be be displaced by distance s.

Potential Energy
The energy possessed by a body due to its position or shape is called its potential energy.
For Example:
Water stored in a dam has large amount of potential energy due to its height above the ground.
A stretched rubber band possesses potential energy due to its distorted shape.
Types of Potential Energy
On the basis of position and change in shape of object, potential energy is of two type:
1. Gravitational Potential Energy:
It is the energy possessed by a body due to it position above the ground.
2. Elastic Potential Energy:
It is the energy possessed by a body due to its change in shape.
Expression for Potential Energy
The potential energy (Ep) is equal to the work done over an object of mass ‘m’ to raise it by a height ‘h’.
Thus, Ep = mgh, where g = acceleration due to gravity.
Law of Conservation of Energy
It states that energy can neither be created nor destroyed, but it can be transformed from one form to another.
The total energy before and after the transformation remains the same.
Proof of Law of Conservation of Energy
Let a body of mass m falls from a point A, which is at a height h from the ground as shown in the following figure:

At point A,
Kinetic energy Ek = 0
Potential energy Ep = mgh
Total energy, EA = Ep + Ek
⟹ EA = mgh + 0
⟹ EA = mgh
During the fall, after moving a distance x from A, the body has reached at B.
At point B,
Let the velocity at this point be v.
We know, v2 = u2 + 2as
⟹ v2 = 0 + 2ax = 2ax [As, velocity at A, u = 0]
Also, Kinetic energy, Ek = 1/2 mv2
⟹ Ek =1/2 m × 2gx
⟹ Ek = mgx
Potential energy, Ep = mg(h – x)
So, total energy, EB = Ep + Ek
⟹ EB = mg (h − x) + mgx
⟹ EB = mgh – mgx + mgx
⟹ EB = mgh
At the end the body reaches the position C on ground.
At point C,
Potential energy, Ep = 0
Velocity of the body is zero here.
So, v2 = u2 + 2as
⟹ v2 = 0 + 2gh = 2gh
Kinetic energy, Ek = 1/2 mv2
⟹ Ek = 1/2 x m x 2gh = mgh
Total energy at C
EC = Ep + Ek
EC = 0 + mgh
EC = mgh
Hence, energy at all points remains same.
Power
The time rate of doing work is defined as power (P).
Power= work/time
Unit of power
sI unit of Power is Joule per second or Js1.
1 watt is the power when 1J of work is done in 1s.
The bigger unit of power is Kilowatt and represented by kW.
1kW = 1000W
Some another units to measure power are:
1 Megawatt = 106 watt
1 horse power = 746 watt
Commercial unit of energy
Commercial unit of energy is kilo watt hour (kWh)
The unit kilowatt-hour means one kilowatt of power supplied for 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 = 3.6x106 J.
Try the following questions:
Q1. Does work have a direction?
Q2. A body of mass 25 g has a momentum of 0.40 kgm/s.Find its kinetic energy.
I hope this helps you .
Mark me as Brainlist.
❤️❤️
Main topics covered in this part of CBSE Class 9 Science, Work and Energy: Chapter Notes, are:
Work
Energy and its forms
Kinetic Energy and its expression
Potential Energy and its expression
Conservation of Energy
Power
Unit of Energy
Key notes for Chapter - Work and Energy, are:
Work (W)
Work is defined as a force acting upon an object to cause a displacement
It is expressed as the product of force and displacement in the direction of force.
W=F x s
Here, W= work done on an object
F = Force on the object
s = Displacement of the object
The unit of Work is Newton metre (Nm) or joule (J).
1 Joule is defined as the amount of work done by force of 1 N when displacement is 1 m.
Sign Conventions for Work Done
when both the force and the displacement are in the same direction, positive work is done.
W = F x s
when force acts in a direction opposite to the direction of displacement, the work done is negative.
W= − F x s
Angle between force and displacement is 180o.
If force and displacement are inclined at an angle less than 180o, then work done is given as:
W= Fs cosθ
If force and displacement act at an angle of 90° then work done is zero.
Necessary Conditions for Work to be done
Two conditions need to be satisfied for work to be done:
Force should act on the object.
Object must be displaced.
Energy
The capacity of a body to do work is called the energy of the body.
Unit of energy = Joules
1KJ = 1000 J
Forms of Energy
The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.
Kinetic Energy
It is the energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.
Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity
Derivation
Let an object of mass m, starts from rest and attains a uniform velocity v, after a force F is applied on it. Let during this period the object be be displaced by distance s.

Potential Energy
The energy possessed by a body due to its position or shape is called its potential energy.
For Example:
Water stored in a dam has large amount of potential energy due to its height above the ground.
A stretched rubber band possesses potential energy due to its distorted shape.
Types of Potential Energy
On the basis of position and change in shape of object, potential energy is of two type:
1. Gravitational Potential Energy:
It is the energy possessed by a body due to it position above the ground.
2. Elastic Potential Energy:
It is the energy possessed by a body due to its change in shape.
Expression for Potential Energy
The potential energy (Ep) is equal to the work done over an object of mass ‘m’ to raise it by a height ‘h’.
Thus, Ep = mgh, where g = acceleration due to gravity.
Law of Conservation of Energy
It states that energy can neither be created nor destroyed, but it can be transformed from one form to another.
The total energy before and after the transformation remains the same.
Proof of Law of Conservation of Energy
Let a body of mass m falls from a point A, which is at a height h from the ground as shown in the following figure:

At point A,
Kinetic energy Ek = 0
Potential energy Ep = mgh
Total energy, EA = Ep + Ek
⟹ EA = mgh + 0
⟹ EA = mgh
During the fall, after moving a distance x from A, the body has reached at B.
At point B,
Let the velocity at this point be v.
We know, v2 = u2 + 2as
⟹ v2 = 0 + 2ax = 2ax [As, velocity at A, u = 0]
Also, Kinetic energy, Ek = 1/2 mv2
⟹ Ek =1/2 m × 2gx
⟹ Ek = mgx
Potential energy, Ep = mg(h – x)
So, total energy, EB = Ep + Ek
⟹ EB = mg (h − x) + mgx
⟹ EB = mgh – mgx + mgx
⟹ EB = mgh
At the end the body reaches the position C on ground.
At point C,
Potential energy, Ep = 0
Velocity of the body is zero here.
So, v2 = u2 + 2as
⟹ v2 = 0 + 2gh = 2gh
Kinetic energy, Ek = 1/2 mv2
⟹ Ek = 1/2 x m x 2gh = mgh
Total energy at C
EC = Ep + Ek
EC = 0 + mgh
EC = mgh
Hence, energy at all points remains same.
Power
The time rate of doing work is defined as power (P).
Power= work/time
Unit of power
sI unit of Power is Joule per second or Js1.
1 watt is the power when 1J of work is done in 1s.
The bigger unit of power is Kilowatt and represented by kW.
1kW = 1000W
Some another units to measure power are:
1 Megawatt = 106 watt
1 horse power = 746 watt
Commercial unit of energy
Commercial unit of energy is kilo watt hour (kWh)
The unit kilowatt-hour means one kilowatt of power supplied for 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 = 3.6x106 J.
Try the following questions:
Q1. Does work have a direction?
Q2. A body of mass 25 g has a momentum of 0.40 kgm/s.Find its kinetic energy.
I hope this helps you .
Mark me as Brainlist.
❤️❤️
Answered by
3
The standard unit used to measure energy and work done in physics is the joule, which has the symbol J. In mechanics, 1 joule is the energy transferred when a force of 1 Newton is applied to an object and moves it through a distance of 1 meter.
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