Chapter 11 – Work and Energy Class 9 Notes Pdf Download

Work and Energy Class 9 Notes

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What is Work?

Let us define Work in a very simple language.

When a force acts on an object or body and this force causes the body to displace, it is said that work is done by the force on that body/object.

The quantity of Work can be obtained by multiplying Force and displacement (dot product).

or, Work = Force × displacement

  • Work is a scalar quantity.
  • Work has magnitude no direction.

Unit of Work

What is the Unit of Work?

The unit of work is newton metre (N m) or joule (J).

What is 1 J amount of work?

Answer: 1 J amount of work is done on an object when a force of 1 N displaces the object by 1 m along the line of action of force.

Example- What is the work done if an object is displaced by 15 m in the direction of a force of 10 N?



Since, the displacement is in the direction of the force applied

Work = Force × displacement

Work = 10 N × 15 m = 150 N m or 150 J (Ans)

What is Energy?

We know without energy, our life is impossible. We need energy in some form, and it has been increasing day by day.

Do you know that the sun is the ultimate source of energy.

Energy Definition

Energy is the capability of an object to do work or you can say energy is the capacity of doing work.

If a body possess energy then it can exert force on another object.

Always remember that the object that which does the work loses the energy and the object on which the work is done gains the energy.

Units of Energy

The unit of energy is same as that of Work i.e., joule (J). The larger unit of energy is kilojoule (kJ).

1 kilo joule (1 kJ) is equal to 1000 joule.

What is 1 joule of energy?

1 J is the energy required to 1 joule of work.

Forms of Energy

The different forms of energy are as follows-

  1. Potential Energy
  2. Kinetic Energy
  3. Electrical Energy
  4. Heat Energy
  5. Chemical Energy
  6. Light Energy

NOTE- Mechanical Energy = Kinetic Energy + Potential Energy

Potential Energy

What is Potential Energy?

Potential energy (P.E.) can be defined as the energy possessed by an object due to the virtue of its position and configuration.

Examples of Potential Energy

Formula of Potential Energy at a height

The formula to find the amount of Potential Energy possessed by an object is-

Potential Energy = m×g×h

here, m = mass of the object

g = acceleration due to gravity (9.81 m/s²)

h = height

Kinetic Energy

What is Kinetic Energy?

Kinetic Energy (K.E.) can be defined as the energy possessed by an object due to its motion.

Examples of Kinetic Energy


Formula of Kinetic Energy

The formula to find the amount of Kinetic Energy possessed by an object is-

Kinetic Energy = ½×mass×(velocity)²

Law of Conservation of Energy

According to the law of conservation of energy-

  • Energy can neither be created nor destroyed
  • Energy may change from one form to another and thus the total amount remains the same or constant.

to be updated

Define Power

What do you understand by Power?

Power is defined as the rate of doing work. It can also be defined as the rate at which energy is consumed or transferred.

Power = Work/time

P = W/t

What is the unit of Power?

Answer- The unit of Power is watt (W).

Define 1 W of power?

Answer- 1 watt is the power of an agent which works at the rate of 1 joule per second. It can be also defined as the rate of consumption of energy by 1 joule per second.


Define Average Power?

Answer- Since energy consumption is different for different time intervals, so Average Power is used.

Average Power = total Energy consumption by the total time taken

Commercial Unit of Energy

Since joule is a very small unit, so we use kilowatt hour (kW h) for large quantities of energy.

Define 1 kilowatt hour?

1 kilowatt hour is defined as the energy consumption for 1 hour at the rate of 1000 joule per second.

  • 1 kW h in joule is equal to 3.6 × 106 J.
  • The electricity bill defines energy consumption in units, here 1 unit is equal to 1 kW h.

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