Class 9: Work, Energy and Power Notes

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Class 9 Work, Energy and Power

Moving to class 9 not just puts you a level up in the academic ladder but also scales the difficulties of subjects taught in the entire session. Science and Maths are considered to be the toughest amongst other subjects. Thus, paying a little extra attention in the class, referring to study notes by experts can help you ace the subjects. Class 9 Work, Energy and Power is an important chapter from the exam point of view. Many competitive exams also have it in their syllabus list. Considering its importance, we have curated a comprehensive blog on Work, Energy and Power notes.

What is Work? 

Work can be defined as the measurement of change in energy whenever an external force is applied to an object. It is considered to be done when the force applied on a body displaces the body from its actual position to a certain distance, and in the direction in which the force is applied. 

We can easily calculate the work done by an object through the product of the Force applied and the Distance it covers. The work done by a body under the constant force F, covering the distance s, can be represented as- 

Work is known to be a scalar quantity and the SI unit of which is Joule (J) whereas the CGS unit is Energy (erg). 
Let us understand the relationship between the SI and CGS units of Work. 

So, 1 Joule = 107 erg

By now you are familiar with what work is and how we can calculate it, but work done by a body at times can be zero as well. Mentioned below are the conditions stating that when and how can be zero.

Work done by a force is Zero, only if: 

  1. A body is not displaced after applying force, i.e., S=0
  2. The body is displaced perpendicular to the direction of the force applied i. e. θ = 90°

The next topic in our work, energy and power notes is Energy, let us understand various insights of this interesting concept. 

Must Read: Class 9 Science: Sound

What is Energy? 

Energy is known as the extent or capacity of a body to perform a given work. Anything that has the capacity to do work is said to possess energy. Energy is always regarded as a scalar quantity. Its SI unit is Joule (J), and the CGS unit is Energy (erg). Energy can neither be created nor can be destroyed and comes in various types and forms such as Mechanical energy (kinetic and potential energy), Solar energy, Chemical energy, Sound energy, Nuclear energy, Electrical energy, etc.

Types Of Energy 

Majorly there are 2 types of energy which we will be discussing in the work, energy and power notes i.e., the Kinetic and Potential energy. Let us understand them one by one.  

Kinetic Energy

Whenever an object possesses some energy because of its motion, it is called the Kinetic Energy. The kinetic energy of an object is given by-

Before moving to the next topic of class 9 Work,
Energy and Power notes, take this Physics Quiz!

Potential Energy 

The energy developed in a body due to the position or its configuration is called the Potential Energy. For Example.: The energy of a compressed string, water collected at height, the energy of a spring in a watch, etc. Potential energy has two different forms:  Gravitational and Elastic Energy which we will be explaining in the work, energy and power notes. 

Gravitational Potential Energy is a type of potential energy that is possessed by a body by virtue of its position above the surface of the earth. It is given by: 

Elastic Potential Energy can be defined as when an elastic body is displaced from its equilibrium position, work is actually needed to be done against the restoring force. The work done is then stored in the body as a form of elastic force, also known as elastic potential energy. It is measured as: 

Must Read: Basic Physics Formulas & Notes for Competitive Exams

What is Power? 

The time rate of the work done by the body is called Power. Power is a scalar quantity, it’s SI unit is a Watt (W). There are other different units of power from which it can be calculated, such as Kilowatt and Horsepower. The relationship between the various units of Power can be seen as-

1 Kilowatt = 1000 Watt 

1 Horsepower = 746 Watt

Power can be measured as: 

Class 9 - Power

Have a look at the Experiment With Diverse Career in Physics!

When under constant force (F), a body is displaced through a distance (s) in time t, where, 

Power P = W/t 

P= F*s/t [ As we know that W= Fxs]

But, we have studied earlier that, 

s/t = v, i.e., the uniform velocity at which the body is displaced, hence, it alters the formula, 

P= F*v = F v cos θ 
(θ is the smaller angle between F and v) 

Practice Questions for Class 9 Work and Energy

  • What is the work to be done to increase the velocity of a car from 30 km h–1 to 60 km h–1 if the mass of the car is 1500 kg?
  • An object of mass 15 kg is moving with a uniform velocity of 4 m S-1  What is the kinetic energy possessed by the object?
  • Find the energy possessed by an object of mass 10 kg when it is at a height of 6 m above the ground. Given, g = 9.8 m S-2 .
  • An object of mass 12 kg is at a certain height above the ground. If the potential energy of the object is 480 J, find the height at which the object is with respect to the ground. Given, g= 10m S-2 .
  • Calculate the work required to be done to stop a car of 1500 kg moving at a velocity of 60 km/h?
  • Two girls, each of weight 400 N climb up a rope through a height of 8 m. We name one of the girls A and the other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the power expended by each girl?
  • A boy of mass 50 kg runs up a staircase of 45 steps in 9 s. If the height of each step is 15 cm, find his power. Take g = 10 m S-2.
  • An electric bulb of 60 W is used for 6 h per day. Calculate the ‘units’ of energy consumed in one day by the bulb. 
  • Certain force acting on a 20 kg mass changes its velocity from 5 m S-1 to 2 m  S-1 Calculate the work done by the force. 
  • A mass of 10 kg is at a point A on a table. It is moved to a point B. If the line joining A and B is horizontal, what is the work done on the object by the gravitational force? Explain your answer
  • A certain household has consumed 250 units of energy during a month. How much energy is this in joules? 
  • An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.
  • A person holds a bundle of hay over his head for 30 minutes and gets tired. Has he done some work or not? Justify your answer. 
  • An electric heater is rated 1500 W. How much energy does it use in 10 hours?

Class 9 Work, Energy & Power: Important Questions and Answers

Here are the important questions and answers for Class 9 Work, Energy and Power:

Does work done depend upon the velocity of the body?

No, work does not depend upon the velocity of a body.

When is 1 joule of work said to be done?

1 joule of work is done when a force of 1 newton acting on a body displaces it in its own direction.

List two essential conditions for work to be done. 

The two essential conditions are 
A force must act 
There should be displacement in the body.

Potential energy is a vector quantity. True or false?


What is Power?

The time rate of the work done by the body is called Power. Power is a scalar quantity, its SI unit is a Watt (W).

What do you mean by the kinetic energy of the body?

Whenever an object possesses some energy because of its motion, it is called Kinetic Energy. 

The total energy when a body falls freely towards the earth remains constant.  True or false?


State the law of conservation of energy. 

The law states that energy can neither be created nor destroyed. It can only change its form.
 In a tug-of-war, one team gives way to the other. What work is being done and by whom?
The winning team does work. The work is equal to the product of the resultant force and the displacement undergone by the losing team.

What is the unit of energy?

Energy is known as the extent or capacity of a body to perform a given work. Anything that has the capacity to do work is said to possess energy. Energy is always regarded as a scalar quantity. Its SI unit is Joule (J), and the CGS unit is Energy (erg)

What will cause a greater change in the kinetic energy of a body? Changing its mass or changing its velocity?

Changing its velocity.

Work and Energy NCERT PDF

Hence, we hope that this blog on class 9 Work, Energy and Power notes helped you understand the concept better. Reach out to experts at Leverage Edu to guide you for the upcoming crucial decisions like stream selection.

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