Work and Energy Class 9th Notes - Free NCERT Class 9 Science Chapter 11 Notes - Download PDF

NCERT notes Class 9 Science Chapter 10 Work and Energy: Download PDF

The Class 9 Science Chapter 10 Work and Energy Notes are a well-organised and simple-to-revise study aid that can enable students to efficiently study to appear in the exams. Developed by the experts in subjects keeping in mind the latest CBSE curriculum, these Class 9 Science Chapter 10 Work and Energy Notes provide important concepts, formulas, diagrams and key points in a clear and concise format. The PDF can be easily downloaded by the students and revised quickly and effectively. The notes are ideal to reinforce the fundamentals and enhance general performance in exams.

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NCERT notes Class 9 Science Chapter 10 Work and Energy

Work and Energy Class 9 Science Notes present a well-organised and easy to learn topic to revise in a short time period, so that it is easy and fast to learn. The Work and Energy Class 9 Science Notes were prepared by the subject experts according to the most recent CBSE syllabus, and they summarised all the significant points, formulas, and concepts in a clear and easily understandable way. They assist in efficient revision of the students, reinforcing fundamentals, and enhancing performance on exams. The notes are the best when one has to learn it at the last minute and develop a good conceptual understanding.

Work

• Not much ‘work’ despite working hard, performing physical or mental jobs, is not considered work concerning this chapter.
• The scientific concept of work-

Work is done on an object when it is displaced by the action of a force.

• Work done by a constant force

Work done = force x displacement

W= Fs {F=force acting on the object, s= displacement caused due to F, W= work done on the object}

• Work done by a constant force can be given by the multiplication of the magnitude of the force and the distance moved in the direction of that force. The SI unit of work done is ‘Nm’ (Newton metre) or ‘J’ (joule). When 1 N of force is applied to displace an object by 1m, 1 J of work is said to be done.
• Work done could be either positive (force applied is in the direction of displacement) or negative (force applied is opposite to the direction of the object’s displacement).

Energy

• The Sun is the ultimate source of energy for sustaining life on Earth. Energy is defined as the capacity to do work. The object which performs work will lose energy, whereas the object on which work is being performed will gain energy. Transfer of energy happens when an object having energy starts to exert force on another object. Energy has the same SI unit as work, i.e., joule (J); this is because energy is measured as the capacity to perform work. 1J work requires an input of 1J energy.
• 1 kJ= 1000J
• Forms of energy: Energy can exist in the form of mechanical energy (potential energy + kinetic energy), heat energy, sound energy, chemical energy, and light energy, and these forms of energy are interconvertible.

Kinetic energy

• It is the energy possessed by an object due to its motion; it is directly proportional to the mass of an object and is proportional to the square of the object’s velocity.
• Let there be an object of mass ‘m', travelling with initial velocity ’u’. If a force ‘F’ acts on it to cause a displacement ‘s’, then its final velocity changes to ‘v’, and the work done on it is ‘W’

W= Fs

From the equation of motion, if a is the constant acceleration of a body;

v²-u²=2as

Rearranging the above equation gives

s=(v²-u²)/2a

Since F=ma, work done can be written as,

W= ma x (v²-u²)/2a

Simplifying the above equation,

W= m(v²-u²)/2

If the object starts at rest, its initial velocity (u) =0

W= mv²/2

Since all work done on the object results in an increase in its kinetic energy,

Kinetic energy= E_k = mv²/2

where E_k = Kinetic energy of the object

m = mass of the object

v= velocity at which the object is moving

Potential energy

• Potential energy can be defined as the energy possessed by an object by virtue of its position or configuration.
• The potential energy of an object at a height:

When an object is raised to a greater height, its energy increases. This is because work is done on it as it is being raised against gravity. The gravitational potential energy is the energy present in such an item. The work done in elevating an object from the ground to a point above the ground is defined as the gravitational potential energy of that object.

Displacement of the object is its change in elevation = h, the force required for this action is mg, where m is the mass of that object, g is the acceleration due to gravity.

W = Force x displacement

W = mg x h

W= mgh

Since all work done on the object results in an increase in its potential energy,

Potential energy = E_P = mgh

Law of conservation of energy

• It is only possible to transform energy from one form to another; it cannot be created or destroyed. Before and after the transition, the total energy of the system is the same. When an object of mass ‘m’ falls from a height ‘h’ to the ground (h=0), its potential energy E_P = mgh gets converted to kinetic energy E_K = mv²/2, no energy is lost in this process.

mgh + mv²/2 = constant, mechanical energy is constant

Rate of doing work

• Power is defined as the rate of doing work or the rate of transfer of energy.
• Power = work/time

P=w/t

• SI unit of power is the Watt (W)
• 1W= 1 Js^-1
• Average power is the total work divided by the total consumed time to perform that work.

Commercial unit of energy

• Since a joule is very small, a larger unit, kilowatt-hour (kWh), is used in commercial applications.
• 1 kWh is defined as the energy that is utilised in one hour at the rate of 1000 Js^-1 (or 1 kW).
• 1 kW h = 3.6 x 10⁶ J
• 1 unit of electricity consumed equals 1 kWh

NCERT Class 9 Science Chapter 10 Work and Energy: Previous Year Question and Answer

The NCERT Class 9 Science Chapter 10 Work and Energy Previous Year Questions and Answers provide students with a focused and exam-oriented resource to understand the types of questions commonly asked in school tests and final exams. These PYQs are used to enable students to recognise key mistakes, enhance their conceptual understanding and their problem-solving abilities. Having properly organised answers according to the recent CBSE rules, they are an effective means of revision and getting higher marks. These questions help to enhance confidence and equip students well to take exams.

Q1: Which of the following are examples of mechanical energy -

1. Kinetic energy, 2. chemical energy, 3. electrical energy, 4. heat energy, 5.Potential energy

Answer:

Mechanical energy is the sum of potential energy and kinetic energy. It is the macroscopic energy associated with a system.

Q2: A ball of mass 1 kg is thrown vertically upward with a velocity of 2 m/s. Then what is the total energy of the ball when it reaches the highest point? (Assume the potential energy of a body at the ground is zero)

Answer:
Kinetic Energy, (K.E.)=1 / 2 x mass x velocity²
KE= 1/2 x 1 x (2)²
KE = 2 Joules
Total energy at any point:
- The total energy of the ball is the sum of its kinetic and potential energy.
- At the highest point, the ball's velocity is zero, so its kinetic energy is zero.
- The total energy at the highest point is equal to the initial kinetic energy, which is 2 Joules.

Therefore, the total energy of the ball at the highest point is 2 Joules.

Q3: If the acceleration due to gravity is 10 m/s², what will be the potential energy of a body of mass 10 kg kept at a height of 2 m? (Assume the potential energy of a body at the ground is zero)

Answer:

The potential energy of a body is calculated by using the formula :

Potential energy = m × g × h=10 x 2 x10=200J

How to Master Class 9 Science Chapter 10 Work and Energy?

Learning Class 9 Science Chapter 10: Work and Energy would be far easier when you are concentrating on concepts rather than memorisation of formulas. This chapter is the foundation of advanced physics, and thus establishing some clarity at this point is highly significant. The right attitude and proper practice can help you get full marks and build your basics to be used in higher classes.

• First, read the chapter on NCERT to get to know the definition of work, energy and power.
• Master all equations like work done, kinetic energy, potential energy, and power and the logic behind each of the equations.
• Study numerical problems using the NCERT textbook, examples and exemplar questions to enhance problem-solving skills.
• Learn the meaning of positive, negative and zero work using practical examples
• Study the law of conservation of energy by using the examples in everyday life, such as a falling object or a swinging pendulum.
• Make clear drawings where necessary, like a force-displacement graph, so that you can easily visualise the concepts.
• Write down brief definitions, equations, and critical points in short notes to be revised in a short time prior to exams.
• Review the chapter several times to keep it accurate and not to make silly mistakes in numerical questions.
• Solve previous year questions, CBSE sample papers to build speed and confidence.

NCERT Notes Class 9 Science Chapter-Wise

In order to revise each topic of the subject in the most effective way, the NCERT Notes of Class 9 of Chapter Science Chapter-Wise offer a complete and structured set of learning materials to the students. These notes are developed by the subject experts according to the newest CBSE syllabus and help in simplifying the concepts by providing clear explanation, diagrams, formulas and main points. They assist in fast revision, reinforcement of concepts, and are more organised in the preparation of examinations. The chapter links allow students to easily learn in a systematic way about any topic they wish.

NCERT Solutions Of Class 9 Subject-Wise

• NCERT solutions for class 9 Maths
• NCERT solutions for class 9 Science

NCERT Class 9 Exemplar Solutions for Other Subjects:

• NCERT Exemplar Class 9 Maths solutions
• NCERT Exemplar Class 9 Science solutions