CBSE Class 11 Physics Revision Notes Chapter 6

Class 11 Physics Revision Notes for Chapter 6- Work, Energy and Power 

Class 11 Physics Chapter 6 Notes provided by Extramarks will help students acquire conceptual clarity on the chapter. Subject-matter experts carefully curated the notes to ease your study process. 

1. WORK 

Work is defined as a movement of a body that travels a certain distance in the direction of the applied force.

However, when there is no movement of the body in the direction of the applied force, there is no work done, which means that work done is zero when there is no movement of the body in the direction of the force.

2.DIMENSIONS AND UNITS OF WORK

Since work = force × distance, the dimensional equation of work is as follows:

⇒W= (M1L2T−2) ×L

⇒W= [M1L2T−2]

There are two types of work units:

1. a) Absolute Units

• Joule: In the SI system of units, the joule is the unit of absolute work. When a force of one Newton moves a body a distance of one metre in the direction of the applied force, the work is said to be one joule.

⇒1joule =1newton × 1metre × cos00=1N

• Erg: It is the CGS system of units’ absolute unit of work, erg. When a force of one dyne moves a body one centimetre in the direction of applied force, it is said to have produced one erg of work.

⇒1erg=1dyne×1cm×cos00=1dyne.cm 

1. b) Gravitational Units

The term “practical units of work” also applies to these.

• Kilogram-metre (kg-m): The SI unit of measurement known as the kilogram-metre (kg-m) represents the gravitational unit of work. When a body travels a distance of 1 m in the direction of a force of 1 kgf, the amount of work is said to be 1 kg-m.

⇒1kg−m=1kgf×1m×cos00=9.8N×1m=9.8joules, i.e.,

⇒1kg−m=9.8J

• Gram-centimetre (g-cm): The CGS system of units uses the gram-centimetre (g-cm) as the gravitational unit of work. A force of 1gf pushes a body a distance of 1cm in the direction of the applied force, which is measured as one g-cm of work.

⇒1g−cm=1gf×1cm×cos00

⇒1g−cm=980dyne×1cm×1

⇒1g−m=980ergs

3. NATURE OF WORK DONE 

Despite being a scalar quantity, work done (W=(Fcosθ)s) can have the value described below:

• When θ is acute, it is called that positive work is performed on the body (<900). Cosθ is positive, and the work done is therefore positive. 
• When θ is obtuse (>900), it is said that the negative work is to be done on a body. It is obvious that cos is negative, and that the effort done is also negative.
• When the force applied to a body or the displacement created, or both, is zero, it is said that zero work has been done on the body. Here, the work done is zero as the angle θ between force and displacement is 900; cosθ=cos900=0.

5. CONSERVATIVE & NON­CONSERVATIVE FORCES 

Conservation Forces

When the work done by or against a force to move a body depends only on the body’s beginning and final positions and not on the type of path taken between the initial and final positions, the force is said to be conservative.

An example of a conservative force is the gravitational force.

Nonconservative Forces

When the work required to move a body from one location to another is dependent on the path used to get there, the force is said to be non-conservative.

Frictional forces, for instance, are not conservative forces.

6. POWER 

Power is defined as the ratio of work done to time spent. It can be represented as:

Power = Rate of doing work = work donetime taken

Watt, represented by the letter W, is the SI system of units’ absolute measure of power.

⇒P=W/t 

⇒1watt=1joule/1 sec 

⇒1W=1Js−1

7. ENERGY 

The capacity or aptitude of a body to perform work is referred to as its energy.

8. KINETIC ENERGY 

The energy that a body has as a result of its motion is referred to as its kinetic energy. A body’s kinetic energy can be derived in one of two ways:

• by the amount of work required to stop it from moving, or 
• by the amount of work required to accelerate it from a state of rest.

9. RELATION BETWEEN KINETIC ENERGY AND LINEAR MOMENTUM 

If a body has mass m and velocity v, its linear momentum is provided by the equation p=mv and K.E., and its key equation is given by the formula:

 KE= ½ mv2= 1/2m (m2v)

⇒KE=p/2m

This connection is significant. It demonstrates how a body needs linear momentum to have kinetic energy. The opposite is also accurate.

Additionally, if linear momentum (p) is constant, 

KE∝1/m

If kinetic energy (KE) is constant, then,

p2∝m or p∝√m

Also, if mass (m) is constant, then

p2∝KE or p∝√KE

10. WORK ENERGY THEOREM OR WORK ENERGY PRINCIPLE 

This concept states that the change in kinetic energy of a body is equivalent to the work done by the net force in moving the body.

As a result, whenever a force is exerted on a body, the body’s kinetic energy increases by the same amount. A body’s kinetic energy falls off when an opposing (decelerating) force is applied to it. The body’s loss of kinetic energy is equivalent to the work it must do to resist the retarding force. 

11. POTENTIAL ENERGY 

A body’s potential energy is the energy it possesses as a result of its configuration or position in a field.

The energy that can be connected to the configuration (or arrangement) of a system of objects that apply forces to one another is referred to as potential energy. There are two main categories of potential energy:

• the potential energy of gravity
• potential elastic energy.

12. MECHANICAL ENERGY AND ITS CONSERVATION 

A body’s kinetic energy (K) and potential energy (V) are combined to form its mechanical energy (E).

i.e., E=K+V

Naturally, a body’s mechanical energy is a scalar quantity expressed in joules.

If the force acting on the system is conservative, we may demonstrate that the system’s total mechanical energy is conserved.

The principle of total mechanical energy conservation refers to this.

13. DIFFERENT FORMS OF ENERGY 

Energy manifests itself in many forms. They include:

• Heat Energy

It is the energy that a body has as a result of the molecules in the body randomly moving.

Frictional force and heat are related concepts. The work done by the force of kinetic friction (f) over a distance (x) when a block of mass (m) sliding on a rough horizontal surface at speed (v) comes to an end is given by −f(x). The work-energy theorem states that ½ mv2=f(x)  and we frequently claim that frictional force causes the block’s kinetic energy to be lost. The block and the horizontal surface, however, show a tiny temperature rise when we closely inspect them. Thus, the work generated by friction is not lost but rather transmitted as the system’s heat energy.

• Internal Energy

It is the entire energy that the body possesses as a result of the unique configuration of its molecules and their sporadic mobility. Thus, the internal energy of a body is the result of the molecules’ combined potential and kinetic energy.

• Electrical Energy

Bells ring, fans spin, and bulbs illuminate due to the movement of electric current. Moving the free-charge carriers through all the electrical appliances in a specific direction requires a certain amount of work.

14. MASS ENERGY EQUIVALENCE 

Einstein made the astounding discovery that energy may be changed into matter and mass into energy. Einstein proposed the equation E=mc2  where,

m – the mass that disappears 

E – the energy that appears

C – the velocity of light in a vacuum.

15. THE PRINCIPLE OF CONSERVATION OF ENERGY 

The overall energy of an isolated system remains constant when all energy sources are taken into consideration. It is impossible to verify the conservation of energy principle. However, this rule has never been proven to be broken.

16. WORK DONE BY A VARIABLE FORCE

When the force is an arbitrary function of position, calculus methods are needed to calculate the work that the force has done. The function of the location x, F(x), is depicted in the picture. The area under the graph from XA to XB is where we will determine the amount of work done by F.

Consequently, the work performed by a force F(x) from a starting position A to a finishing point B is given by,

XB

WA→B= ∫ FXdx

XA

17. CONSERVATIVE & NON­CONSERVATIVE FORCES 

When a mass is moved between two points, the net work against the force depends only on the locations of the two points and not on the path taken.

Non-conservative forces are those that do not meet the aforementioned requirements. The two most typical types of non-conservative forces are friction and viscous forces.

18. DYNAMICS OF CIRCULAR MOTION 

1. Force on the Particle – Acceleration of the particle in uniform circular motion is of magnitude v2/r and is pointed in the direction of the centre. A force of magnitude mv2/r aimed at the centre is therefore required to maintain a particle’s circular motion. This is a centripetal force that is directed toward the centre. 
2. Main steps for analysing forces – Think of two axes: one parallel to the radius of a circle (i.e., in the direction of acceleration) and the other perpendicular. Resolve each force into its parts. There is no net force along the perpendicular axis. Net force along the radial axis (towards the centre) is equal to mv2/r=mω2r.
3. Fundamentals of Force Analysis in Non-Uniform Circular Motion – After all tangential and radial forces have been resolved, the net tangential force is equal to Ft. Net tangential force =Ft=mat

Net radial force = Fr=mar=mv2/r

CBSE Class 11 Physics Notes Chapter 6 Work, Power and Energy 

The chapter on work, power, and energy is one of the most essential chapters in physics that deals with mechanical principles.

Every concept in the chapter on work, power, and energy is explained in great detail in the notes. Additionally, comprehension will help in describing and computing motion and its effects on various objects.

Chapter 6 Physics Class 11 Notes 

• Define Work.

Work is the quantity of energy that moves when a body is displaced by an outside (external) force that is transmitted in the direction of the displacement.

• Define Power.

The speed at which work is accomplished can explain the concept of power. Power is equal to Work/Time.

• Define Energy.

It is possible to define energy as the capacity to perform work. There are numerous ways that energy can be expressed. Kinetic, thermal, potential, electrical, chemical, nuclear, etc. are the most often used types of energy.

Notes of Work Energy and Power Class 11

Some important questions under Chapter 6 Physics Class 11 Notes include:

Q1. Friction causes the aircraft casing to burn up. Find the energy that is obtained and needed for the casing to burn.

Due to friction, the mass of the rocket decreases when the casing is burning. The law of energy conservation states that no reaction is beyond it.

So, ETotal = Potential energy + Kinetic energy = mgh + ½ mv2

A decrease in total energy is observed as a result of the aircraft’s mass being reduced. Because of this, energy is necessary to burn the casing that was produced by the rocket.

Q2. Find the amount of potential energy that a ball with a mass of 5 kg has stored if it is positioned on a surface that is 3 metres higher.

Given, m = 5 kg

h = 3 m

We know that g = 9.81 m/s-2

So, Potential energy = m * g * h = 5 * 3 * 9.81 = 147.15 J

Work Done by a Variable Force and Conservative and Non-Conservative Forces 

These two sections, CBSE Class 11 Physics Chapter 6 Notes, explain how variable forces operate and how conservative and non-conservative forces differ.

Power And Energy 

The ideas of power and energy, along with their quantifiable units, expressions, etc., are covered in this portion.

Kinetic Energy, Relation Between Kinetic Energy and Linear Momentum 

Students will learn about the various real-world applications of kinetic energy in this lesson, as well as how to calculate a body’s K.E. and how it relates to linear momentum.

Work-Energy Theorem or Work-Energy Principle 

According to the work energy concept, the effort required to move a body is equal to a change in kinetic energy. This section lays out a detailed analysis of the theory.

Potential Energy 

Potential energy is another important subject. Read the notes provided by Extramarks and become acquainted with the many various types of potential energy, including elastic and gravitational potential energy.

Mechanical Energy and its Conservation 

The definition of mechanical energy is nothing more than the accumulation of a body’s kinetic and potential energy. 

Different Energy Forms, Mass Energy Equivalence and Principle of Conservation of Energy 

The various energy sources, including thermal energy, internal energy, etc., are covered in this section. Additionally, students will learn about Einstein’s mass-energy equivalency equation and the energy-conservation concept.

Work Done by a Variable Force and Dynamics of Circular Motion 

The final section of the notes covers concepts like the work that force does from point A to point B and forces involved in uniform and non-uniform circular motion.