CBSE NCERT Notes for Class 9 Science Chapter 11 Work and Energy

Work And Energy Class 9 CBSE Notes – Chapter 11

CBSE Class 9 Science notes will assist students in studying the topic thoroughly and clearly.

These CBSE Class 9 Science notes were written by subject experts who made the study material very basic, both in terms of language and format.


Work is considered to be completed when an item is moved from its original location in the direction of the applied force.

Two requirements must be achieved in order for work to be accomplished from a scientific standpoint.

  1. An item should be subjected to some kind of force.
  2. A displacement of the item is required.
  3. If any of the requirements listed above are not met, work will not be completed.

The work performed by a force on an item is equal to the force’s magnitude multiplied by the distance travelled in the force’s direction.

Let a constant force, F act on an object. Let the object be displaced through a

distance, s in the direction of the force. Let W be the work done. We define work to be equal to the product of the force and displacement.

Work done = force × displacement

W = F s


The SI unit of work is the newton-metre (N-m), sometimes known as the joule (J).

Thus, 1 J is the amount of work performed on an item when a force of 1 N displaces it by 1 m along the force’s line of action.

Work is a scalar quantity; it has no direction and only magnitude.

1 joule = 1 newton x 1 metre 1 J= 1N-m


  1. When the force F and the displacement s are in the same direction (the angle between the force and displacement directions is 0°), the work done by the force is positive, i.e. the force generates work.
  2. When the force F and the displacement s are in the opposite direction (the angle between the force and displacement directions is 180°), the work done is negative, i.e. work is done in opposite to the force.
  3. When the force and displacement are perpendicular to one another (the angle between the force and displacement directions is 90°), the work done is zero.


It is the capacity to do tasks. It is usually required while undertaking any kind of mechanical activity.

Energy is a term that refers to a thing that is capable of doing labour.
The object that does the work loses energy, whereas the object that performs the task gains energy. The energy contained in a thing is quantified in terms of its ability to do work.

The SI unit of energy is identical to that of labour, namely the joule (J).

To do one joule of labour, one joule of energy is needed. kJ is a bigger unit of energy.

Forms Of Energy
The sun is our primary natural source of energy. Additionally, we may get energy from the nuclei of atoms, the interior of the Earth, and the ocean tides.
Mechanical energy (the total of potential and kinetic energy), heat energy, chemical energy, electrical energy, and light energy are all forms of energy.

Kinetic Energy : The energy that an item has as a result of its motion is referred to as kinetic energy.

  • It is denoted by the SI unit joule (J). The kinetic energy of a body moving at a certain velocity is equal to the effort required to achieve that velocity.
  • The kinetic energy of an item rises in relation to its speed.
  • A bullet fired from a gun may penetrate a target due to its kinetic energy.
  • Using a moving hammer, a nail is driven into the wood. It has kinetic energy, or the capacity to do labour, as a result of its motion.
  • The kinetic energy of all objects of mass m travelling at a constant velocity v is defined as,

Calculation Of Kinetic Energy : The kinetic energy of an item is defined as the amount of work it can do before coming to a complete stop. Consider a mass m object travelling at a constant velocity u. It is acted upon by a force F, which displaces it over a distance s and accelerates it to a velocity v.

The Formula Can Produce Several Significant Results

  • When an object’s mass is doubled, its kinetic energy is similarly doubled.
  • When an object’s mass is half, its kinetic energy is likewise halved.
  • When an object’s speed is twice, its kinetic energy is quadrupled.
  • When an object’s speed is half, its kinetic energy is reduced by one-fourth.
  • Large items moving quickly have more kinetic energy than small objects moving slowly.

Potential Energy : Potential energy is the energy that a body has as a result of its change in position or form. It is represented by the SI unit joule (J).

We may define potential energy as the energy that a body has as a result of its position or configuration, e.g. a stretched rubber band, spring, string on a bow, etc. Now, we may say that even when a body is motionless, it retains energy.

  • Examples of potential energy include the following:
    • Water contained in a dam contains potential energy due to its elevation.
    • A stone on the top of a tower has potential energy as a result of its height
    • A wound spring in a watch contains potential energy as a result of its form change.

When an item is elevated over a specific height, its energy rises. This is because effort is done on it in opposed to gravity as it is elevated.

The gravitational potential energy stored inside such an item is the energy present.
The gravitational potential energy of an item at a location above the ground is defined as the effort needed to raise the object against gravity from the ground to that point.


Consider an item of mass m that is located on the Earth’s surface at a location A. Its potential energy is 0 in this state, and its mass mg works vertically downward.

To move the item from location B to position B at a height h, we must apply a force equal to mg in the upward direction.
Thus, work is performed on the body in opposition to the force of gravity. Therefore,

Law of Conservation of Energy

Energy cannot be generated or destroyed; it can only be converted from one form to another. The total energy before and after transformation is always constant.


Consider the mass m of an object at position B. It is free to fall from a height (h) above the earth, as seen in figure.

  • At B

At the start, the potential energy is mgh and the kinetic energy is 0 (due to the fact that the velocity of the object is zero). e.g., PE = mgh and KE = 0.

  • at A

As it falls, the potential energy in the object is converted to kinetic energy. If v is the object’s velocity at a given moment, its kinetic energy is equal to —

  • At point C

As the object continues to descend, the potential energy decreases and the kinetic energy increases. When the item is on the verge of touching the ground, h = 0 and v is the highest.

  • Thus, the object’s total potential and kinetic energy would be constant at all sites, i.e. PE + KE = constant.


One type of energy may be transformed into another type of energy, a process known as energy transformation. When an object is dropped from a height, its potential energy is converted to kinetic energy continually.

When an object is thrown upward, it continually transforms its kinetic energy to potential energy.

  • Green plants generate their own food (which is stored as chemical energy) via the process of photosynthesis.
  • When we throw a ball, the muscular energy held in our bodies is transformed to the ball’s kinetic energy.
  • Potential energy exists in the wound spring of the toy vehicle. When a spring is released, its potential energy is converted to kinetic energy, which causes the toy vehicle to drive.
  • A stretched bow stores potential energy. As it is released, the stretched bow’s potential energy is converted to the kinetic energy of the arrow, which accelerates ahead.


1.Electric motorElectrical energy into mechanical energy.
2.Steam engineHeat energy into kinetic energy.
3.Electric generatorMechanical energy into electrical energy.
4.Dry cellChemical energy into electrical energy.
5.Solar cellLight energy into electrical energy.

Rate of Doing Work: Power

The speed at which work is performed or energy is moved, utilised, or changed into another form is referred to as power.

The SI unit of power is the watt, which carries the symbol W in honour of James Watt. The greater rate of energy transfer is expressed in kilowatts (kW).


The term “average power” refers to the ratio of total work performed to total time taken. An agent may conduct labour at various rates and at various time intervals. In this case, average power is calculated by dividing total energy spent by total time consumed.


The joule unit is insufficiently tiny to represent huge amounts of energy easily. As a result, a larger unit of energy is used. The kilowatt-hour (kWh) or Board of Trade (BOT) unit is the commercial unit of electric energy.

It is the amount of electric energy utilised in one hour by a 1000 W appliance.

The kilowatt-hour is the unit of energy measurement used in homes, industries, and commercial facilities. In our monthly bills, the amount of electrical energy used is stated in units.

NCERT questions & answers from Work And Energy

A force of 7 N acts on an object. The displacement is say 8 m in the direction of the force. Let us take it that the force acts on the object through displacement. What is the work done in this case? (CBSE 2011)


Here, Force, F = 7 N

Displacement, S = 8 m

Work done = F x S = 7 x 8 = 56 J.

When do we say that work is done? (CBSE 2012)

Answer: When a force moves an object from its original position, the object has done work.

Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Answer: W = FS

A pair of bullocks exerts a force of 140 N on a plough. The field being ploughed is 15 m long. How much work is done in ploughing the length of the field? (CBSE 2010)


Here, Force, F = 140 N

Displacement, S = 15 m

Work done = F x S = 140 x 15 = 2100 J.

What is the kinetic energy of an object? (CBSE 2011, 2012, 2014, 2015)

Answer:The energy that an object has because it is moving is called it’s kinetic energy.

Write an expression for the kinetic energy of an object.

Answer: K.E. of an object = 1/2 mv2, where m is the mass of the object and v is the speed of the object.

What is power?

Answer:  Power is the rate at which work can be done.

Define 1 Watt of power.
Define SI unit of power.

Answer: Power of an agent is said to be 1 watt if it does 1 joule work in 1 second.

Define average power.

Answer: Average power is defined as the ratio of total work done by an agent to the total time taken.

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