Work and Energy Class 9 Notes

Work and Energy Class 9 Notes – Work is the amount of energy transferred to or from an object by a force that acts on the object as it undergoes a displacement in the direction of the force. It is a scalar quantity that is measured in joules (J) in the SI system.

Depending on the direction of the force and the object’s displacement, work might be positive, negative, or zero. Positive work is done when the force and displacement are in the same direction, negative work is done when they are in opposite directions, and no work is done when there is no displacement.

Work and Energy Class 9 Notes

Conditions need to be satisfied for work to be done

The following requirements must be met for work to be done on an object:

  • Force must be applied on the item.
  • There must be a displacement of the item in the force’s direction.
  • The object must be displaced as a result of the force.

No work is done on the object if any of these requirements are not met.


A system’s capacity for work is defined by its energy. It is a scalar number that can take on a variety of shapes, including kinetic energy, which is the energy of motion, potential energy, which is the energy associated with position, thermal energy, which is the energy connected with temperature, and electromagnetic energy (energy associated with electric and magnetic fields). Although energy can be changed from one form to another, it cannot be produced or destroyed.

Forms of Energy

There are many forms of energy, including –

  1. Potential energy – energy associated with position, such as the energy of an object held above the ground.
  2. Kinetic energy – energy of motion, such as a moving car or a rolling ball.
  3. Thermal energy/ Heat energy – energy associated with temperature, such as the heat generated by a fire.
  4. Chemical energy – energy stored in chemical bonds, such as the energy stored in food or batteries.
  5. Electrical energy – energy associated with electric fields, such as the energy used to power a light bulb.
  6. Electromagnetic energy/ Light energy – energy associated with both electric and magnetic fields, such as radio waves or X-rays.
Kinetic energy
kinetic energy
Kinetic Energy

The energy that an object has as a result of motion is known as kinetic energy. It is defined as one-half of the mass of an object multiplied by the square of its velocity. The equation for kinetic energy is as follows:

KE = (1/2) * m * v^2

where KE is the kinetic energy, m is the mass of the object, and v is the velocity of the object.

The greater the mass and velocity of an object, the greater its kinetic energy. For example, a heavy truck moving at high speed has more kinetic energy than a small car moving at the same speed. Kinetic energy is a scalar quantity and is measured in joules (J) in the SI system of units.

Factors affecting kinetic energy

Here are the factors affecting kinetic energy in a short point wise format:

  • Mass: the kinetic energy of an object is directly proportional to its mass.
  • Velocity: the kinetic energy of an object is directly proportional to the square of its velocity.
  • Direction and angle of motion: the direction and angle of an object’s motion can affect the work done on the object by external forces, which can in turn affect its kinetic energy.
  • External forces: external forces acting on an object can increase or decrease its kinetic energy depending on the direction and magnitude of the force.

Potential Energy

An object can store potential energy as a result of its placement or arrangement within a force field. This means that even when an object is not moving, it has the capacity to perform work. The most prevalent kind of potential energy is gravitational potential energy, which is the power an item has as a result of its elevation above the earth. Elastic potential energy, or the energy held in a stretched or compressed spring, is an additional illustration of potential energy.

potential energy
Potential Energy
Potential Energy of an Object at a Height

If an object is lifted to a certain height, it gains potential energy because work is done against the force of gravity. This potential energy is a form of energy that is stored in the object due to its position in relation to the ground. The amount of potential energy an object has at a certain height depends on its mass, the height it is lifted, and the strength of the gravitational force. Therefore, when an object is raised to a height, the work done to lift it is equal to the potential energy it gains at that height.

W = F.s
F = ma
In the case of increasing the height, F = mg
Therefore, W(P.E) = mgh
ΔPE=mg(h final−h initial)

Law of Conservation of Energy

Energy cannot be created or destroyed, but it can be changed from one form to another, according to the rule of conservation of energy. The total energy of a system is made up of kinetic energy (KE) and potential energy (PE). The following is the formula for total energy:

Total energy = KE + PE

When a ball falls freely from a height, it starts with only potential energy, which is equal to the product of its mass, acceleration due to gravity, and height. As the ball falls, this potential energy gets converted into kinetic energy due to its velocity. By the time it is about to hit the ground, it has acquired a certain velocity and therefore has kinetic energy. However, the total energy of the ball, which is the sum of potential and kinetic energy, remains constant throughout the fall. This means that the potential energy lost by the ball is equal to the kinetic energy gained by it, and the total energy of the ball is conserved.


Power is the rate at which work is done or the rate at which energy is transferred. Mathematically, power is defined as the amount of work done or energy transferred per unit time.

Power = Work done / time


Power = Energy transferred / time

The SI unit of power is the watt (W), which is defined as one joule of work done or energy transferred per second.

Commercial Unit of Energy

The unit joule is too small to express large quantities of energy. Therefore, a bigger unit of energy called kilowatt hour (kW h) is used. One kilowatt hour is the energy used in one hour at the rate of 1000 J s–1, or 1 kW.
1 kW h = 1 kW x 1 h
= 1000 W x 3600 s
= 3600000 J
1 kW h = 3.6 x 106 J

In households, industries, and commercial establishments, energy consumption is usually expressed in kilowatt hours. For example, the electrical energy used during a month is expressed in terms of ‘units,’ where 1 unit is equal to 1 kilowatt hour.

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