CBSE Class 9 Science Chapter 11 Revision Notes

Chapter 11: Work and Energy Revision Notes

Work Done

  • The product of the amount of the force applied on the body and the displacement in the direction of the force is defined as work done on an item. F.s = W
  • The work done is 0 if a force acting on a body causes no displacement. For instance, pushing against a wall.


  • The ability to work is defined as energy. It has the same unit of measurement as work.
  • Joule (Nm) or Kgm2s2 is the SI unit of energy or work.
  • Light, heat, chemical, electrical, and mechanical energy are all examples of energy.

Forms of energy

(i) Kinetic energy (K.E)

(ii) Potential energy (P.E)

Kinetic energy

  • The energy of motion is referred to as kinetic energy.
  • The energy possessed by a body as a result of its motion, which can occur at any level from the atomic to that of a whole organism.
  • This is not an exhaustive list of kinetic energy examples.
  • Atomic motion is referred to as electrical motion.
  • The movement of waves (electromagnetic or radiant)
  • The movement of molecules (thermal or heat)
  • Object movement is referred to as motion.
  • Sound is the movement of waves through space.
  • Thermal/heat energy is commonly referred to as “internal energy” by engineers, while “kinetic energy” is only used in the context of motion.

Potential Energy (Stored energy or gravitational energy)

Potential energySource:

  • The ability to perform labour as a result of one’s position or arrangement.

  • As a result of its location or elastic source, an item can store energy.

  • At the highest HEIGHT, potential energy is at its highest.

  • The conversion of one kind of energy into another is known as energy transformation.

    The following are some examples of energy transformation:

  • Food is ingested and turned into motion in order to participate in sports or take an exam.

  • Radiant – Plants absorb sunlight and transform it into energy for growth.

  • Electrical – When energy is supplied to an oven, it is transformed to thermal energy, which is used to heat our meals.

Types of Energy

  • There are two types of energy sources: renewable and nonrenewable.
  • A renewable energy source is one that is renewed continuously and quickly by natural processes.

The following are some examples of renewable energy sources:

Biomass — The utilisation of a living or formerly alive creature as a source of energy.

Hydropower is the energy generated by the flow of water.

Geothermal – Using heat from deep under the Earth or from the atmosphere near seas to heat homes and other structures.

Wind turbines — wind turbines are used to generate power.

Solar– The use of the sun as a heat source, such as to heat a room within a house, and so on.

Conversion of Energy

  • Fossil fuels are a type of energy that comes from the decomposition Chemical, heat, mechanical, and electrical energy are all sources of energy.
  • Solar cells are made up of solar cells that absorb sunlight and convert it into electricity.
  • Turbines powered by wind Mechanical Electrical
  • When an item is lifted to a greater height, its energy rises.
  • The ground level or zero level determines the potential energy of an item at a height.


  • Work is completed or energy is consumed at a certain rate.
  • The fundamental unit of Power is **Watt (P=w/t). **
  • One joule of labour per second equals one watt.

Types of Power

  • Electrical power is a type of energy that is used to perform tasks.
  • Mechanical power is a type of energy that is used to perform tasks (linear, rotary)
  • Fluid power is based on the transmission of energy between liquids (hydraulic) and gases (gaseous) (pneumatic)
  • The rate at which we consume energy is referred to as power.
  • Work or Energy / Time = Power



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