NCERT Class 9 Physics Formula and Important Terms Board Sample Problems Part 4 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Work and Energy

Work and Energy
S. noTermDescriptions
1WorkWork done by force acting on an object is equal to the product of force and the displacement of the object in the direction of the force.

Work, which is the product of force and displacement, has only magnitude and no direction. it is a scalar quantity

The unit of work is Newton meter (N m) or joule (J) .

The work done by a force can be either positive or negative or zero.

2Positive workIf a force displaces the object in its direction, then the work done is positive


3Negative workIf the force and the displacement are in opposite directions, then the work is said to be negative.

4Zero WorkIf the directions of force and the displacement are perpendicular to each other, the work done by the force on the object is zero
5Work when displacement at an angle with ForceIf displacement d of any object makes an angle with the force F acting on it as shown below in the figure, then the work done by the force is

6EnergyThe capacity of an object to do work is called energy of the object.

The energy possessed by an object is measured in terms of its capacity of doing work.

The unit of energy is, therefore, the same as that of work, that is, joule (J) . 1 J is the energy required to do 1 joule of work.

7Kinetic EnergyKinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.

kinetic energy possessed by an object of mass m and moving with uniform velocity v is

8Potential EnergyPotential energy is the energy stored in the body or a system by virtue of its position in field of force or by its configuration.

Potential energy is denoted by letter U.

Potential energy for a body placed height H above from ground is given by

9PowerIt is defined as rate of doing work or work done per unit time by an object.

SI unit of power = SI unit of Work done/SI unit of t

= J/s

= watt, W

1W = 1J/1s

Definition of SI unit of power — Power of an object or agent is said to be 1 watt when it does 1 joule of work in 1 second.

Law of Conservation of Energy

Statement 1 — Energy can neither be created nor destroyed, but can be changed from one form to another.

Statement 2 — When one form of energy is changed or transformed into other forms of energy, the total energy of an isolated system remains the same.

Example: Case of Oscillating Pendulum

  • In case of an oscillating pendulum. When a pendulum moves from its mean position to either of its extreme positions, it rises through a certain height above the mean level. At this point, the kinetic energy of the bob changes completely into potential energy.
  • The kinetic energy becomes zero, and the bob possesses only potential energy. As it moves towards point mean position its potential energy decreases progressively.
  • Accordingly, the kinetic energy increases. As the bob reaches point Mean position its potential energy becomes zero and the bob possesses only kinetic energy. This process is repeated as long as the pendulum oscillates.


  • Total energy at Extreme Position = PE
  • Total energy at Mean position = KE
  • In between Total energy = KE + PE
  • So total energy remains conserved

Units of Power

1 horse power: 746 W

1 kilowatt: 1000 w

1 MW: 106W

Commercial Unit of Energy KWH (Kilo Watt Hour)

Definition of 1 kWh

A kilowatt hour is the amount of electric energy used by 1000 W electric appliance when it operates for 1 hour

Relationship between joule (J) and kilowatt-hour (kWh)

1 kWh

Forms of Energy

Forms of Energy
Mechanical energySum of kinetic and potential energy.
Thermal energyEnergy possessed by an object due to its temperature.
Chemical energyEnergy released in chemical reactions.
Sound energyEnergy of a vibrating object producing sound
Electrical energyEnergy of moving electrons in a conductor connected with a battery
Solar energyEnergy radiated by the sun.

Interesting Facts About Energy

Interesting Facts About Energy
S. noPoints
1The word energy comes from the Greek word Energies.
2Stretched rubber bands and compressed springs are examples of elastic potential energy.
3During chemical reactions, chemical energy is often transformed into light or heat
4The mechanical energy of an object is associated with the objects՚ position and motion
5James Joule is the physicist who discovered the relationship between the loss of mechanical energy and the gain of heat.
66 Energy cannot be destroyed or created — only transformed.
77 Albert Einstein defined energy as mass multiplied by the speed of light squared, or
8A “watt” is a unit of power that measures the rate of producing or using energy. The term was named after Scottish engineer James Watt (1736 - 1819) , who developed an improved steam engine. Watt measured his engine՚s performance in horsepower. One horsepower equaled 746 watts.