CBSE Class 9 Science Notes Chapter 10 Work and Energy

Work is a fundamental concept in physics that describes the transfer of energy when a force acts upon an object and causes it to move. It is a measure of how much energy is transferred by the force. Energy is the capacity to do work or produce change.

1.0Concept of Work

• Work is done on an object when a force causes a displacement of the object.

Conditions that must satisfied for work to be done:

1. A force should act on an object.
2. The object must be displaced.
3. The direction of Force must not be perpendicular to the displacement

Mathematical Definition of Work

1. A constant force is applied in the direction of the displacement of an object.

Work = Force ✕ displacement

W = F ✕ s

2. A Constant force is applied at a certain angle with the direction of the displacement of an object.

W = Fs Cos $\theta$

• Work is a scalar quantity.
• SI Unit of work is Joule.
• CGS Unit is Erg

Some important Points related to work

1. If $\theta$ = 0° then Cos 0° = 1 and W = F ✕ s
2. If $\theta$ = 90° then W = 0
3. If $\theta$ = 180° then Cos 180° = -1 then W = -F ✕ s

Concepts of Positive and Negative Work

• When angle between the force and displacement is acute i.e. 0°< $\theta$ < 90° The work done is positive.
• When angle between the force and displacement is obtuse i.e. 90°< $\theta$ < 180° The work done is negative.
• Whenever force is in the direction of motion, velocity of the object increases and work done is positive.
• Whenever force opposes motion, velocity of object decreases and the work done is negative.
• Work is positive when force and displacement are parallel to each other and that are in the same direction and work is negative when force and displacement are antiparallel to each other.

Condition for Zero Work

2.0Energy

• Energy is the capacity to do work, its unit is the same as that of work that is Joule(J).
• IJ is the energy required to do 1 Joule of work. Sometimes a larger unit of energy called KiloJoule(kJ) is used. IKJ = 1000J

3.0Forms of Energy

The world around us offers energy in various forms, including potential energy, kinetic energy, thermal energy, chemical energy, electrical energy, and light energy.

1. Mechanical Energy-The capacity to do mechanical work is called mechanical energy. Mechanical energy is of two types:

(a) Kinetic energy       

(b) Potential energy

Mechanical Energy = Kinetic Energy + Potential Energy