# CBSE – 9th Standard Science ## CBSE – 9th Standard Science

### Class 9 Chapter 11 – Work & Energy

• Doing anything is termed as ‘Work’.
• For work to be done a force should act on an object, and the object must be displaced.
• We define work to be equal to the product of the force and displacement.
• Work done by force acting on an object is equal to the magnitude of the force multiplied by the distance moved in the direction of the force
• Work has only magnitude and no direction.
• The unit of work is newton metre (N m) or joule (J).
• 1 J is the amount of work done on an object when a force of 1 N displaces it by 1 m along the line of action of the force.
• Work done is negative when the force acts opposite to the direction of displacement.
• Work done is positive when the force is in the direction of displacement.

ENERGY

• The Sun is the biggest natural source of energy to us. Many of our energy sources are derived from the Sun.
• An object that possesses energy can exert a force on another object.
• The energy possessed by an object is thus measured in terms of its capacity of doing work.
• The energy of an object is its capacity for 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.
• Kilo joule (kJ) is the larger unit for energy.
• 1 KJ equals 1000 J.  1000 J = 1 kilo joule(KJ)
• There are different forms of energy. They are heat energy, light energy, electrical energy, chemical energy (potential energy+kinetic energy) etc.

FORMS OF ENERGY

• Mechanical energy (potential energy + kinetic energy)
• Heat energy
• Chemical energy
• Electrical energy
• Light energy.

KINETIC ENERGY

• The kinetic 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 is the energy possessed by an object due to its motion. The kinetic energy of an object increases with its speed.
• The work done is equal to the change in the kinetic energy of an object

POTENTIAL ENERGY

• Potential energy is energy that is stored within a system
• The force is often called a restoring force.
• PE = W = F x d = m a h
• Familiar examples of potential energy:
• A wound-up spring
• A stretched elastic band
• An object as some height above the ground
• The gravitational potential energy of an object at a point above the ground is defined as the work done in raising it from the ground to that point against gravity.
• The potential energy of an object at a height depends on the ground level or the zero level you choose.
• An object in a given position can have a certain potential energy with respect to one level and a different value of potential energy with respect to another level.

LAW OF CONSERVATION OF ENERGY

• According to this law, energy can only be converted from one form to another; it can neither be created nor destroyed.
• The total energy before and after the transformation remains the same.
• The law of conservation of energy is valid in all situations and for all kinds of transformations.
• During the free fall of the object, the decrease in potential energy, at any point in its path, appears as an equal amount of increase in kinetic energy.

POWER

• Power measures the speed of work done, that is, how fast or slow work is done.
• Power is defined as the rate of doing work or the rate of transfer of energy.
• The unit of power is watt [in honour of James Watt (1736 – 1819)].   