- Chapter 1 – Matter in our surroundings
- Chapter 2 – Is matter around us pure
- Class 9 chapter 3 – Atoms & Molecules
- Class 9 Chapter 4 – Structure of the Atom
- Class 9 Chapter 5 – The fundamental unit of life
- Class 9 Chapter 6 – Tissues
- Class 9 Chapter 7 – Diversity in living organisms
- Class 9 Chapter 8 – Motion
- Class 9 Chapter 9 – Force and laws of motion
- Class 9 Chapter 10 – Gravitation
- Class 9 Chapter 11 – Work & Energy
- Class 9 Chapter 12 – Sound
- Class 9 Chapter 13 – Why do we fall ill
- Class 9 Chapter 14 – Natural Resources
- Class 9 Question Bank Chapter 1 – Matter in our surrounding
- Class 9 Question Bank Chapter 2 – Is matter around us pure
- Class 9 Question Bank Chapter 3 – Atoms and molecules
- Class 9 Question Bank Chapter 4 – Structure of the Atom
- Class 9 Question Bank Chapter 5 – The fundamental unit of life
- Class 9 Question Bank Chapter 6 – Tissues
- Class 9 Question Bank Chapter 7 – Diversity in living Organism
- Class 9 Question Bank Chapter 8 – Motion
- Class 9 Question Bank Chapter 9 – Force and laws of Motion
- Class 9 Question Bank Chapter 10 – Gravitation
- Class 9 Question Bank Chapter 11 – Work and energy
- Class 9 Question Bank Chapter 12 – Sound
- Class 9 Question Bank Chapter 13 – Why do we fall ill
- Class 9 Question Bank Chapter 14 – Natural Resources
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.
- 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.
- 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 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 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)].