Work and energy class 9 notes, Class 9 science chapter 11 notes

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9 Class Science Chapter 11 Work And Energy Notes

ClassClass 9
Chapter Chapter 11
Chapter NameWork And Energy
CategoryClass 9 Science Notes

Work and energy class 9 notes, Class 9 science chapter 11 notes. here we will be learn about Work, joule, positive, negative and zero work, energy, mechanical energy, kinetic energy, potential energy etc.

Class 9 Science Chapter 11 Work And Energy Notes

📚 Chapter = 11 📚
💠 Work And Energy 💠

❇️ Work :-

🔹 When a force acts on an object and the object shows displacement, the force has done work on the object. 

  • Work = Force x Displacement (F x S) 
  • For doing work, energy is required. 
  • In animals, energy is supplied by the food they eat. 
  • In machine, energy is supplied by fuel. 

🔶 Not much work inspite of working hard :- Reading, writing, drawing, thinking, and analysing are all energy consuming. But in scientific manner, no work is done in above cases.

❇️ Work is said to be done when :-

  • a moving object comes to rest. 
  • an object at rest starts moving. 
  • velocity of an object changes. 
  • shape of an object changes.

❇️ Scientific Conception of Work :-

🔹 Scientific Conception of Work is done when force is applied on a body and when that force produces motion under its influence.

❇️ Condition of Work done :-

  • Force should be applied on the body. 
  • Body should be displaced.

❇️ Work Done by a Fixed Force :-

🔹 Work done in moving a body is equal to the product of force and displacement of body in the direction of force. 

  • Work = Force × Displacement 
  • W = F × S 
  • Work is a scalar quantity.

❇️ Unit of work :-

🔹 Unit of work is Newton metre or Joule. 

❇️ Joule :-

🔹 When a force of 1 Newton moves a body through a distance of 1 metre in its own direction, then the work done is known as 1 Joule. 

  • 1 Joule = 1 Newton × 1 metre 
  • 1 J = 1 Nm

❇️ factors of work done :-

🔹 The amount of work done depends on the following factors :-

🔶 Magnitude of force :- Greater the force, greater is the amount of work & vice-versa. 

🔶 Displacement :- Greater the displacement, greater is the amount of work & vice-versa.

❇️ Negative, Positive and Zero Work :-

🔹 Work done by a force can be positive, negative or zero. 

🔶 Positive work :- Work done is positive when a force acts in the direction of motion of the body.[ Fig. (a)] (θ = 0°). 

  • θ = angle between direction of force applied & the motion of body.

🔶 Negative work :- Work done is negative when a force acts opposite to the direction of motion of the body. (θ = 180°)

🔶 Zero work :- Work done is zero when a force acts at right angles to the direction of motion. (θ = 90°)

❇️ Energy :-

🔹 The capacity of doing work is known as energy.

🔹 The amount of energy possessed by a body is equal to the amount of work it can do. Working body loses energy, body on which work is done gains energy. 

The sun is the biggest source of energy. 

Most of the energy sources are derived from the Sun. 

Some energy is received from nucleus of atoms, interior of the earth and the tides.

🔹 Energy is a scalar quantity.

❇️ Unit of Energy :-

🔹 The SI unit of energy is Joule (J) and its bigger unit is kilo joule (kJ). 

  • 1 kJ = 1000 J 

🔶 Joule energy :- The energy required to do 1 Joule of work is called 1 Joule energy.

❇️ Forms of Energy :-

🔹 Main forms of energy are :-

  • Kinetic energy 
  • Potential energy 
  • Chemical energy 
  • Heat energy 
  • Electrical energy 
  • Light energy 
  • Sound energy 
  • Nuclear energy 

❇️ mechanical energy :-

🔹 Sum of kinetic energy & potential energy of a body is called mechanical energy. 

🔹 The energy possessed by a body on account of its motion or position is called mechanical energy.

❇️ Kinetic Energy :-

🔹 The energy of a body due to its motion is called kinetic energy. 

🔶 Examples of kinetic energy :-

  • A moving cricket ball 
  • Running water 
  • A moving bullet 
  • Flowing wind 
  • A moving car 
  • A running athelete
  • A rolling stone 
  • Flying aircraft

🔹 Kinetic energy is directly proportional to mass and the square of velocity.

 â‡ï¸ Formula for Kinetic Energy :-

🔹 The kinetic energy of a moving body is measured by the amount of work it can do before coming to rest. If an object of mass ‘m’ moving with uniform velocity ‘u’, it is displaced through a distance ‘s’, Constant force ‘F’ acts on it in the direction of displacement velocity changes from ‘u’ to ‘v’. Then acceleration is ‘a’. 

  • Work done, W = F × S
  • F = ma

❇️ Potential Energy :-

🔹 The energy of a body due to its position or change in shape is known as potential energy. 

🔹 Examples :- 

Water kept in dam :- It can rotate turbine to generate electricity due to its position above the ground. 

Wound up spring of a toy car :- It possess potential energy which is released during unwinding of spring. So toy car moves.

❇️ Factors affecting Potential Energy :-

🔶 Mass :-  P.E. ∝ cm

  • More the mass of body, greater is the potential energy and vice-versa. 

🔶 Height above the ground :-  P.E. ∝ h (does not depend on the path it follows) 

  • Greater the height above the ground, greater is the P.E. and vice-versa. 

🔶 Change in shape :- Greater the stretching, twisting or bending, more is the potential energy.

❇️ Potential Energy of an Object on a Height :-

🔹 If a body of mass ‘m’ is raised to a height ‘h’ above the surface of the earth, the gravitational pull of the earth (m × g) acts in downward direction. To lift the body, we have to do work against the force of gravity. 

  • Thus, Work done, W = Force x Displacement 
  • Or W = m × g × h = mgh 

🔹 This work is stored in the body as potential energy (gravitational potential energy). 

  • Thus, Potential energy, Ep = m × g × h 
  • where g = acceleration due to gravity.

❇️ Transformation of Energy :-

🔹 The change of one form of energy to another form of energy is known as transformation of energy.

🔹 Example :- A stone on a certain height has entire potential energy. But when it starts moving downward, potential energy of stone goes on decreasing as height goes on decreasing but its kinetic energy goes on increasing as velocity of stone goes on increasing. At the time stone reaches the ground, potential energy becomes zero and kinetic energy is maximum. 

🔹 Thus, its entire potential energy is transformed into kinetic energy.

❇️ Law of Conservation of Energy :-

🔹 Whenever energy changes from one form to another form, the total amount of energy remains constant. 

“Energy can neither be created nor be destroyed.”

Although some energy may be wasted during conversion, but the total energy of the system remains the same.

❇️ Conservation of Energy during Free Fall of a Body :- 

A ball of mass ‘m’ at a height ‘h’ has potential energy = mgh. 

As ball falls downwards, height ‘h’ decreases, so the potential energy also decreases.

Kinetic energy at ‘h’ is zero but it is increasing during falling of ball. 

The sum of potential energy & kinetic energy of the ball remains the same at every point during its fall. 

½mv² + mgh = Constant 

Kinetic energy + Potential energy = Constant

❇️ Power :-

🔹 Power is defined as the rate of energy consumption.

🔹 Power = Work done / Time taken or p = w/t


  • P = Power
  • W = Work done 
  • t = Time taken

🔶 Unit of Power :-

🔹 SI unit of Power is Watt (W) = 1 Joule/second. 

  • 1 watt =  1 joule/1second or 1W = 1j/1s

🔹 Power is one Watt when one Joule work is done in one second. 

  • Average Power = Total work done or total energy used / Total time taken

❇️ Power of Electrical Gadget :-

🔹 The power of an electrical appliance tells us the rate at which electrical energy is consumed by it. Here, when work is done, an equal amount of energy in consumed. 

🔶 Bigger unit of Power :- Bigger unit of power is called Kilowatt or KW. 

  • 1 Kilowatt (KW) = 1000 Watt = 1000 W or l1000 J/s

❇️ Commercial Unit of Energy :-

🔹 Joule is very small unit of energy and it is inconvenient to use it where a large quantity of energy is involved. 

🔹 For commercial purpose, bigger unit of energy is Kilotwatt hour (KWh).

❇️ 1 KWh :-

🔹 1 KWh is the amount of energy consumed when an electric appliance having a power rating of 1 Kilowatt is used for 1 hour. 

❇️ Relation between Kilowatt hour and Joule :-

🔹 1 Kilowatt hour is the amount of energy consumed at the rate of 1 Kilowatt for 1 hour. 

  • 1 Kilowatt hour = 1 Kilowatt for 1 hour 
  • = 1000 Watt for 1 hour 
  • = 1000 Watt x 3600 seconds (60 x 60 seconds = 1 hour) 
  • = 36,00,000 Joules

🔹 So 1 KWh = 3.6 × 10⁶ J = 1 unit

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