Work and Energy Class 9 Notes Science - Chapter 11

Chapter: 11

What Are Work And Energy ?

Work

  • When the force acts on an object or body and the object or body moves or shows displacement, then the force has done some work on the object or body.
  • Mathematical formula: W = F Ɨ S or Work done = Force Ɨ Displacement
  • If the displacement is zero, then work done will also be zero, W = 0.
  • If work done is in the direction of force, then W = F Ɨ S.
  • If work done is in the direction opposite to the force, then W = ā€“F Ɨ S.
  • Work done has only magnitude but no direction (scalar quantity)
  • SI unit = Joule.
  • 1 Joule = 1 Newton Ɨ 1 meter
  • 1 Joule work is said to be done when 1 Newton force is applied on an object and it shows the displacemnt by 1 meter.

Energy

  • Capacity of a body to do work.
  • SI unit: Joule

Various Forms of Energy

Potential Energy (PE) Kinetic Energy
  • Energy possessed by an object by virtue of its position/configuration
  • Energy possessed by an object or body by virtue of its motion.
  • Gravitational potential Energy: Work done in raising an object from the ground to a point above the ground against gravity EP = mgh
  • KE (EK) =(1/2)mv2
where m = mass of object where m = mass of object or body
g = gravity v = speed of object or body
h = height of object
  • KE of an object increases with its speed.
  • Unit: Joule

Law of Conservation of Energy

Energy can neither be created nor be destroyed, it can only be transformed from one form to another.

  • Sum of the kinetic and potential energies of an object is called its mechanical energy.

Total Energy (Before transformation) = Total Energy (After transformation)

lawconservation
lawsconservationenergy

$$\text{PE}=\text{mgh}\\ \space \text{KE}=0\\ \space \text{KE}=\frac{1}{2}\text{mv}^2=\text{increases} \\ \space \text{PE}=\text{mgh}=\text{decreases} \\ \space \text{KE} \gt \text{PE}\\ \space \text{Ground level}$$

$$\text{PE}=\text{mgh}\\ \space \text{KE}=0\\ \space \text{KE}=\frac{1}{2}\text{mv}^2=\text{increases} \\ \space \text{PE}=\text{mgh}=\text{decreases} \\ \space \text{KE} \gt \text{PE}\\ \space \text{Ground level}$$

lawsconservationenergy

$$\text{PE}=\text{mgh}\\ \space \text{KE}=0\\ \space \text{KE}=\frac{1}{2}\text{mv}^2=\text{increases} \\ \space \text{PE}=\text{mgh}=\text{decreases} \\ \space \text{KE} \gt \text{PE}\\ \space \text{Ground level}$$

Commercial Unit of Energy

  • Kilowatt hour (KWh)
  • Energy used in 1 hour at the rate of 1KW is called 1KWh
  • 1 KWh = 3.6 Ɨ 106 J
  • 1 Unit = 1 KWh

Power

  • Rate of doing work
  • Power =(Work/Time)Ā  āˆ“ P =(W/T)
  • SI Unit = Watt =(Joules/Second) ā‡’ 1 Kilowatt = 1000 Watts; 1 Kilowatt = 1000 J/sā€‹