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 × S or Work done = Force × 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 × S.
 If work done is in the direction opposite to the force, then W = –F × S.
 Work done has only magnitude but no direction (scalar quantity)
 SI unit = Joule.
 1 Joule = 1 Newton × 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 




where m = mass of object  where m = mass of object or body 
g = gravity  v = speed of object or body 
h = height of object 


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)
$$\text{PE}=\text{mgh}\\ \space \text{KE}=0\\ \space \text{KE}=\frac{1}{2}\text{mv}^2=\text{increases} \\ \space \text{PE}=\text{mgh}=\text{decreases} \\ \space \text{KE} \gt \text{PE}\\ \space \text{Ground level}$$
$$\text{PE}=\text{mgh}\\ \space \text{KE}=0\\ \space \text{KE}=\frac{1}{2}\text{mv}^2=\text{increases} \\ \space \text{PE}=\text{mgh}=\text{decreases} \\ \space \text{KE} \gt \text{PE}\\ \space \text{Ground level}$$
$$\text{PE}=\text{mgh}\\ \space \text{KE}=0\\ \space \text{KE}=\frac{1}{2}\text{mv}^2=\text{increases} \\ \space \text{PE}=\text{mgh}=\text{decreases} \\ \space \text{KE} \gt \text{PE}\\ \space \text{Ground level}$$
Commercial Unit of Energy
 Kilowatt hour (KWh)
 Energy used in 1 hour at the rate of 1KW is called 1KWh
 1 KWh = 3.6 × 10^{6} J
 1 Unit = 1 KWh
Power
 Rate of doing work
 Power =(Work/Time) ∴ P =(W/T)
 SI Unit = Watt =(Joules/Second) ⇒ 1 Kilowatt = 1000 Watts; 1 Kilowatt = 1000 J/s