# Work and Energy Class 9 Notes Science - Chapter 11

## What Are Work And Energy ?

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## 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 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)

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

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

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

Commercial Unit of Energy

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