Work and Energy Class 9 Notes Science - Chapter 11

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 E_P = mgh	KE (E_K) =(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)

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