According to Law of Conservation of Energy
When energy changes form from one form to other
Total Energy remains constant
There is no loss or gain of energy
In easy language
Energy can never be created nor be destroyed
It only changes from one form to other
Example 1  Falling brick  Law of Conservation of Energy
We know that
its potential energy gets converted into Kinetic energy
There are different stages in this case as follows
Brick About to be thrown
At the time, when the brick is about to be thrown, it has Potential energy (Suppose 30 Joules) but 0 kinetic energy
So its total energy is 30 + 0 = 30
Brick Falls Down
As it falls down ,its potential energy keeps on decreasing/(Suppose it becomes 20 joules) but Kinetic energy goes on
inreasing(Suppose it becomes 10 Joules),
However total energy stays equal at 20 Joules + 10 Joules = 30 Joules
Brick Falling Down even further
As brick falls down even more,its potential energy keeps on decreasing (12 joules) and Kinetic Energy goes on increasing at 18
jOules
But total Energy stays at 12 + 18 = 30 Joules
Brick before Hitting Ground
Just before hitting the ground, its potential energy becomes 0 but kinetic energy becomes the highest (30 joules)
So total Energy stays at 0 + 30 = 30 Joules
Case  Potential Energy  Kinetic Energy 
Total Energy

Brick before throwing  30  0  30 
Brick Falling  20  10  30 
Brick Falling after Some time  12  18  30 
Brick Just before reaching ground  0  30  30 
Note
In all above cases,total energy remains constant at 30 Joules
This is because of law of conservation of energy which states that
When energy changes form from one form to other
Total Energy remains constant
There is no loss or gain of energy
Example 2  When a ball is thrown up
When a ball is thrown up, its speed decreases
When a ball is thrown up, it is thrown with some initial velocity.
Hence, it has some kinetic energy but no potential energy.
Ball About to be thrown up
At the time, when the ball is about to be thrown, it has kinetic energy (Suppose 30 Joules) but 0 potential energy
So its total energy is 30 + 0 = 30
Ball goes up
As it goes up ,its kinetic energy keeps on decreasing(Suppose it becomes 20 joules) but potential energy goes on
inreasing(Suppose it becomes 10 Joules),
However total energy stays equal at 20 Joules + 10 Joules = 30 Joules
Ball goes up even further
As ball goes up even more,its kinetic energy keeps on decreasing (12 joules) and potential Energy goes on increasing(18 joules).
But total Energy stays at 12 + 18 = 30 Joules
Ball when it reaches the highest point
Just when it reaches the highest point, its kinetic energy becomes 0 but potential energy becomes the highest (30 joules)
So total Energy stays at 0 + 30 = 30 Joules
Case  Potential Energy  Kinetic Energy 
Total Energy

Ball About to be thrown up  0  30  30 
Ball goes up  10  20  30 
Ball goes up even further  18  12  30 
Ball when it reaches the highest point  30  0  30 
Questions
NCERT Question 6  The potential energy of a freely falling object decreases progressively. Does this violate the law of conservation of energy? Why?
NCERT Question 10  An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is halfway down.