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 half-way down.

1. Class 9
2. Chapter 11 Class 9 - Work and Energy (Term 2)
3. Concepts

Concepts

Class 9
Chapter 11 Class 9 - Work and Energy (Term 2) 