Concepts

Class 9
Chapter 8 Class 9 - Motion (Term 1)

## What is Velocity?

Velocity of body is

Distance travelled by body per unit of time in a given direction

Velocity = Distance travelled in a given direction/Time Taken

= Displacement/Time Taken

## How is velocity different from speed?

Example 1

Suppose a person travels 6 km in 2 hours towards east

Speed = Distance/Time = 6/2 = 3 km/Hour

Velocity = Displacement/Time = 6/2 = 3 Km/hour towards east

Example 2

A car travels 40 km from Delhi to Noida in half an hour towards east

It comes back from Noida to Delhi in another half an hour

Speed = Distance/Time = 80/1 = 80 km/Hour

Velocity = Displacement/Time = 40 - 40/1 = 0/1 = 0

## Difference between speed and velocity

 Speed Velocity It is Distance travelled by body per unit of time It is Distance travelled by body per unit of time in a given direction It only has Magnitude It has Magnitude as well as direction Formula is Formula is Speed =Distance/Time Taken Velocity =Distance in a given direction/Time Taken =Displacement/Time Taken Average speed of Moving body is never 0 Average velocity of Moving body can be 0

Note

Are speed and velocity always equal in Magnitude?
No
They may or may not be equal

Example - Speed and Velocity equal in Magnitude

A car travels 40 km from Delhi to Noida in half an hour towards east

Here Speed = Distance/Time = 40/0.5 = 80 Km/Hour

Velocity = Displacement/Time = 40/0.5 = 80 km/hour towards east

Here ,Magnitude of speed and magnitude of velocity are equal(both are 40)

Example -Speed and Velocity not equal in Magnitude

A car travels 40 km from Delhi to Noida in half an hour towards east

It comes back from Noida to Delhi in another half an hour

Speed = Distance/Time = 80/1 = 80 km/Hour

Velocity = Displacement/Time = 40 - 40/1 = 0/1 = 0

here,Magnitude of speed is 80 but magnitude of velocity is 0

So both are not equal

Summary

Hence,we can say that average speed of moving body can never be 0 but average velocity of moving body can be 0

(Example-In case of Round Trip,there is no displacement,so Displacement is 0,so velocity will also be 0)

## Questions

Q 1 Page 102 - Distinguish between speed and velocity 