Velocity

Velocity is a speed of a body with its direction of motion. It changes when either speed or direction of motion changes.

When we say that a bus is moving at a speed of 40 km/hour there is no information about the direction of its motion. When we also inform the direction with the speed, then it is called velocity.  
 

Velocity changes if either magnitude or direction changes. So if we say a bus is moving at speed of 40 km/hour towards the north, we are talking about its velocity because it has speed in a specific direction.

Velocity remains constant when speed (its magnitude) and direction do not change during the motion. 

Here are the few examples of the velocity change. You can get an idea of speed through the reading of speedometer and direction can be visualized through the bumps on the road.

(i)  Speed changes but direction remains fixed 


In this case bus is moving in a straight line but its speed is changing.


(ii) Speed is constant but direction changes


In this case bus attians a speed initially then covers the rest of journey with the constant speed. There are ups and downs on the road, so its direction is changing. 

(iii) Both speed and direction change

 

In this case both the speed and direction is changing.