# Basic Physics lesson-2 : Speed and velocity

## Introduction

When a body moves from one place to another we say that is had some speed with which it travelled from first point to the last point. If we recall in the earlier article Basic Physics lesson-1 : Distance and displacement, we learned about distance and displacement. In continuation to that and in the same way here in this article we would now learn about speed and velocity.

## Physics

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## What do you understand by speed?

When a person moves a distance 'd' in time 't' then we say that his speed is d/t and we can represent it in meter per second or meter per minute or km per hour or miles per hour whichever unit is convenient to us. Speed is a scaler quantity as it does not have the element of direction in it. The person can move with a constant speed or varying speed during the covered distance. So by dividing the distance with time what we get is actually the average speed over that stretch. Let us make it more clear with an example. A person walks in the city in zig-zag lanes covering a total distance of 3 km in 2 hour. So we can calculate his average speed as 3/2 = 1.5 km per hour or 3000/120 = 25 metres per minute. So speed can also be thought as the rate of distance covered. For example if a person covers 100 meters in 2 minutes then his speed is 100/2 = 50 meters per minute.

## What do you understand by velocity?

Once we add the element of direction to speed it would become a vector quantity having a value as well as direction. In the earlier article Basic Physics lesson-1 : Distance and displacement, we learned that displacement had a value as well as a direction. Velocity is defind as the rate of change of displacement. For example if a body has a displacement of 2000 meters in 10 minutes say in East direction then the velocity is 2000/10 = 200 meter/ minute in East direction.

## Application

Knowledge of velocity of a body over a time interval can be used to find the displacement. If velocity is constant then it is very simple and by multiplying it with the time we get the displacement. If velocity is varying then we can use the graphical area method to find it.

Let us go through one example to make it clear. Suppose a body is having a velocity of 8 meters per minute and gradually it increase to 25 meters per minute in a duration of 20 minutes. Now the problem is to find out the displacement in this 20 minute time. One method to solve for this is to draw a graph between velocity and time. Let us plot velocity in Y axis and time in X axis. For our convenience we have assumed that at time equal to 10 minutes it was having a velocity of 8 meters per minute and after 20 minutes that is at time equal to 30 minutes it has attained a velocity of 25 meters per minute. Once the graph is plotted, we can simply find the area of the shaded part with dashed lines as depicted in diagram-1 which will give the displacement in meters. I have hand drawn it just to encourage the students to draw it themselves. Computation of this area will give us the displacement in meters. One can try it and the answer is 420 meters. Please note that in this example the velocity is gradually increasing from 8 m/mt to 25 m/mt which is represented with a straight line. In case the velocity is changing in some other functional way or irregular way then either we have to use a graph paper to compute the area or use integration method to find it. Anyway, integration techniques would separately be dealt in some future lesson.

## Diagram-1

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## Conclusion

Speed is the scaler quantity which gives the rate of distance covered by a moving body while velocity gives the rate of change of displacement. Velocity is a vector quantity and has a specific direction element embedded in it.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

**© 2020 Umesh Chandra Bhatt**

## Comments

What would be extremely helpful is a couple of bulletpoints of real life applications for how you would use this. Well presented.

Very precisely taught!!

It cleared small yet useful concepts of Physics specially this topic which is bound to us in day to day life!!

Keep posting conceptual lessons in future too!!

My brain just got blown!!

I did speed and velocity in school but never used a graph to calculate distance. Interesting. I still remember s=ut+1/2 at^2.

I will be back. Worthy of a re-read.

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