# Basic Physics lesson-2 : Speed and velocity

*Umesh is a freelance writer contributing his creative writings on varied subjects in various knowledge and educational sites in internet.*

## Introduction

When a body moves from one place to another we say that it 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

## 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. We can always convert the value from one unit system to another as conversion factors are available and can be used easily 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. The path need not to be a straight line, as it could be a combination of straight and curvilinear parts or simply a zig-zag one. So, in any case 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. Let us take example of an athlete taking part in 800 metre race and completing it in one and half minute. His speed would be = 800/1.5 = 533.3 metre/minute. That translates to about 32 kilometer per hour. That is almost like the speed of a car when we drive through the town market!

## 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.

When a body is not moving in a straight line then each small part of its movement can be represented by a very very small (infinitesimal) velocity vector having a value equal to the speed at that point and direction of movement at that particular place. These small value vectors having different different directions together constitute the trajectory of the movement of that body and is the fundamental concept behind the velocity as a vector.

## 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 in this series.

## Diagram-1

## Some interesting points

When we are travelling in a train, we are just sitting in our compartment and from our point of view our velocity is zero as we have no displacement with respect to the train which is the frame of reference for us while travelling. But an observer from outside will see the train and it's passengers moving with a good speed. So perception of speed is a relative phenomenon in many such situation. Another example often quoted is the movement of Earth around Sun in almost a circular orbit completing one round in 1 year time but we do not feel it because we are a part of the Earth only like a person sitting in a train.

Bodies and objects can have different speeds depending on their masses and force applied on them to make them move. Heavy objects cannot be moved so easily while lighter bodies can gain speeds with relatively lesser force.

When we throw a body up in the air, it comes back to us. Why? Because of the gravitational pull of Earth which is strong enough to attract it back. Then how we are sending the rockets to Moon or Mars. Would not they fall down? The answer to this question lies in the concept of escape velocity. Actually, the gravitational pull decreases when the bodies are present at greater distances from each other. So, what we do is that with powerful thrust engines we fire the rockets and once they acquire a high velocity then they go out to bigger distances and Earth's attraction on them diminishes considerably and the rockets then move on without falling back. To achieve this the critical velocity to be attained is called escape velocity (literally to escape from the grip of Earth) and is quite good in value and is actually about a whopping 11.2 kilometers per second!

## Rate the article

## 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

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on May 14, 2020:

Jim, thanks for your nice comment. Also for visiting.

**Jim Henderson** from Hattiesburg, Mississippi on May 13, 2020:

Enjoyed. Physics is a great subject, one that forces me to think. Some of it often seems counter intuitive, oftentimes because I failed to consider all of the factors involved.

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on May 07, 2020:

Fusrya, thanks for visiting and sparing your valuable time. I enjoyed your hilarious note much. Keep in touch. Stay blessed.

**Fusrya** on May 07, 2020:

Very nice article sir!

Hope you also like my geeky physics fact - "If i die on my birthday in the same hospital where i was born, then my average velocity through life on earth would be zero"

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on May 04, 2020:

Aruj, thanks for sparing time. Appreciate.

**Aruj** on May 04, 2020:

Very well written article. The diagram Is an added bonus. It helps in internalising the concept of velocity and displacement.

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on May 03, 2020:

aktiwari1201, thanks a lot for your lovely comment. Appreciate much.

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on May 03, 2020:

SKMUNSHI, thanks for sparing your time for going through the article and also for your valuable suggestions. People like you are a great source of encouragement for me. Highly appreciate.

**aktiwari1201** on May 03, 2020:

Nicely explained with the help of diagram. People without science background will be benefitted with this comprehensible article. Pl do post more such articles. It refreshes our old memory and makes us nostelgic.

**SKMUNSHI** on May 03, 2020:

Nicely articulating the concept of speed ,velocity and distance. Illustrations with diagram make it more interesting. One more graph between distance and time to explain a derived quantity velocity may be added in the next lecture.

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on May 03, 2020:

Sangita, thanks a lot.

**Sangita** on May 02, 2020:

Very well explained - the illustration makes it very clear. Thanks

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on March 04, 2020:

Flourish, thanks for your kind words and happy to see that it has practical utility. Thanks for sparing your time.

**FlourishAnyway** from USA on March 04, 2020:

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

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on March 02, 2020:

Anurag, thanks for the nice comment. Appreciate.

**Anurag** on March 02, 2020:

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!!

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on March 01, 2020:

Clive, thanks for visiting and sparing your time.

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on March 01, 2020:

DreamerMeg, you have reminded me those 2-3 equations relating initial velocity, final velocity, acceleration and distance with each other. I would be preparing a lesson on that separately. Thanks for your interest shown. Really appreciate.

**Clive Williams** from Jamaica on March 01, 2020:

My brain just got blown!!

**DreamerMeg** from Northern Ireland on March 01, 2020:

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.

**Umesh Chandra Bhatt (author)** from Kharghar, Navi Mumbai, India on March 01, 2020:

Eric, thanks for your interest. Really appreciate.

**Eric Dierker** from Spring Valley, CA. U.S.A. on March 01, 2020:

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