 and welcome to this session on Newton's laws of motion. So till now we were discussing about various types of forces and hence we concluded that Sir Isaac Newton came up with his famous three laws of motion. But before we take up laws of motion given by Sir Isaac Newton, it is also important to understand what happened before Newton. So who all were there, who all scientists were there who have given laws of motion and what was world following till then, till 1642 as in the year when Newton born and it was late 1660s when Newton came up with his laws of motion. So before that what was happening is what we are going to discuss in this session. So there was a philosopher named Aristotle and he was a Greek and he existed in 4th century BC. So he was from 384 to 322 BC. Just a trivia for you Aristotle was also the teacher of Alexander, so the famous king of Greece. So he was the person who was a philosopher, he was a polymath basically. So he was an engineer, a philosopher, a physicist and all at that point in time and he came up with his laws of motion from the observations which he had around him at that point in time. So this is we are talking about 4th century BC. Now what was his laws of motion and what was the elements of his laws of motion, let us discuss that. So what he thought was the substances making up the earth were different from the substances making the heaven. That means earth is made up of different type of material and heaven, the other part of the universe is made up of some other material. And hence the laws governing the motion of the heavens were different set of laws than those that govern motion on the earth. What it means is whatever we are observing in earth or whatever laws of motion are there here on earth, it will be different from whatever is happening let us say on Mars or Venus or even Sun and any other part of the universe. That is what he said. Then what he was talking about is this also that dynamics is the branch of physics that deals with motion. So this is you can now see the history of the word dynamics. So this is coming from that time and it was branch of physics that deals with motion was primarily determined by the nature of the substance that was moving. So hence as you can see if the substance is different, the laws followed or obeyed by that particular substance will be different. So that is what Aristotle is saying. So here you can see there is an apple tree and there are raindrops which are about to fall from clouds. So let us say the apple is falling from the tree and let us say the raindrops are falling from the cloud and what Aristotle is suggesting is since apple and water are made up of different substances, hence they will obey different laws of motion. This is what he is proposing at that point in time. Now you can reason out why is he saying that and what is the logical sense to all of that and today we know that all of this is not true but at that point in time we cannot expect that there are enough experimental tools or even for that matter there was no methodology of researching about all of that. Fair enough. So let us go further into it. So what is he is mentioning further? So he is saying that a stone fell to the ground because the stone and the ground were similar in substance. So that is what now he is saying. He is saying that a stone is going to fall on the ground because stone and the ground are made up of similar material. Similarly, smoke is rising away from the earth because in terms of the four basic elements and what are those basic elements you would have heard that fire, earth, water and all these, there are four basic elements which were there during that time they used to believe that everything is made up of those four basic elements. And now Aristotle is saying that it was primarily, the smoke is primarily air that is the substance of smoke and substance of air is same and hence smoke will go towards air. So that is what Aristotle is suggesting. It is as good as saying that like is attracting like. Now similarly, the most contentious part of his theory and today or later on when Galileo refuted it and when Aristotle, our Isaac Newton also refuted it was that objects move as long as they are pushed. That means if something is moving that means it is constantly being pushed. And today we know that you don't require force to keep something moving. You only require to start and stop it. If there is no friction, it will continue moving. So this is what we know that but earlier that in that point in time when in 4th century BC his observation is that the particle are moving or the objects are moving because a constant force is being applied onto it. Now he could not see that there is something called friction which actually opposes the motion and later on it was clarified by Galileo. Now then came Galileo. So you can see that till 15th or 16th century BC, there is no much or not much development in terms of laws of motion. And this is the time when Europe is undergoing some kind of a transformation when renaissance is happening other things in the European society where they are challenging all the beliefs that were established during that time. So Galileo comes and he challenges Aristotle in laws of motion. So he is an Italian astronomer, physicist and engineer. So you can see that during early days people were involved in more subjects than what today we people are doing. And he refuted Aristotle in laws of motion. He said that whatever Aristotle is suggesting is not correct. And he asked this question that if push is required to continue motion then what pushes an arrow once it leads the bow. So what is that thing which is making the arrow go on continuously in one path if the archer has released the string or the arrow has left the bow? Then who is pushing that arrow? That was his question. He proposed his own ideas and he said that he proposed the theory of inertia which we now know as Newton's first law of motion. And he said that a body continues to be at rest or uniform motion because of its inertia. There is something called inertia which keeps a body at rest or uniform motion. So that was he suggested. And this theory became the foundation of Newton's laws of motion. We all know that the first law is nothing but which explains inertia. So anything which is at rest will continue to be at rest. Anything which is undergoing motion, uniform motion it will continue to do so till an unbalanced force is acting externally on it. So we will deal with them in detail when we are taking up Newton's laws but this is what Galileo has observed. So Galileo was very famous for his thought experiments. Now what are thought experiments? You actually concede the idea. I don't really do it but you concede the idea and with logic you try to prove the idea. So what did he suggest? He basically took the inspiration from a pendulum. He said that he said that if there is a pendulum and it is hanging from the ceiling. And let's say this is the bob and you take the bob in one direction and release it. So what happens? It starts swinging, isn't it? And interestingly it raises to the same level from ground. So let's say this is the ground level and this is the extreme position. So the bob comes here and the bob comes here. So in both the cases the extreme positions, the height attained from the ground are same. That's what he observed. So he tried to replicate this experiment using a hemispherical bowl and rolling a bowl on that small marble on that and he suggested that if this is the ball here, if released it will come and it will stay here. And it was common observation if there is no friction between the surface of the ball and the hemispherical bowl. So this is what he said that if the bowl is there it will come back come to the same height and it will stop there. Now he suggested what if we have a different setup. Let's say this kind of a setup is there and now we have extended the path of the bowl which is going to roll over and basically it's an extended hemisphere as you can see. Then here also he observed that the bowl when released goes to the same height. So it goes to the same height. Now it takes more time and it is covering a larger distance but again, because there was no friction he observed and later on you will also study under something called conservation of energy that the potential energy is going to be same initially and the final or total energy rather is going to be same initially and finally now since the kinetic energy initially is zero in this case and finally also it is zero because it has attained the same height it is coming to a halt momentarily so hence the initial potential energy and final potential energy must be same and hence it attains the same height. So if there is no friction between the two surfaces then the bowl is going to come up to the same height. Now he stretched his imagination and he said now let's go to this kind of a setup. This kind of a setup. Now what you can see is only the first half of that extended path for the rolling bowl. The other half is not shown in this diagram and imagine that it is infinitely long. So if it is infinitely long then the bowl which is rolling will never be able to come to the same height. It will continue moving like this. It is going to continue move continuously like that. So it will never stop or it is trying to attain the same height but it is never getting that height. So it will continue to move. So imagine that towards a long distance. After a long distance it will see the upward curve but then every time it reaches the upward curve you take that curve away from the bowl. So this bowl is going to continuously move. Isn't it? That is what Galileo observed through his thought experiment. So guys what is the summary of the entire discussion then? So we have two stalwarts. One was Aristotle and another was Galileo and there they are around what some two centuries almost two million almost two millennia away. So Aristotle is saying force is required to continue uniform motion but he could not attribute the slowing down of moving object to friction. So he could not see that objects are slowing down because there is something called friction which is acting between the two surfaces. While Galileo observed that force is required only to start and stop the uniform motion you don't need to apply force continuously to maintain the motion. For example a spacecraft which is moving in space right now for example you know you would have heard of the spacecraft Voyager 1 and Voyager 2. They have crossed the solar system now so they are moving on their own. So we don't need to really apply any effort to make them move. So how are they moving on their own? So that's what Galileo observed that if there is nothing to stop them they are going to continue with the same velocity. And finally he said that an object slows down because an external force either friction or air resistance acts on it. So this is the summary of the two people or the laws of motion given by the two people. And then finally who came into the picture? Sir Isaac Newton. So it was him who took the baton from Galileo and came up with his three laws of motion. Not only he gave the qualitative analysis or qualitative view of laws of motion he also gave mathematical view of that. So he explained the three laws using mathematics. And while doing so he contributed significantly in the field of calculus. So today we know that calculus. He was one of the early contributors other being got rid of limits. So these two people are known for developing the body of initial body of mathematics calculus. And now today we are using this subject in multiple other fields as well. And interestingly Newton was born in the year when Galileo died. Okay that was the year 1642. So 1642 was the year when Newton comes on earth and you might be astonished to know that when he was giving this laws of motion it was a plague outbreak in London and because of which he had to go to his hometown and there is where he had a lot of time to brood over all these natural phenomena and finally he came up with lots of lots of laws and theories in physics as well as mathematics. I hope you got the glimpse of the historical background of laws of motion. Now in the next session we are going to discuss Newton's laws of motion one by one in a much detailed manner. Thanks for having patience to go through all this and keep watching. Thanks a lot. Bye bye. Take care.