 So you saw in the initial part of the video that a rocket was going up right and that's the story today. So we are going to talk about how did that rocket go up and what made that aircraft when it landed stop and on the runway. So let's talk about Newton's law of motion and this particular topic is going to explain everything we just saw in the video. So what is Newton's law of motion and why we are taking a first law of motion? How is it related to the concept of inertia which we studied in the previous session? Let's try and you know understand all once again. So you watch this video and you saw that the aircraft was being lifted up and there was a rocket. The rocket there is a lot of exhaust gases which is coming out and by the previous knowledge of action reaction that is Newton's third law which you would have studied or you would have had some idea about this law. You know that when the exhaust gases come out what they do they force the rocket upwards is it. So here again that force is the agent which is pushing this rocket up towards the sky. Similarly when you saw the aircraft landing back at the runway. So what was that agent which caused it to stop? If that agent was not there for example you can see that parachute opening up. So this is basically to give the aircraft a bit of air resistance or wider the surface area more is the air resistance you can see it here the parachute being opened up here. So this is for giving the aircraft air resistance and this resistance which is a force will lead to stopping of this vehicle or the aircraft. If this resistance was not there or if the brakes were not applied what would have happened? This aircraft would have gone straight without stopping and probably would have met with an accident. So that is not a good thing to expect and hence we have provided or the scientist who have created or the technologist who have created this aircraft they have ensured that this aircraft stops when the pilot wants it to stop and hence they have given adequate measures for it and if you really realize that we are using the very concepts the very basic concepts of inertia and Newton's first law in this application as well. So now let us go to some more use cases or examples. So look at this picture here the child is going to kick the football the ball is lying on the ground and after some time in the moment he kicks it the body moves or the body starts moving. So what is that thing or agent which is leading to the motion of an object so as you understood last session in the last session that the ball is having an inertia and that inertia is inertia of rest. So this object does not want to move on itself so hence the external agent if it disturbs it then it starts moving and now that what we are seeing here. So let us see what is happening now look at this the boy comes and kicks the ball and the ball starts moving. So this is what we now know that what and we are investigating what is causing the ball to change its state of rest and start moving. So that agent is nothing but the force which the kick applied on to the ball. Now in the second diagram you can see and this is this picture there is a vehicle and you all know that the vehicle continuously needs some engine power to make sure that it runs continuously. So why does it need an engine power to continue in motion because there is something called ground resistance in terms of friction. So the friction will cause retardation and hence the car will come to a stop. So hence what do we now know so to maintain the motion of the vehicle against the resistance of ground friction or let us say even the air resistance what do we need we need a force. So this brings us to the you know the concept of Newton's first law and Newton's first law of motion is actually the reason because of which a body which was lying at rest needed some external agency to set in motion and vice versa that is something which was moving it with or moving in uniform motion or under uniform motion had to be brought down to rest by again applying some external agency. So let us first define the Newton's law so though it looks little you know terribly long but do not worry we are going to break it into small parts and then understand one by one. So clearly talks about a body at rest so all of us know what is rest so it is again a relative term in the chapter of motion you understood rest is not absolute it is relative so we are definitely talking about a frame of reference where we are seeing or observing an object to be at rest similarly something at rest or more body in uniform motion. So body at rest will remain at rest so it is it has the tendency to stay like that it has the inertia it has the power to resist the change. So body at rest will remain at rest and a body in motion will continue in motion here motion means uniform motion just to clarify uniform motion is what we are talking about uniform motion so something which is undergoing uniform motion will continue to do so and not uniform motion guys you know a body moving with a constant speed in one direction or in other words you know body undergoing motion with constant velocity right constant velocity that is called uniform motion. So a body undergoing uniform motion or at rest will continue to do so in either case right unless it is compelled by an external force right unless it is compelled by an external force to change its state of rest or of uniform motion so now you can relate the the rocket was lying idle so it needed some exhaust power to start moving similarly the aircraft which was landing needed some resistance power of the parachute to stop right so some again so air resistance was useful there the air resistance was external force we will talk about external force internal force also but external force has in something which is out of that body or out of that system so hence if you look at that aircraft which was landing the air resistance was outside that aircraft is in it so that was external to the object similarly the football went into motion because an external force that is the force of the kick given by the child drives it right similarly that vehicle needs constant engine power to overcome the resistance of the road that is the friction and that friction again or the air resistance which is also applicable in this case to overcome these external forces now these are the forces which are external to the object under consideration so hence unless that external force is applied the body will neither change its state of rest or of uniform motion so I hope you understood this part so something is at rest let's say a book is kept at the on the on the tabletop you want to move it you need to apply an external force right similarly if someone is you know moving in constant or uniform motion to stop it you need to apply break externally so this is what is called or this is what is the definition of Newton's first law you all know that Newton has given three laws in motion so hence this is the first law sometimes it is or actually it is also Galilean law of inertia which is in another words Newton has described now you can see where this is coming from so this is the 17th century book with Newton wrote and here you can see the the first law and the second law this book is written in Latin so that time the language of science was Latin so this book is written in Latin and you can see this is the first law and this is the second law which you can see here this is the copy of the page from the book called this right philosophy naturalist principia mathematical right so this was this book was written by Sir Isaac Newton now what does it mean so does it mean that a body which is at rest or undergoing uniform motion then no force is acting on it so is it is it that that there is absolutely no force acting on it and the body is at rest or meaning thereby something is at rest that means no force is acting on it or something is moving with uniform motion then also no force is acting on it actually not in every case so that is what we are going to discuss now so and answer as you see is not not yes it's no right so there are two cases which are possible okay so two cases where you know the body will be at rest or uniform motion what are these two cases case one is very clearly when there is no force acting on the object if there is no force acting on the object the body which was at rest the book which was lying at the table will continue to do so right or you know and then second case where the forces are balanced there are multiple forces acting on it but if you add all the forces put together and you know that forces are vector quantity so if you add all the vector quantum vector forces the sum is 0 so in either cases in either of these cases what will happen the body if it is at rest will continue to be at rest and if the body is undergoing uniform motion it will continue to undergo uniform motion okay now let's study these cases one by one so first case is when no force is acting on the object is it really possible that no force acts on the object any ever so the most ideal ideal case it is a theoretical case you know it it is you know possible in theory but practically speaking we don't see we don't get to see that in our day to day life so there is no object in the universe which is not under influence of any force so right now when we are sitting at our homes though we must be appearing to be stationary with respect to the earth but the forces of sun the moon the gravitational attraction is definitely acting on it acting on us so hence there is no object in the universe on which the force or some influence from some other object in the universe is not acting on it so it's a most ideal case theoretical case so to say so a spacecraft we're watering in the expanse of empty space let's say you imagine a very you know empty space with you know almost almost you know billions and billions of kilometers or light years for that matter distance between two celestial body then that could be a closer approximation to this case but even that is not possible because theoretically speaking some bit of influence of gravity or other electromagnetic influence will definitely be there so it is a most ideal case so we don't get to see this in our day to day life okay so closer approximation as I told you a spacecraft wondering in the space so if you know of aircraft which was sent or a spacecraft rather sent in 1977 that was called Voyager 1 and Voyager 2 they are still in use or they are still sending back the information from outer space now around 2013 or 14 I am not very sure of the date but these two spacecrafts have crossed the limits of our solar system so NASA had sent this in space to one you know to let's say do more probing out in outer space and primarily to find out if there is existence of life in outer space so if you consider this particular probe or this particular spacecraft in space and there is no influence of anything else you know not Earth gravity not even you know any other planets gravity there is theoretically but it's very minimal but yes there are there there is definitely some influence of gravitational forces of other celestial bodies but again it doesn't require any you know any external force to push it and it's can't constantly going away from our earth and it is definitely sending some you know great pictures taken from you know out of let's say outside the boundary of solar system and all so if you get a chance please visit the NASA's website where they have you know shared all these imageries which they we these spacecrafts are sending and that's really awesome pictures you can have a look so the spacecrafts are moving away from us without any external force acting on them so there is no fuel in terms of there is definitely fuel to do some course correction but there is no propellant in in to push it away from us so it is going on on its own and it has been going on so let's say now it's more than 40 years which this probe is going on continuously so hence here again you can see Newton's first law is in application why because there is if something is going in undergoing on uniform motion if you don't apply any external force it will continue to do so that explains why Voyager 1 and Voyager 2 are constantly moving away from us without any you know propellant on it or propellant which is pushing it away from us now let's take the case to where it's not the case where none of the forces are acting there are many forces which are acting but the catch is all these forces cancel out so let us consider this bookline and you know at rest on the table so this is the book which is lying over here and then let's understand what all forces are acting on the book so if you look closely these are the forces one the weight which is pulling it downwards the weight or the gravity is pull it is pulling it downwards and the normal force which we studied in previous sessions the normal force is trying to push it upwards the normal is being applied by the table and the weight is nothing but the pull applied by the earth so these are the two forces now not apart from the apart from this information this information is also there that normal is upward you can see this arrow here normal forces upward and the weight is downward now these two forces when added vectorially that is if you consider them to be vectors and add them together the sum is going to be 0 that is the force of normal is balancing out the force of gravity so these two forces are balancing each other out hence we say the body is at equilibrium but what do we say we say the body is at equilibrium now this is called mechanical equilibrium guys what is equilibrium what types are equilibrium of we will see later but right now this is mechanical equilibrium right and this is a two force system so two forces are acting and they are balancing each other out okay now let's and now you can see also you can I have written this equation where normal force minus the weight I am just talking about the magnitude of the forces they are the sub if you minus them subtract them you will get 0 because their magnitudes are same okay now let's consider another example where this is a swing you use it in the park children use it for entertainment and enjoyment during their evening you know outings so let us consider this swing and I'm going to consider the plank as the body okay this plank wooden plank is there now it is at rest it's given and what all forces are acting on the wooden plank let's start let's find out so if you look at it there are tension in the four links you can see there are four links and T1 T2 T3 T4 are the tensions which are there on these links can you see that guys this one T1 T2 along this links which are there right T3 T4 and then finally W so these are the total forces now if you see they are not in the same direction in the sense T1 T2 are not vertically opposite to W there is some angle if you can see there is this vertical angle and there is some angle between the T2 and the vertical direction so hence they are not vertically aligned but one thing is very clear if you add all of them vectorially that is the vector sum of all of these is going to be 0 okay so this is what I have written T1 plus T2 plus T3 plus T4 plus W if you look at these arrows above these letters it means that they are vector quantities and hence vectorially if you add you will see all of them will add up to 0 so multiple forces are acting on it it's not that no force is acting multiple forces are acting but the thing is even if multiple forces are acting the body can be at rest or can be undergoing uniform motion only condition is some of them must be 0 and again this body is at an equilibrium multi-force equilibrium because lots of forces are acting still the body is at equilibrium is it this is what is called balanced forces so these all put together are balancing each other out now let's take an example problem to understand so the example problem is a body of weight 5 Newton is kept on a smooth horizontal table the same example which we just illustrated find the force exerted by the table on the body so what do you need to do is first draw the diagram and this is called the free body diagram guys so we'll take up free body diagram in a different session but what do we do we draw the diagram and we identify all the forces acting on the body so and when we identify we make sure that the forces are shown or displayed at the point of application so wherever for example the weight acts from the center of mass here and the normal is acting at these locations right and hence you don't see this arrow is behind the white arrow okay so but then the force force is being acting from here is it so this is the thing so normal is there weight is there there are these are the two forces and Newton says that if the body is under rest is at rest then the total force acting on it must be 0 because there is no either the case is no force case or some of all the case forces will be 0 so hence normal and weight if added vectorially we should be 0 or they should be balancing each other out so normal must be balanced weight and hence we can write normal minus this 5 Newton weight will be equal to 0 so normal is equal to 5 Newton force so don't get confused between these two symbols this stands for normal because Newton is the unit of force we saw earlier and Newton and capital N represents Newton so hence don't get confused here so hence I have used a different font for normal and this font is for Newton as in the unit of force so in n minus 5 Newton is 0 so normal force must be equal to 5 Newton force is it so hence why have I used negative sign here and the previous slide if you look at it let me go to the previous slide here all are sum all our pluses now this pluses are because I have used vector notation so whenever you're using vector notation you will learn later that then you have to use plus but if you strip off the vector symbol and you are trying to only work with the the magnitude of the forces then you can use the minus sign that is and in other words you can say this normal and then 5 Newton force this normal force minus that 5 Newton force must be 0 the net force must be 0 so this is what it is right so hence it will be so this is n is equal to or the normal is equal to 5 Newton force okay so this is what is about Newton's first law in the subsequent sessions what we are going to do is we are going to solve a lot of numerical problems and that will make things much much clearer I hope you understood this concept first law of motion Galileo started working on it carried forward by Newton now the same force the same laws will become the foundation for the subsequent laws which Newton gave later okay so with those words we will close this session and see you in the problem solving session after this one thank you