 okay so today's session is on free body diagram okay so I have taken free body diagram just to you know introduce to you how how can we use the concept of vectors in the free body diagram okay just like you have taken example of projectile motion to understand how vectors can be used to analyze the projectile motion okay similarly I'm going to take free body diagram to make you understand how can we use the concept of vector that we have just learned on to this okay from where we work from like where we will then integral part of this chapter laws of motion okay have you seen that equation force will do mass demonstration okay so we are going to learn how to apply force will do mass time acceleration okay that force is net force okay so we're going to write that equation now just like g is equal to u but then when we use g is equal to u plus a t in projectile motion we have used in x-axis and then long y-axis yes or no right similarly we are going to use force is equal to mass demonstration along x-axis and then long y-axis and if it is required along z-axis also okay but before you would write that equation before you even write that equation you need to do some basic analysis as in what are the forces acting on the body which directions there the forces are acting so that basic analysis what we are doing 90% of that is free body diagram only fine so if you do this properly physics okay because laws of motion is the basic chapter of the physics if you're good at laws of motion your entire physics becomes very easy but if you're bad at laws of motion it becomes extremely difficult fine so you cannot afford to be bad at you know drawing free body diagram or this chapter laws of motion okay so we are going to do this law of motion again when we start class 11 syllabus we'll be doing laws of motion again but right now I'm just introducing to you a section of laws of motion which is free body diagram so that we can understand how can you use vectors there okay but before we even get into free body diagram alright we need to understand what are the forces what are the properties of forces what are the laws which forces or any object in motion follows okay so write down the motion I'm not going to complete this chapter I'm just taking the relevant portion of this chapter which in which we can use the vectors law of motion as a name suggests it is not which an object in motion follows understood if object is under motion that object must follow these laws which we are going to talk about okay now there is the concept of object at rest doesn't exist actually fine so you might like you know any object at rest can be thought of moving with respect to some other object like all these moving you might think that your address you're not okay so when I say laws of motion these laws are also valid for an object which is at rest fine so you might have the motion since from your class 9th or 7th also at times Newton Newton or the person who has formalized these laws in a systematic way and he has put across it as first law of motion second law of motion and third law of motion right can you tell me what is the first law of motion what it is anybody remain in rest but we haven't see the thing is I know you haven't you have this concept of force already you know it but the way I started teaching in class 11 that I'm not assuming you know anything okay I'm not even assuming you know what is force till we introduce what is force let's not use these terms fine so can you define the first law of motion it introduces tendency of an object isn't it it tells you the behavior of an object and what is the object what is the definition of an object anything that has mass anything that has mass first law talks about the behavior of it Newton's first law of motion and I'm just writing first-law motion it talks about behavior what is this behavior all about the basic behavior of anything any mass it doesn't want to change that and that is also like you know for any you don't want anything to be changed in your life also at times okay the same kind of behavior every mass has whatever the mass is doing the mass wants to do it okay but and that is its behavior all right the object right now the object or mass are we using object and mass interchangeably okay object doesn't it's state whatever state the object is in the object wants to be in that state okay now when I say state two kinds of states all right state of rest right on the concept but then you remember you know that when Newton was formulating laws you know he was treating rest an object at rest means the object is at rest with respect to earth we are sitting we are standing on the earth okay we are at rest so that that is what it means object at rest okay so object at rest is a state of rest and state of uniform motion these are the two states so if these two states are there in an object object doesn't want to change are you getting it so you don't have to do anything for example it is like this okay if you do not make any effort object that's what it means okay and if uniform motion means uniform velocity not even direction is changing getting it direction and magnitude should be constant uniform motion means uniform velocity right now direction is unchanged and constant speed okay constant speed motion in a straight line is a uniform velocity okay so if object is moving a straight line you don't need to make any effort in doing so why because that's what if some some object is sliding on the earth support on the floor some object is sliding and then stops after some time why that happens this object is moving velocity V this object will stop some after some time all that wants to move isn't it but why it is stopping because somebody is making effort what should happen with this block they keep on moving the friction a little bit the masses for further move away that tells you the behavior of the mass that you know if ground lessens its effort of stopping this mass or it completely removes its effort then the mass will keep on going fine so this tells the you know behavior of the mass getting it now let's talk examples okay and you have any examples of a day-to-day life where you observe Newton's first law one example I tell you for example where when you have a dust from a drug what you do beat it beat it right so when you beat the rug what happens is that the loose particles of the dust they want to stay wherever the rug suddenly shifts when you beat it the dust particle they fall down this is the example of Newton's first law give me another example like when you're sitting in a like car which I'm which are not move and start suddenly starts because you want to stay in that position but you are moving back now you said because you can't stay there for you are you correct so car moves ahead but your upper body so your lower body is moving with the car but upper body wants to remain there itself okay so that's how you feel as if you're falling back okay what else what are example one example smart phone only last year last week when you play the car I stacked the coin one after the other and you hit the bottom of the coin with a very high velocity what happens to the upper coins they stay like that only and they fall vertically down so the upper coins wants to maintain their position wants to remain at rest also remain as it is okay so that's why they they don't fall down but if you put the striker a little slowly the upper one also falls down why so because then there's like enough time to transmit that force to the top correct so there then the lowest lowest coin gets sufficient time to affect the motion of the upper coins getting it but if you don't give it time what else but if there is no friction suppose between the lowest coin and the upper coin then it doesn't matter yeah what else think of tell you there's so many examples so many don't think of okay tell me one thing suppose one example is this when you when you shake a tree I mean not a branch of a tree better example when you shake a branch of a tree the the fruits which are I mean the fruits which are heavy or which are right they they fall down right but the fruits which are like not right they will not fall down why they want to stay right they want to stay there but then the pump to stay there yes or no but the heavy one wants to stay more that's kind so this behavior these two behavior is stronger for heavier mass density of behavior also I mean these two be here will be definitely there for each and every object but the intensity or behavior is more for heavier mass so this behavior is proportional to the mass yes or no to check this you can think of a light object at rest you can easily make it move but if there is a heavy object when you push it it is difficult to getting it so that tells you that the heavier object has more intense behavior okay and since this behavior is very common across the universe this behavior is okay so we name this behavior and the name of this behavior is inertia so inertia is more for the heavier mass but mass is not inertia mass is a representation of the inertia and you think of another example give me couple of balls get off the bus then so suppose you have this bus so bus is moving don't do that okay but suppose you are out of your mind and you jump okay which side you will fall forward or backward why because you are moving forward when you jump your feet come to rest keep on moving so you fall forward so if you don't want to fall then what you should do keep running keep running in this direction only what will happen then is that your upper body lower body all are moving with the same velocity okay so you should run with the velocity of the bus only for for a couple of seconds okay but if you run with more velocity of velocity of bus you'll fall backwards okay so you should calculate first and then jump this is not safe you might be safe that you you know you're not toppling but from behind some some other vehicle might be coming okay so you have to do a lot of other calculations as well don't know that any other example check out there are stacks of books stacks of books and there's a piece of cloth that is at the bottom you have to pull it very fast one more like they set the plates on the dining table and the dining cloth is there if you pull the dining cloth really fast the plates but the clothes should be you know completely flat and okay then what else that's it I mean let's move forward so there's so many examples of this okay in fact you know how to take motion that we were doing some of my fighter aircraft velocity of the bomb initially same as the ball wants to continue moving with the same velocity with which it was like when it is in the plane getting it so there's so many examples and anyway so when we proceed with this chapter we will come to know ways other examples and last one that is coming in my mind is this suppose an object is moving in a circle circuit motion okay and there's a string attached to it this mass is moving a circle and a string is attached to it suppose the string breaks how the muscle go and gently why tangentially that's because that was its velocity at that point in time it tries to maintain that velocity but if there is a string that way so it's velocity changes and it makes a kind of circle but if string is cut nobody is there to pull and gently it tries to maintain its velocity at that point in time fine so like this you know the first law of motion but you should be more aware of it you know you should know that okay fine I'm using first law of motion fine then you'll feel a lot more in control when you solve numericals getting it down the second law of motion second law of motion is the most important law in the physics class 11th and 12th okay which looks very simple force is equal to mass and excitation but don't go by how simple it looks it will make your life for two years okay so lot of tricky questions can be made out of just application of this simple equation okay so be very watchful and be be very observant of whatever is discussed in this chapter okay so second law of motion tells you that there's a behavior and then in state what is the state two states okay the second law of motion tells you that how much effort you should make to change to change the state it quantifies the amount of what you should be making to change the state of the object here first but if you want to change the state how much effort you should be making that Newton's second law tells you getting it now Newton's second law is force equal to mass immaculation if mass is constant but that's that law is not God has a mass immaculation it is our own creation getting into the power 3 nobody's going to in the simplistic equation possible more simple equation which is force equal to mass immaculation okay but whatever in sync with what is the observation you should not say that force is equal to acceleration divided by mass why because you know that more is the mass more force you need that kind of basic observation must be taken care of so let's talk about few observation then we can arrive at the Newton's second law okay observation number one two masses this is smaller mass this is bigger mass moving with the same velocity moving with the same velocity so where you have to make more efforts to stop the bigger mass simple now since we are talking about the effort let's try to let's give this a name a formal name because effort is a very subjective name fine so we call effort in physics as force okay so going forward when I say for when I say force it quantifies the amount of effort okay so force required the masses more what is the assumption you are stopping both the masses in same time same time okay in two seconds suppose you stop this and in two second you stop that also so effort required here is more okay so from this you can say that force is directly proportional to mass times a you can say mass to the power a isn't it even that is correct a can be one a can be two a can be three but a cannot be minus one minus two understood so this is observation one observation two suppose you have two mass I mean two equal masses this is moving with hundred meter per second it's like a bullet and this is moving with one meter per second which will take more effort to stop of course the higher velocity okay so what the force is proportional to velocity here according to observation what it is velocity you so you can like change with change in velocity change in velocity suppose I change hundred meter per second from hundred to just 99 then it's the same then it is same okay but I am changing more the velocity of suppose I change one meter per second this side to hundred meter per second backwards so the change is 101 getting it so it is changing velocity the force is proportional to yes or no okay so force is proportional to change in velocity to the power b you can say observation three two equal masses two equal velocities this year stopping quickly this year taking time to stop where you take more we have to make more effort here so so lesser time you take to change the velocity more is the is when when a critic catches the ball he does like this he follows the motion of the ball okay so that the effort required by his hand to stop the ball is less okay so is inversely proportional that's a delta t raised to power c but it so these are the three observations which must be true whatever equation you come out of delta t is see t equal to 0 is a hypothetical concept t is equal to 0 you can say the beginning of time or whatever you can say delta t is amount of right now time is can become 10 a.m. 20 minutes so delta t is finite time managed initial time that is delta so that's a common way of writing delta t if I just say t it means why it cannot be one by delta t if you say you're you're making it specific why it should be one by delta t if you say one by delta t then I should ask why one by delta t I am saying that it can be anything b c can be anything one two three four five why it cannot be one by delta t square all you have determined by the observation is if delta t is less four should be more that's all I really you have what it is first of all you don't even know how to calculate force force is an effort you're quantifying the effort but what it is it is creation of our it is our own creation that one Newton is this much so these are the three observation if you combine see you know if any of these three suppose m is 0 what is 0 if delta t is infinite effort is 0 fine so if any of these three if m is 0 this condition happened condition I will be able to write force as proportional to m to the power delta v to the power b divided by delta t raised to power c getting it must be true okay now it is up to us to take whatever a b c you want to take so what we do we assume a simple possible most simple possible relation what is that constant proportionality one b is equal to one c is equal to one fine all right so we have assumed that force which is quantification of effort is m times delta v by delta t and by the way this represents the average force the on an average this much force is required any doubts nothing so where's the constant constant is one you have assumed it to be one you assumed it to be one fine it is up to me to define what is force is equal to find force equal to this m into delta v by delta t nobody can say something is wrong with that but everybody has come to this understanding that let's assume a simplest possible relation which is this getting it this is the average force suppose you want to find out force at that instant or what does it mean instantaneous force you have to find out instantaneous rate of change of velocity which is acceleration understood if you combine everything you will get force is equal to mass times acceleration acceleration is instantaneous rate of change of velocity and these are vector quantities acceleration force they are vector quantities capital M which is mass is a scalar so scalar times vector is another vector force okay now here throughout we have assumed mass is constant getting it mass need not be constant for example rocket it is fired it moves up but force applied is down gravity is acting down still it is moving up why because mass is not constant when your balloon inside balloon there is air if you release the balloon it should come down because of gravity but it goes up the mass system will not talk about it right now okay that will deal in when we talk about laws of motion chapter again right now our focus is whatever is required for application of vectors okay so we're assuming mass as constant and 99% of the cases mass will be constant only 99.5% you can say fine all right so copy this down all of you quickly let me know if you have any doubts no doubts okay now what is the equation force vector is equal to mass times acceleration vector right this is a vector equation just like v is equal to u plus 80 all the vector equation from x axis and x axis along x axis like that we have divided the equation into two along x axis equation along y axis the equation okay same thing over here we can say that force in x axis is equal to mass time acceleration along x axis what is good if you write like this I mean how this will help this is a vector equation this is a scalar equation scalar equation we have been solving since our childhood x plus y is equal to 2 x minus 3 y is equal to side so what is x and y okay so we'll be able to solve it like a scalar equation if you write like this that is why fine otherwise you'll get a vector equation which is a little tricky to deal with okay where fx and fy will be equal to m into a y in this chapter it will be two dimensional in nature okay so everything can be described but by just taking two coordinates x and y okay there will be rarely a chance that along z axis also something happens fine so whatever needed to be covered will be covered in x and y only there will be cases where z axis is also involved that will take up later on right now our focus is again application of vectors okay so we are taking only two dimensional problems fine now tell me if multiple forces are acting suppose multiple forces are acting something to write then we can add it up simple if you add it up split it into their components and then add the x and the y correct good so this is summation of all the forces this is summation of all forces along x axis summation of all the forces along y axis okay but experience only net force yes or no consideration can object has multiple forces so why it can't have multiple object doesn't know multiple forces are acting on it already multiple forces similarly acceleration can be split into two or three things okay there are cases in which you have to add acceleration for the same object along x axis and then say that this is the equation along x axis I'll just give you an example for example if this is the case that you know this object is moving with acceleration some acceleration all right even this object is moving this object's acceleration is a1 with respect to this object so total execution will be a1 plus a5 so we'll come back to this no need to worry about it but as I'm telling you that there exist such cases where you split the acceleration also and then when you write equation you add it up fine so don't think that acceleration will be always one vector in the diagram there can be multiple vectors which represent the acceleration then you have to take component of acceleration also along x axis and along y axis now these things which I'm telling you you'll only understand when you solve questions which we'll be doing it later okay but that's just that I'm telling you for our information okay fine so this is the second law of motion this gives you an equation the most important part first law and third law doesn't give you equation second law gives you an equation okay so this is about this second law of motion now let's talk about the third law of motion what is third law of motion about reaction okay but then have you thought about like what does it mean what do it mean so you are you have the basic understanding of what does it mean so let's try to you know get into the depth of it and try to understand more about the third law of motion because the third law of motion is where the most of the student makes error okay they think that entire mechanics is just forward to mass acceleration and they forget to apply Newton's third law usually okay so third law of motion says I mean the statement could be many you know the way you explain it the the most common explanation of third law of motion is every action has equal and opposite reaction okay what is action action we thought see when I say action action has a wrong connotation also action when you say it it signifies some motion some movement but even if there is not a movement the force which is applied is called the action it is not a movement which called the action every force if you repeat the statement every force has equal and opposite force simple every force has equal and opposite force so so if there is a mass if there is a mass for this mass if somebody applies force F so this force has equal and opposite force so there will be any opposite force applied on the mass also is that true the mass correct so the person applies F on the mass mass applies opposite force on the person let's try to put it in a more simple manner draw two masses two objects object A and object B object A applies a force on B as FBA let's say F only then B will apply force on A F simple getting it if A applies force on B in this direction B will apply force on A in opposite direction this is what Newton third law is getting it so if there is a force in this universe somebody might be applying it somebody something will be applied that force will be applying that force yes or no so on that object or person there will be equal and opposite force also applied so in this universe total forces zero zero zero okay so say about third law of motion is that right now everything exists in pairs all the forces exist in pair car engine generates force like car engine where is the opposite car engine generates force where on the wheels like the reason why I can't say over it is because of the opposite force no no no he's saying something else he's saying something else you car engine applies force on the wheel wheel applies force on the engine trying to let's say one example I'm trying to twist this remote okay I am experiencing a force you you will experience a force when interested yes or no but you are applying force how come you are experiencing the force you are experiencing the force because this is applying force on you simple a force you should ask yourself a force if you're not able to find the pair you're doing it wrong you'll get a wrong answer getting it simple thing when you draw a force in a diagram that this is the force yourself where is a pair of this force oh there is this pair so I'm ignoring it let's move forward now if you're not able to answer that question you're doing it wrong simple okay now tell me if I'm standing here what are the forces acting on me gravity is acting on me so where is the pair of that force no who is applying gravity on me earth earth so I should apply a force on the yeah other applying force on me right me or mg on me so I should apply mg in a poet that's standing here on you there is mg force so on earth also there will be mg force this is a pair things are very very simple so keep it simple don't complicate in your head so where is applying mg simple whatever we have learned nothing else okay I'm standing on the floor floor is applying force on me upwards yes or no and you are also applying force on the floor downwards the floor applies and on you and you are applying normal reaction or the force on the floor also yes or no floor it's very weak so I will break because you're applying force on it these are action reaction pair mg mg n a 9th equal opposite pairs what about the air here air is pushing you down you can say that you're also pushing the air up if you don't push the air up air will come down okay but we are ignoring these side effects okay we are ignoring the air drag and all those things and any of the doubt anything else no examples of third law of motion it is very very common so you shoot a bullet what happens yes the gun the gun is applying force on the bullet so the bullet applies force on the gun so you feel the push backwards okay so that like when you're launching a lock it like you're like spelling all the gases downwards so the force is downwards so pushing the gas downward gas is pushing out okay okay ball is applying force on the wall and wall applies force on the ball the boat moving in water the boat exerts the force on the water and the water exerts the reaction force of the boat how you're exerting force on the water the boat exerts the force on the water body how with the no with the weight okay very correct correct so weight weight is balanced by the force due to the water when you're playing cricket ball is coming fast then you can see the back like when you walk you use that you push the ground and the ground push the ground correct when you're walking you're actually pushing the ground backwards and ground is pushing you forward and you're walking so when you jump then then you will create a force on the ground when I'm jumping which direction I'm pushing the floor down floor pushes you up what else so if if you if you were on a frictionless surface and you were to throw something really heavy forwards you would start sliding backwards like I saw this in this yeah it is possible yeah it happened in fact in the space now people do it yeah yeah like what else when you swim you're pushed the water back water pushes you forward okay so Newton's third law is very very common very common okay so when we will solve problems we will keep that thing in mind okay so when you solve problem we'll be representing the forces so every time you represent the force ask yourself where is the pair of this force and then you proceed forward okay right so these are the three laws of motion which I mean which you should understand in a great depth okay more understanding will be developed when you will solve problems in class 11 it will never happen that you understand everything by reading theory and then you solve problem that will never ever ever will happen your 70 to 80 percent of learning will happen when you solve problems so do not I mean what what happens is that when we go back home we start reading book let me complete the chapter then I'll start solving problems no start solving problems and then read the chapter when you're stuck somewhere reading it that's the way you have to learn that's the only way to learn your school exam your ut is or whatever it is do not read the chapter again and again solve problems after the problems after problem because 11 12 is about application of knowledge it's not about knowledge just they're not going to tell you they're not going to ask what are the returns three law of motion lays down no they're going to ask you how you apply that so you need to learn that that will be learned only when you solve problems okay