 So, in laws of motion, in laws of motion, had we discussed all the laws or you, if you suggest I will start from the beginning of this chapter, not a problem, in two hours kind of we can try to finish it. So, in the chapter before laws of motion, which was understanding movement, understanding motion, we had an application come understanding on the lines of equations of motion, right or not. Equations of motions were done, clear, right, fine. We discuss about v is equals to u plus a t, right, s is equals to u t plus half a t square, okay, and v square is equals to u square plus 2 a s. Now, these equations were telling us that where was the body, where will it be, what is the time required for moving from here to here, what is the acceleration in changing the velocity and all that. We applied mathematics to in order to understand the past, present and future position of the body and what state of motion was it, what is uniform motion, what is non-uniform motion, what is non-uniform motion with uniform acceleration, what is non-uniform motion with non-uniform acceleration, we did their graphical representation and all those things, right or not. Now, in this chapter primarily we will understand or we will try to understand, okay, what is the thing which brings a body into motion, what is the reason why a body moves, okay, why you get up and start from home and reach school, right. So, why you do things like you are here and how you make things happen and the answer to it was actually the single word force, right, sure. And Uncle Newton as per, as he was a follower of great people, okay, like Aristotle, Archimedes, like Galileo and all, sure. So, he concluded several concepts which were related to why body moves and consolidated them and collated them in order to make three primary understanding. These three primary understandings are called as laws of motion, law number one, law number two and law number three, clear. So, law number one talks about inertia, clear. Law number two talks about definition of force and law number three talks about action and reaction, right or not, clear. Is it visible to everyone? You are 12 here, where are the others? How will you know you are at home? Now, first law stated that if there is an object kept which is stationary, it won't move until unless it acted upon by a net external force, right or not, clear. In the very same way, if there is an object moving with a uniform speed, okay, it cannot be brought to rest, it cannot be brought to rest until unless acted upon by a net external force, clear or not. So, in order to change the state of motion of a body, you will have to apply some force, sorted, clear. This is Newton's first law and what is inertia? The tendency of a body to retain its initial position or any state, initial state of motion. That's what is called inertia. A rolling ball likes to keep rolling, clear or not? If you are sitting or sleeping, you like to do that, you really like to do that, okay. Now, the second law gives a calculation of how much force, okay, of how much force is required to bring what magnitude of change. So, we started understanding few things, see that the required force is directly proportional to mass, heavier object will lead, say for example, two objects are rolling toward you, one is of 2 kg, one is of 1 kg, then which one will require more force and the obvious answer is the one with 2 kg because it's heavy. In the very same way, it is also directly proportional to the one coming with higher speed. Say for example, two objects of same size, same mass are coming toward you, one with 5 meter per second and one with 1 meter per second. Which one will require more force to stop? You will say, okay, obviously 5 meter per second. Beyond that, it was also understood that, say for example, you are going in your car, okay and your friend also is going in the very parallel road in your car, then at some distance, you both observed an obstacle, okay, something like a speed breaker or a signal and all, clear? He has only 5 seconds to stop his car and you have 15 seconds to stop his car, you stop your car, sorry, not his car. So, which among you, the one having 5 seconds or the one having 15 seconds will need more force and your answer to it is, sir, the one having only 5 seconds to stop the car will need more force. That means, force required is inversely proportional to time, lesser time mean more force. Combining these three statements, it was brought that, okay, force is nothing but that to change, that to what? Change, what amount of change you have to break, clear or not? Sure, no doubt. Now, this quantity was defined as momentum of a body and it is generally denoted by small p, it is a vector quantity, okay? Sure, some 5 second what? Sir, sleeping or watching TV, what is this 5 second I couldn't get? Yeah, there is a lag of not only 5 second, it will be somewhere around 10 or 15 odd seconds, okay? So, not a problem, okay? We are anyway here. So, momentum, got it? m into v is defined as momentum of a body, clear? The mass and the velocity, clear? So, it was brought that, okay, force is the second law, rate of change of linear moment, clear? dp by dt, d by dt is rate of change with respect to time, clear? Sure, no doubt, fine, clear? Now, the 5 second is the answer, okay, got it, got it, sorry. This is Newton's second law, clear? Now, from here many applications came up, like, if you are stating that force is change in momentum upon changing time, then, then, then, then, then, it mean that if you not apply any force, then change in momentum will also be equal to 0, yes or no? Clear? If you not apply any force, if force is 0, then, force is your time and change in momentum, force into time will be 0, that is, change in momentum, because it's 0, sure? And from here comes principle or law of conservation of linear momentum, law of conservation of linear. What does law indicates or states? It states that, if there is no force acting on a system, then its momentum remain conserved. If no force is acting on a system, then its momentum remains conserved. We are not at all talking about energy now, that will be discussed with you in the next chapter. But as of now, if no force acts on a system, then the momentum of the system remain conserved, clear? Sure? The d by dt, yes, it is differentiation, but in your 9th standard school level, that is, CBC slivers, differentiation is not there, we will cover it post completion of a school slivers. For you, the better expression is change in momentum upon time or change in time. Well, change in momentum upon change in time, clear? Sure? Fine? Cool? So, law of conservation of linear momentum states that, until the next force is acting on a system, its momentum will remain conserved. And there comes the few concepts and examples, like concept of the recoil of gun. We have kind of discussed about it, I guess, because I had shown you videos also, clear? Now, what happens, suppose gun is in this box, this box has several masses in it, like the bullets, ok? As you trigger, you are actually externally not applying any force, you are just opening a pin, right or not? So, as you trigger, one part of this mass comes out of the system and moves ahead with a velocity v, for example, velocity of bullet, clear? That means, initially, as the gun was at rest, initially, as the gun was at rest, ok? Sure? Fine? The total momentum P initial was 0. Now, finally, when you are observing, clear? The gun and this whole system has one momentum forward, which is mass of one bullet into velocity of bullet. Now, as no system is applied on the, no force is applied on the system, this momentum or this m into v has to be kept constant as 0. That means, by default, there has to be a momentum this way, so that this can be cancelled. So, no doubt, clear? So, the momentum of bullet is this, momentum of gun will be this way and that will be nothing but mass of gun into velocity of gun, right or not? So, you write an expression that the recoil of gun is mass of gun into velocity of gun is equals to mass of bullet into velocity of bullet. I hope it is clear? Sure? Fine? Sure? Sure. Very good. Easy? No doubt on a question and solve, I will need the answer. Say for example, there is a gun of 2 kg. From this gun, one bullet of 10 gram mass, one bullet of 10 gram mass is fired with a velocity of 30 meter per second. With what velocity will the gun go backward? Please calculate. Hello? Yes, I will need the answer for this. 1.5 meter per second. So, 2 kg becomes 10 to the power 6 grams, yes or no? Am I clear? No, right? 10 to the power 3, okay? Sure? Into velocity of gun is equals to mass of bullet into velocity of bullet. Clear? Sure? So, velocity of gun is equals to 300 upon 2000, which is very obviously 1.5 meter per second. Sure? Clear? Now, next question. Here, what we have done is, we have considered the mass of the bullet negligible with comparison to mass of the gun, okay? Else, you will have to subtract some grams from it and then there will be some complications in the calculation. Understood the recoil thing? Fine? Sure? In the very same way, the second law also explains the concept of impulse. Now, what is impulse? When a force is applied, when a force is applied for a very short duration, for a very short duration, we call the person is impulsive, he plays impulsively because just for a sudden some variation will come. Now, what is impulse? The understanding is the example is catching a ball or the shock ups. How? What we do while catching a ball? Say for example, this person is there, the great Mahendra Singh Dhoni and he is catching a ball. Okay? Sure? What he will do? Post catching the ball, he will bring his hands back, right or not? Why? Because he is increasing the time, he is increasing the time of application of the force. See, the force applied by the ball will remain the same. The momentum carried by the ball is same, the mass of the ball and the velocity with which it is coming. But when we increase the time, when we increase the time, then we know that momentum upon time is force. So, while increasing the time, we reduce the force and that is what you do. You reduce the impact of the force. Clear? Sure? Fine? No doubt? Clear? Okay? So, easy? Now, so what is impulse generally stated? It is stated as force multiplied by time. Clear? Sure? Sure? Shock ups, what they do? As the spring is there, okay? Sure? So, as the force is exerted on the spring, it takes a while to get compressed and by that it reduces the impact. If you will apply some force on a nail, okay? To stick it in a wall, sure? Clear? To nail it in the wall actually. Fine? You have to apply it impulsively. If you will just keep the hammer touched with the nail for a long time, even you are applying force, it won't get into it. You need it to be of short duration. You don't slap someone for a long duration. You just instantly slap or touch his face. What is inch punch? You punch when you come into contact for a very short duration of time. What is collision? Two objects come into contact for a very short duration of time. If the contact is made for a longer duration, it won't apply that much force as it will apply if it is a shorter duration. Got it? Clear? This is about second law. Second law also explains to you as we said it is what amount of force will bring, what amount of change. So, if I'll ask you that on a platform, here you have a fur and here you have a hammer. You are at a very high point, okay? You are at a good height. If you leave them both together, drop them both together. If you drop them both together, if you will drop them both together, okay? Sure? Who will reach to the ground first? I am waiting for your answers. Thore ka hammer? No, Thore ka hammer is not there. It's magnetic. It's not a free-falling body. It's a controlled body. Okay, if there is hammer, then okay, fine, fine. You guys are kind of correct because what you have observed is correct that when air is there, so because of air friction, because of air friction, okay? Sure? This fur does not fall straight. It will go like this. Yes or no? And then reach to the ground. Whereas this hammer goes straight, that's why hammer reaches first. But if it is vacuum then, but suppose if it is vacuum then, so let me explain you by showing it. Okay? I hope it's clear to understand and all, right? Yeah, gravity is everywhere, dear. Gravity is like don't compare vacuum and space as same thing. Vacuum is different, space is different. Okay? Gravity is not in space. Okay? Sure? Clear? No, we will understand about gravity in the chapter following this. Okay? So, clear with Newton's second law. Sure? Sorted? Fine? Now, two things are very clear. First thing is what brings change into the state of motion of a body? It is the force. Second thing is, second thing is what amount of force, what amount of force? So, f is equals to mv by t, change in momentum upon time, whatever amount of momentum you want to be changed in whatever amount of time, that amount of forces applied. But the third question is from where is this force coming? And that's the third law. It states that you are not directly doing everything, every action of yours has an equal and opposite reaction. Clear or not? Fine? For example, if there is an object kept on a table, then this is applying a force of mg downward. The table is as a response applying a force upward. Clear? Sure? Fine? You are hitting by the hammer, there is a hammer. Okay? You are hitting by the hammer on the floor. The floor is pushing back the hammer with the same speed. Okay? You want to be seated on the chair. What happens is this chair pushes you back and that's why you don't keep yourself at one place for a long duration of time. Got it? Sure? Clear? Now, so every action will have an equal and opposite reaction. This is what is Newton's third law and there are wide applications of it. Rocket propulsion. Okay? And all. Sure? I'll show you a video on that line to make you understand how it happens. Okay? Okay? So understood, like what is happening over there? What was the scene? What happened and all? Okay? Sure? So it was from this box, the exhaust was coming out and this mass into velocity upon time that is force is giving this whole mass some velocity. Clear? Every action will have an equal and opposite reaction. When you walk on ground, okay? Sure? This is your foot forward. Clear? This is your foot backward. Because this backward foot applies the force on earth in this direction, you move forward. Okay? So every action will have an equal and opposite reaction. When you have to jump, you just don't go up. You first go down and as a response to it, you go up. Clear? So every action will have an equal and opposite reaction. Clear? So for swimming, you push the water back, the whole body of yours gets pushed forward. Sure? Every action will have an equal and opposite reaction. Got it? Easy? Sure? Fine? So now, don't feel surprised but yes, this is it with the chapter you have in the ninth grader physics. Okay? But we are going to cover much more than this. I'll just show you the NCRT of ninth standard so that it will be easier for you. Forces and laws of motion. Okay? Clear? Balance and imbalance forces. First law of motion. Clear? This we kind of have seen. If you remember, we have seen instead of this, we have seen a better one, the sharpshooters. Right? Clear? Some magical shots were just demonstrated by them and we could easily understand the inertia by that. Okay? This I showed you the video by recorded by myself. Clear? Now then comes the second law of motion to change in momentum upon time. Okay? Sure? This catch example thing. Clear? Fine? Laws of motion, third law. Every action will have an equal and opposite reaction. Recoil of gun. Walking on, like the conservation of momentum we discussed about. Clear? Fine? Recoil of gun. Okay? Don't worry. It is not possible for me to not to do numerical science. Don't get proper sleep if you're, if I'll not irritate you guys with numericals. Okay? So we'll do that. But also while discussing all these things, from childhood you have been understanding the fact that F is equals to MA. Here we never discussed about F is equals to MA. How it came is nothing but the second law. Second law is change in momentum upon time. Okay? Now, when you push a body, a rigid body, especially a rigid body, that's what we are talking about. It smart doesn't change it. So change in momentum can also rate a mass into change in velocity upon time. And you know that this jubbi is nothing but rate of change of velocity, which is acceleration. So what it is force is equals to mass into acceleration. Easy? Sure? Clear? Now beyond this we are going to talk about two applications of laws of motion and existence of some natural forces. One is tension and the other one is friction. Tension and friction. Clear? Now this is the beginning of my level of physics. Okay? Let's, let's, let's face this. Till now, even I was getting bored of whatever and teaching you simple things. This also you know. Right? Now I'm teaching you something and you will need, you'll not need Shridharacharya thing that is the knowledge of quadratic equation as you did in laws of, like understanding motion and all. We'll understand some simple thing. So I'll ask you a simple question that, okay, if this is a ceiling and then there is a string, there is a bob mass hanging, then what is the force on this bob? And your answer will be sir, there is an mg force downward. Apparently you also know that there is a stretch in this string and this stretch is nothing but to balance this downward thing. Okay? So there will be a tension in the string which is the stretch and that will be nothing but mg. Fine? Clear? Sure? Now, now, now, now. If I'll ask you that there is a pulley and across this pulley goes a system of mass connected with each other through a string. This is m1 and this is m2. Now which one is heavier m1 or m2? m1, m2, m1, m2, m1, m2. Learn this principle for life just by a single picture you can't say whether the sun is rising or setting. Just by a single success you can't say whether this person will be successful in life or will not be successful in life. Because both a rising sun and a setting sun, at one instant of time look same just by a single picture you can't determine, but by a sequential arrangement of pictures you can. In the very same way in this case, just by looking at it you can't say that m1 is heavier than m2. It may appear that this system is going downward. It may be a case that this system is going up. Got it? Sure? So you can't say whether m1 is greater than m2 or m2 is greater than m1 just by the single picture. Now, my question was not this at all. Actually I had to ask is the string stretched? Your answer is yes. The string will be stretched. Is the stretch same throughout? Your answer will be yes. It will be equally stretched throughout. That mean throughout this string the tension is same. Tension is supplied by this mass as well and tension is supplied by this mass as well. Sure? Any doubt that there is tension throughout the string and the two tensions which I am marking are same? Sure? No doubt? Fine. Okay, cool. So now understand this. Suppose this whole system is going as you consider m1 is heavier, still fine. Suppose this whole system is going like this, right or not? From here to here, right or not? Sure? So if the acceleration of this block is A, will not be the case that the acceleration of this block will also be A? Yes it will be. Why? Because they are connected bodies and last chapter while understanding relative motion we understood this. They are connected bodies. All the compartments of a train they are connected move with same accelerations, not the case that the block one is moving with some different acceleration, another block is moving with some different acceleration. In that case the train won't move together, right or not? So as they are connected system with a non-stretchable string then surely they will have same acceleration. Clear or not? Clear. How are the tensions same? Okay, you have a doubt. Dear understand this. The tension will get equally distributed. Tension is the force in the string. So it will get equally stretched. It doesn't happen that there is a string tied to one end and you are holding it in one hand. It has different stretch at different levels. No. I don't know. It will have same stretch throughout it. So in a string the tension remains same. Yes or no? If you are satisfied then please else I will have to re-explain it. You don't know. Not at all. You don't know what the two masses are. They can be equal or unequal. But as there are masses and it is hanging these two masses are experiencing downward force, right or not? This is M1G and this is experiencing M2G. Right. Fine. So there is a load coming. There is a load. There is a load on the string. That's why there is a stretch. M1G is being experienced this way. M2G is being experienced this way. As we are considering estimating that okay M1 is greater than M2 then okay this will go this way. Right or not? And there is no problem in estimating because at last I will prove you that tension is independent of the two masses and their value. Like of the acceleration not the two masses sorry. Tension is independent of their value of A. Even if acceleration is not there the tension will remain the same. Sir break. Singles. Anyways I will see something if something can be done. So now understand this. This whole system is going like down okay. On that line if acceleration A is related with M1 can I say the force experienced by mass M1 is M1A mass into acceleration. But is it the actual applied force you will say no because the actual applied force by S is nothing but naturally the actual applied force is M1G yes or no. And also there is a tension in the string T upward which is opposing M1A that's why minus. On similar lines if you will see M2A it is acting upward. Now who is in support of M2A and the answer to it is tension. Who is in opposition with M2A? The answer it is M2A. These kind of equations when you make they are called free body equations. What is a free body equation? Writing the force equation for one particle or one object at a time. Writing the force equation for one object or particle at a time is called its free body equation okay. So easy. Now these two equations are two linear equations where variables are A and T yes or no. Because other things when we make the system M1, M2 and G so anyway is known by us. Other things are known to us right. So can you please solve these two equation and find the value of A and find the value of T please solve. Solve these two equations and find the value of A and T. G is known Nare. Solve linear equation in two variables you know how to solve simultaneous pair of linear equations. If you don't just comment here so I will start doing it. No M1 is not equal to M2 okay. Chalo fine. I will do it. No problem. No problem. Relax, relax, relax. Suppose C, C, C, C simple. These two equations are there. If you will directly add these two equations what will you get? If you will directly add these two equations what will you get? M1 plus M2A okay is equal to T and T got cancelled. M1G minus M2G. So what is A equal to? A is nothing but M1G minus M2G upon M1 plus M2G. Clear? So now on the very same line if you will put this value of acceleration A in any of the two equations okay fine. Then you will get the value of T like first equation if I will substitute. M1 into instead of A M1G minus T. Clear? Opening the bracket. This dabbha goes here. This term goes inside okay. So M1 square G minus M1 M2G is equals to M1 square G minus M1T plus M1 M2G minus M2T. Yes or no? Sure. I am raising this area okay. Clear? Fine. So if you solve further you get this M1 square G and M1 square G got cancelled. Clear? Send this this way and bring all these two Ts this way. So you will get M1 plus M2T is equals to 2 M1 M2G one time and second time. Clear? So T is equals to 2 M1 M2G upon M1 plus M2. Understood or not? Now this is we are talking physics till now we were joking only okay. Now we are talking physics. Is it sorted and clear? The expression for tension and the expression for acceleration okay. Sure? Very beautiful. So now see understand this 2 M1 M2G upon M1 plus M2. Clear? Sure? So there is there is there is no dependence on the acceleration of the tension. Tension is independent of the acceleration right or not? Clear? Because it is natural until unless you are applying some force and bringing some extra acceleration there this tension naturally is independent of the acceleration of the system. Whether the system is moving down toward M1 or down toward M2 or it is starting the tension will remain as 2 M1 M2G upon M1 plus M2. Got it? So now the same system understand this keep this notion in your mind very clear okay. Sure? And I am asking you a different question now. I will erase the first case for that okay and you have to answer this. Keep the same strategy is same what you followed here but the new question is different. The question is suppose there is a horizontal floor at the edge of this horizontal floor there is a pulley. Across the pulley now there is a string going on. One mass is hanging suppose M1 and the other mass is on the floor of the table M2. There is no friction on this table it is a frictionless surface as of now. Sooner I will bring friction as well into action but now okay as of now there is no friction clear. M1 is hanging M2 is on table. Can you please tell me what is the tension in the string for your help? I can explain that there will be tension here and there will be tension throughout this string the tension is like that and it will be same. So can you please make these kind of free body equations and tell me the answer? Make make make make. I will not wait for more than half a minute okay. You can tell me there itself they are getting or not and also meanwhile answering this please tell me can I involve a bit of trigonometry here please please please can I can I involve a bit of trigonometry? I am having a high feeling that I should annoy you a bit more. So please help me out allow me to use trigonometry a bit please please please okay. Aiden don't take these kind of risk in public they may hit you at school okay. Such a big no. Aniket Aditya K and Sanjana also said a big no. Aniruddha also no. Aniket what exactly? Big no. Swalpa trigonometry please swalpa. Aniket if you have such feeling of seeing Aiden on Tuesday don't declare these things on public portal right. Yes M2G can not cancel itself out but M2G is downward on the floor. So the normal reaction from the floor will cancel M2G okay. M2G see M2G is here right. Here down M2G clear and that will be cancelled by normal reaction which is again M2G. We are moving this is being moved on this direction horizontal M2G will not have any influence of it until unless there is friction. If there will be friction M2G will have a participation if there is no friction as of now like. So there will not be any involvement of M2G got it okay. I will solve this Akshay okay. May I erase this a bit okay. Sure I am just erasing this schematic representation. I will let the steps be here or I will erase this part okay. I will just make the equation they will help you out see understand. Free body equations. So on M2 suppose M2U starts so you say okay suppose the system is going this way and this way with an acceleration A. So for M2 the total force acting is mass into acceleration but this force is an outcome of supportive forces which is tension and there is no opposing forces because there is no friction. Also M2G does not have any participation and on the second line M1 the total force acting on M1 is M1A supported by M1G and opposed by tension. By solving these two equations can't you tell the value of A and T please say very good A is equals to M1G upon M1 plus M2 very correct aniket A da aniket M aniket Gupta's answer is correct aniket M you don't have to bring T you have to remove it for while calculating A whereas while calculating T you don't need A so as aniket Gupta has answered A is nothing but M1G upon M1 plus M2. So by substituting this value over here I can get T is equals to M1 M2G upon M1 plus M2 yes or no yes or no yes or no good good good good good good good good many of you are answering Priya Krishnan who is Priya Krishnan might be the name of someone's parent I am not a problem not a problem not a very good very good very good very good got the value of tension as we will see here there were two strings okay so the answer was two M1 M2G upon M1 here there was only one hanging part so answer is one M1 M2G upon M1 plus M2. Now tell me please please suppose this becomes a case like there is a horizontal table and on this horizontal table there are two blocks block one and block two and there is a string connected what is the tension in this string no what is the tension in this string so zero zero zero until unless you apply some force the tension in the string will remain zero beautiful beautiful beautiful you guys got it no no no you guys got it very good very good very good okay sure but I am quite a devil now I will put some force and ask you the question that suppose on that horizontal table that board was looking beautiful but let it be you have a mass here another mass here these two are connected by a string you are pulling this block with a force F you are pulling this block which is mass M1 with a force F sure there will be tension here there will be tension here yes or no please answer what is the tension in this string and also with what acceleration will the objects move please what kind of treat you want a answer answer answer two M1 G upon two M1 G upon M1 is what M1 and M1 will get cancelled and there is no G where is G G is downward I told you know both the G's will get cancelled by the normal reaction so suppose okay a few of you are answering I am happy with that suppose because of this force this blocks go with an acceleration A as they are corrected they will go same so dear now I am telling you the simple way of this A is equals to total force upon total mass if you remember in first case A was equal to M1 G minus M2 G upon M1 plus M2 in second case A was equal to M1 G upon M1 plus M2 why see in first case this is being pulled by M1 G down and this is also being pulled by M2 G down so total force is the difference of these two upon total mass in case two this is only being pulled by M1 G down because there is no M2 G participation so total force upon total mass now similarly total force upon total mass because force is nothing but mass into acceleration isn't it simple sure cool so on that line you got A beautiful now you will write the equation like M2 A is equals to supportive force minus opposite force no opposite force so from here T equal to F into M2 upon M1 plus simple sure no doubt easy clear okay fine so this is about tension in the strength now let's discuss about friction do you guys have energy do you guys have energy are very good I was not at all expecting this from you good good good so now there is a rain see there are few people and they are asking please continue sir I can't disappoint them now Gupta ji what is the formula for tension it's not one formula for different cases tension is different okay you can remember for a two-string system it is two M1 M2 G upon M1 plus M2 for a single-string system it is M1 M2 G upon M1 plus M2 okay for horizontal if there is no force no F is zero but if there is F then the answer will be F onto the mass not applied by force upon M1 plus M2 understand how see you had M1 here and M2 here you applied the force on F that's why the tension came as M2 into F upon M1 plus M2 suppose if you would have applied the force on M2 then tension would have been M1 into F upon M1 plus M2 you getting my point sure yeah Aditi which part I have just now re-explained the whole thing of tension all the cases see the board good good sanjana good sanjana good sanjana good sanjana understanding easy sure very good very good now good question by Sankaran see I don't know who this person is sorry you might be using one of your parents main idea I guess right so I'll explain the case now suppose very good question suppose sir both side we are applying some force so we are applying a force F1 on mass M1 and there is a force F2 on mass M2 because of this there is a stretch here right or not now consider one case let's suppose either F1 is greater than F2 or F2 or greater suppose this is going the whole system is going this way right or not you have to understand that it will go at one side or not clear so no doubt fine acceleration is there so you'll write the equation what are the equations that M1 a will be equal to F1 minus t M2 a will be equal to minus F2 please be careful here M2 a is in this direction that was supportive force will be t and opposing force will be F2 clear so fine okay now if you'll add the two equations we will get M1 plus M2 a is equals to F1 minus F2 isn't it the same acceleration is equals to total force upon total mass clear is equals to F1 minus t so nothing this part goes here this goes in M1 F1 minus M1 F2 clear is equals to M1 F1 minus M1 t plus M2 F1 minus M2 t clear so so again see here it's very easy M1 F1 and M1 F1 got cancelled this fallout t and this fallout t came this side so you got P M1 plus M2 equal to look here M1 F2 is positive and M2 F1 is also positive M1 F2 plus M2 F1 upon M1 plus M2 again what is this F2 is the force acting on M2 so that's why M1 is multiplied by it not applying force right or not and F1 is the force acting on M1 so what is applied multiplied with M2 not experiencing force like that clear easy simple it is the combination of both the cases now clear sure fine check this this M1 and M2 has already gone here so I can erase this from here where you guys there you don't get lost total because these two forces sanjana are opposite no F1 and F2 are opposite right so F1 minus F2 is the total force or if you will just add the two equation F1 minus F2 by mathematically only it will come no need to apply concept answer so okay I will re-explain and you know don't worry I am re-explaining this stuff listen here see here you got this equation you got this equation right or not M1 a is equals to F1 minus t F1 minus t and M2 as this is going this way the supportive minus opposing right or not so clear fine now add both the equations so you will get M1 a plus M2 a is equal to F1 minus F2 the two t's will get cancelled clear so a is equal to F1 minus F2 by M1 plus could substitute a in any equation so if you substitute a in any equation if you substitute a in any equation okay so what will you get say for example in equation 2 now suppose I am substituting in 2 second one then M2 into a what is a F1 minus F2 upon M1 plus M2 okay is equal to t minus M2 this goes here this goes in so M2 F1 minus M2 F2 is equals to M1 t minus M1 F2 plus M2 t minus M2 F2 yes or no now this M2 F2 negative and M2 F2 negative is cancelled sure fine this M1 F2 comes that side so M2 F1 plus M1 F2 is equal to t into M1 plus M2 so t is equals to M2 F1 upon M1 F2 plus M1 plus M2 am I clear now got it very good very good very good everyone of you getting so now now I will not get into the friction thing else you will just seal out at me can I use throat as a trigonometry please please show to the trigonometry show to small cell of fine fine fine not using not using not using cell fine fine fine fine fine so this was about tension okay there is a basic understanding of friction that only one thing and I will just put it on the conclusion that if there is a box or is if there is an object kept on a surface download force is mg and that is what it received as a normal reaction at this point of time the friction on the block is 0 because the block is not tending to move capital F is forced show to F is friction so friction is 0 because there is no force even if you will just touch it with a force to push whether the box moves or not whether the box moves or not if it is trying to move also then there will be a friction okay because there is a because there is a force it it is not the case that whenever the box will slide whenever the box will slide then only the friction will act no even the box tend to slide mean there is a friction or actually why the block is just not starting to slide and just only trying to slide is the reason is the friction okay so that's it that's it that's it about this comprehensively we will discuss in the next class don't worry about it show to friction yeah show to F okay fine so this was I guess my first like online lecture with you sorry for the hindrance which happened in the beginning you did not get the sound and all but hey inertia inertia we will discuss and we will talk about it okay that's why there are three cases of friction static friction kinetic friction and limiting friction and all that covers inertia as well okay so so we will talk about the formula mu into n what is n why n is used why not directly mg is used when n is equal to mg what why is it harder to pull and then to push and there will be a minor trigonometry applied sorry sorry friction I can't avoid that but there will be an application of trigonometry in friction okay tension twice as you guys told so I obeyed you okay so sorry for that what the beginning thing happened but yes I hope post that the connectivity was continuous and all clear I'll just cover up the whole thing by a simple case okay sure and then you can see to doubt things and all okay