 fine so these are the three laws of motion okay now let's talk about the kind of forces that we will be dealing with fine so in mechanics we will be dealing with couple of forces again and again so we should know the behavior of these forces how these force behave what are the properties of these forces then only we can make use of the properties to solve the equations and things like that fine so can you tell me all the forces you know in general you don't need to worry about whether it is used in mechanics or not tell me few forces gravitational one by one okay gravitation let me write down you also write gravitational force then electrostatic electric force or electrostatic strong and weak force nuclear forces okay watching this video and no learning from the video don't sit with a popcorn and watch it like a movie okay you should also write down while you watching treat as if you're sitting in a class tell me magnetic force what else friction that a thrust what is that buoyant force buoyancy okay a first can be due to various reasons the currency is incorporated there itself what else tension how can you forget tension tension is due to tension forces you just ring okay so you'll see string is very common in this chapter so that force you must account for what else just like tension force there is one more very popular force spring not many forces they're just handful of them but if you get into the depth of it if you become physicist let's say okay you will have this understanding that only four forces exist in nature four fundamental forces okay this is a electromagnetic force electrostatic in magnetic then you have gravitation weak and strong okay your friction is electrostatic force only but we will not treat it like a lecture electrostatic force will treat it differently okay I'm just telling you for your knowledge that there exists only four fundamental forces and every the forces there are a form of that force okay I just to give you an example when when you're pushing one hand again I experience a force because electron from this hand electrons on this hand repels the electrons on that hand the electrostatic force fine don't actually touch the electron from this hand doesn't touch the electron of that hand but you will still feel it because it is a force which you're feeling we as a humans a constant velocity that is our behavior we are also masses but we can feel the change in velocity if change in velocity after you suddenly feel so what whatever we are feeling is the force electron from this hand touches the proton of that hand then you will not be able to take out this usually then it becomes like this you'll be like this throughout your life it'll become like one single limb starting from here to here okay okay so in in class 11th in class 11th our focus are these forces 1 and 5 there is a separate chapter on buoyancy and all these forces so I will not count this this will be taken like a separate chapter okay I am going I'm talking about the forces which will be taking up for solid objects there are only five in class 11th in class 12th you have electrostatic you have magnetic force also these two forces also okay but in class 11th only these five forces are there taking one by one these forces and understand their behaviors got it no it is not there because they they are short range forces their expression is very complicated and the way they behave okay and anyways this doesn't affect our day-to-day life right so we don't study but if you want to do research on it you can spend your half of life doing research on just that you know but then pick your research properly suppose you want to take up the physics as a research area in your life but the thing is that don't take some random topic for example one of my friends senior had taken this the electron getting trapped in a timer or something yeah it sounds very cool and you can you know it will be very interesting to do that research but once you complete that research after five year when you come out from college with your PhD the market value of that is nothing no nobody will bother to give you job because there is no market existing for that fine so whatever you do you should also think what difference of it in current scenario fine so keep an eye on that even if you have decided okay not to spend a single penny rest of your life and you'll dedicate your life to them good of the mankind but then also you know have this thing when back of your mind whatever I'm doing is it relevant okay you but then there are people who have like who has spent their entire life physics research what is theoretical physics yeah there are no experiments to prove what is right what is wrong you just imagine and then math using mathematics you come to some conclusions that all those things mathematical expressions so right now in when when Einstein was dying he was trying to I mean at that during his later part of the this thing life he was doing a research where he was trying to unify all the laws of physics physics is very unstructured one set of laws for bigger masses moving with normal velocities one set of laws for speed of light one set of laws for subatomic particles one set of laws for other kind of scenario but then there is no unified law what is true here suppose I like finding ages half MV square I cannot write half MV square kind of energy for let's say some object which is movie speed of light okay so whatever I'm using here I can't use it there that doesn't happen in mathematics okay so that that's what the Einstein was trying to do he was trying to unify everything so that there's a unified theory theory of everything have you heard of it the quest of Einstein towards a later part of his life then Stephen Hawkins was also trying to do the same thing he came off with string theory but then that has its own limitation okay there are challenges in theoretical physics also which you can take up to do something on that but our focus right now is class 11th okay so let's take one by one these forces and study the behavior of it and the one thing you will notice with forces is that so forces one type of force doesn't require any physical contact for number gravity wherever I go even if it if I'm there there will be a gravity force acting on me fine and one set of forces will need a contact those five which I have ticked only one is non-contact which is gravity every other force needs a contact but that I can't take that that's not class 11th fine we'll talk about electric and magnetic force in class 12th not now fine so write down one by one we'll be taking these forces the first one is gravity force see when gravity forces right now the assumption is the object is near the earth surface okay write down the assumption okay the mass is near the earth surface and if a mass is near the earth surface sphere or things like that no okay whenever I say that there's a mass connected to the string and it is put on the inclined plane and things like that I don't I should I will not be stating that mass is near the earth surface fine that is an assumption if it is not near the earth surface it will be stated separately so right now what till we don't say that mass is near the earth surface or not we'll assume that a mass is near the earth surface okay and if mass is near the earth surface like for example projectile motion okay there'll be a force there will be gravity force how much m into g m into g okay so magnitude of the force right now magnitude magnitude is mass times acceleration due to gravity mg on any mass of mass m kg's okay the direction will be no can I say along negative y axis is the force mg it depends how you take your x and y axis okay I can take my y axis to be horizontal but whatever is your x y axis doesn't matter mg force will be always vertically down the mass is kept what the mass is doing nothing it doesn't matter when you look at the mass if mass is there the first thing you should do is draw mg force down that's step number one in terms of magnitude as well as direction both are fixed so this is gravity for next force right down on the other surface to be friction also fine normal force is the normal two-surface getting it normal force is the perpendicular compact forces between the two surfaces friction is a tangential component of the contact force between the two surface getting it direction is normalization if normal is not there simple normal reaction represents the contact between the two surfaces if normalization is not the direction is perpendicular to both the surfaces for example I'm standing here like this where the direction normal reaction on me upwards upwards downwards getting it now can you tell me in these cases draw these cases you have to show the normal reaction force show the normal reaction on this mass and on that mass this is a sphere and this is a ladder which is placed on the wall in the floor on these you can show the gravity force as well as normalization both the forces direction of mg force will be this is mg present forces of with an arrow because it's a vector normalization on n will be mg sign we have to split it into its confidence normalization I'm saying normal normalization be like this this is the normalization no this is mg write it down this entity is a component of mg normalization normalization is a separate force mg is a separate force two separate forces fine okay there will be mg force like this there's a mg then from here there's normalization what else there's anyone which way like towards the center yes yeah why perpendicular now but it is not perpendicular here now perpendicular here is horizontal yes or no not defined this is the edge it's not a surface contact fine we'll come back to that I'm just talking about the directions right now but you're going perpendicular to the sphere only we are saying perpendicular to both the surfaces which is not defined we will take the perpendicular to the there will be mg mg down let us say what else are you sure or perpendicular to the rod exactly which one the second one the second one no the first one is correct because there's an edge contact for the ladder but the for the edge perpendicular is not defined but for this this surface perpendicular is clear which is horizontal so even for the bottom one even for the bottom one normalization is vertically and you will notice that normalization always force write it down it's a push force pushes the mask this way normalization here pushes this way they have that way push push the direction what about magnitude within this there's no reaction as long as this floor doesn't break so normal reaction actually in a way depends on the strength of the material okay alright infinite also to determine it is an unknown but won't normally like how much of a force you apply that much of a normal force you get right so if I jump from this height on the floor I'm applying a lot more force than just my weight then I hit the floor you have to determine Newton's laws of motion equation mass and maceration one of the force will be normal reaction and that will be an unknown okay and you'll be able to getting it I time recession is zero so next force will also be zero so you write the equation you get the answer for the magnitude for the normal action so this is for the normal force we also say that this is normal reaction no normal reaction so suppose you are standing okay you're still on you and he's acting down so if normal is zero it should go down with expression G right but you're not going down with efficient G so there is a force which is balancing MG acting on you which is normal reaction okay next one write down the spring force we call it tension due to the strength is called tension okay to push something I cannot I cannot tie a string and push using the string okay so string cannot apply push right string force or tension will always pull something but I can't push okay so it's always a pull force write down direction string there will be a force and it will be up to determine because it can be anything it's an equation fine what the component come in your mind if there is a string so what other thing is used with this string string system right regularly huh no have you seen someone drawn water from the well have you seen the well how many of you not seen the well everybody seen the well you haven't seen have you seen water in the well or it was like a dry well you see a pulley on the well and there people come with a string with bucket attached with that string so they they put bucket goes down and then they pull the string down and the bucket moves up there's a lot in like this and bucket with the water comes up have you seen that okay so how come that is happening I'm applying force down and pull is moving sorry the bucket is moving up that that doesn't happen usually right so that cannot happen you see without pulley you suppose pull is not there then what you do you pull the string like this up and the bucket will come up fine if you're not careful you will also fall on the bus on the well only okay so in order safely you have a pulley and then you take the exam support and then you pull it you to change the direction of the string force which is tension there you can change the direction of the string force okay so we have to discuss little bit about pulleys also because it comes with the string the purpose of the pulley is first change the direction of tension as a by the way tension is represented by a letter T capital T okay just like normal is represented by letter capital N tension is represented by capital T okay to use as use to multiply the force multiply the effect of the force you can say it can so happen that you are applying let's say 5 Newton of force using some pulley arrangement you can lift off 20 Newton of the weight force multiplier now how that happens let's see all of you clear about it these things okay now let's talk about pulley and string system a little bit more because it is very common I had more police okay so what we are going to discuss is the ideal case what is the ideal case there is no friction mass less string frictionless pulley and string so there is no friction the string will not be able to rotate the pulley itself basically string slides on the pulley imagine a case there's a pulley and the friction is not there between string and the pulley it is a slide on it pull you will not rotate fine so this is a case we are talking about and in this case this there is one thumb rule same string same tension if if there is a single string it has multiple pull you it goes like this and then comes down like this throughout the string tension force will be same if it is a single string there is one more assumption here that it is in extensible the string is fixed so usually this is the case with the string so if this is not stated we'll assume that is correct for that particular case okay it need not be stated if there is other situation to be written that it has friction or it can be stretched fine otherwise you assume this so same string has the same tension this is the thumb rule for example let's take this example if I pull it down with a force of F thumb with a force of F I will be pulled by tension it will be equal to F equal opposite force you don't throw it off okay T is equal to F okay now side will be pulled by what force this force will be what why tell me with what force this this side is pulled up by a force of tension this part okay so it will be pulled on by tension T okay tension will pull everything that is connected to it this tension is for that point this tension is for this point this tension is for that point this tension for that point every point will be pulled ready now you can see that you are applying F force which is equal to T on this pulley there will be 2 T acting which is 2 times F okay so this 2 T can lift off mass M if you have this arrangement your F can lift off 2 times M and you can see how nicely the pulley has you know twisted the direction of force so if there is no pulley the force will just be like this you know on a straight line but say there's a pulley that the tension forces transmitted with a this kind of system is very you know very common when you solve problems this this to this T is on that point this T is on that point if you just look at the pulley this pulley this is the T and this is the T understand more when you solve problems any of the string force what is the other force friction okay right now friction of force friction is a force that acts tangents tangent to the surface of contact okay friction tries to oppose oppose what motion what kind of motion relative motion between the two surfaces if two surfaces are moving together the same velocity there is no friction fine but if one surface tries to move with a different velocity there will be friction friction opposite is tending motion tending motion means that the motion may not be there itself example when I'm pushing this this thing suppose I'm pushing it with some force this is not moving is there a friction which is the preventing force to move how much is the friction equal to my force in opposite direction that is why this is not moving suppose friction is less than what I'm applying the force then they should move are you getting it yes now if I increase the force then also it is not moving so has friction also increased friction has adjusted its value to be equal to my force getting it so but then if I apply the large order was now it is moving the friction is that it adjusts adjust its value till some point and then becomes constant ready so there are two cases of friction static friction is when there is no relative motion and then the kinetic motion when there is a relative motion between the two surfaces so we'll first take up the static friction see if you do again this chapter you feel lot in at lot more ease so no motion no actual relative motion but the object is trying this example let's say this is mass M you're applying force F okay friction reward which direction opposite direction in this way tangent to the ground friction okay so friction will adjust its value to be equal to capital F if this static friction okay so the graph if you plot will be somewhat like this it's a 45 degree graph and this is the case till this point it will be static this is friction force and this is force which you are applying both are equal object starts move it's called dynamic friction there is a relative motion and then so object friction is slightly dip and then becomes constant like this okay you have any doubt yeah sir you said that after a certain point the friction will stop increasing rate but then when you move an object really fast it heats up so doesn't that mean that there's more friction with more speed heat what? when you move an object really fast on the surface it generates heat right yeah so that means that there's more friction now than there was when you were moving at slow speed no it doesn't mean that so the thing is that when you move fast it is getting generated heat is getting generated because friction whatever work is done against the friction that is getting converted into the heat okay faster you move faster you're doing the work faster it is getting converted into heat okay so it doesn't mean that friction force has increased it means that you're doing the work faster that's all coming to this case friction will be equal to the force applied to a small dip okay why there is a that that is actually because the inertia itself you can at the microscopic view the surface contact is like this imagine to be like this when this surface tries to move against that surface you know there will be this case and after that point when it is about to move suddenly you know these these edges may break off or there because of inertia it there will be slight dip against whatever resistance you're feeling okay we can only talk about it in a qualitative manner I can't prove this to you is in mathematics okay I only give you an example like this right understood so this is an observation that there will be a dip after which the friction becomes constant okay now this value this constant value is is called the kinetic friction fk you can say f small fk okay kinetic friction is represented by letter small fk okay people also write it as f with a subscript of small r fr okay kinetic friction is dynamic friction this common friction friction could be static fs or fk so so fk is fixed for any surface I'm coming to that have you written this should be proposing to what service not exactly yeah the roughness roughness surfaces what else what else it is proposing to what else just I mean take any example on your desk itself you can do so much if you if you just touch the surface and move my hand which will be less but if I press it hard and then move hand it will be more right so what is that force which if I'm pressing what force is increasing normal reaction normal reaction okay so friction is proposing to normal reaction also written as roughness is represented by mu which is efficient which is a constant okay between f and normal reaction okay don't speak ahead of me if I ask you then please tell me so mu is the constant of proportionality between friction and normal reaction okay the magnitude of the friction force it depends on the roughness which is represented by mu and normal reaction and the name of this mu is coefficient of friction coefficient of friction and see that the maximum value of static friction is more than the maximum value of kind of friction yes or no so the maximum value of static friction can be represented at coefficient of static friction times normal this is a maximum value the static friction will be less than or equal to this so this is for static friction case and for the kind of friction this this will be mu k but suppose in the question only one mu is given coefficient friction then you assume both are same this is very very small difference friction is a self-adjusting force mu times n is its maximum value it's not its only value okay if relative motion is not there friction can be anything between 0 and mu n fine new representing roughness yeah so then how is roughness changing and like between the first equation like that thing with see when when it starts moving the roughness little bit changes because then it is not exactly it is roughness what you feel suppose there's interlocking between these two surfaces this surface and this there is interlocking so you feel it is more rough okay but if suppose one surface is moving on the other surface like this you feel lesser roughness okay so let's not get into the microscopic view of it okay because this will be not used actually but then yeah it is good to understand that the roughness changes when you have static or kinetic this thing okay any other doubt so it doesn't depend on the surface area at all it doesn't depend on the area see the thing is I understand what you're trying to say actually if there are two objects okay sir more the surface enough contact actually means that mass is more normal reaction is more so you're trying you're I think confused between normal reaction and the area of the contact yes sir but what if it was like the exact same mass one of them is like a tiny marble and what is this big sheet so so then normally actually will be the same right it will be equal because it's at that one center of gravity correct so and only small area normal reaction will be higher but if it a bigger sheet normalization will be distributed throughout isn't it so then it's not that everywhere same amount of normalization is applied if it is a bigger sheet yes the direction of friction is what opposite to that opposite to the relative tending motion wherever the object is trying to move relatively the friction is opposite to that trying to move so I think we have done a lot of theory so let's take a small break then I'll introduce to you friction spring force and then we'll solve only problems