 Hello guys, so I can see if you people are there online already so we'll wait a few more to join so in case you have any topic you know for discussion you can tell me that we'll start from there because last time we did a full revision on all the topics so if you have any particular question today to be discussed please let me know you can always take a picture and post it in the group what's that group and I'll take it from there and I'll explain so otherwise if you have any other topic also to be revisited please tell me that so here we have you know students from all the three npses so depending upon the time availability we'll be trying we'll be trying to cover as many topics as we can so feel free to post your queries any particular any particular topic you want me to start off with any one of you please post it in the comment section you can do that yeah any any topic guys please please tell me so that I can you know start explaining that so where do I start from which which topic do you want me to cover first or I'll then otherwise I'll do a general revision if you don't have any any particular topic so in physics so there are two sets of people who are there with us right now so one is our nps nps kormangla and hsr so they have same syllabus nps r and r has a little bit different syllabus but there is lots of concurrence okay so let's let's first see what all were the comment topics so one is your first chapter is motion then second chapter was laws of motion newton's laws newton's laws of motion and third one is oh I have not shared my screen yet just a share the screen first wait a minute yeah so I will be sharing my screen yeah so I hope all of you are able to see my screen now in a few moments yes so I was explaining that we have two sets of students here first set is going through the topics they have to cover is motion newton's laws of motion and I think there is the gravitation and pressure small bit and nps r and r apart from all these they had units of units and then uh what about this they have turning forces I think these are the total there's a total syllabus to be called you know there's a total syllabus of physics in both the all the schools yeah so r and r has these two topics extra if I am missing anything you can just post it okay so what I'm going to do is I'm going to solve one worksheet so that you know all the entire syllabus is covered okay give me a moment so I'll do is I'll give you a worksheet worksheet to solve okay so let's start with motion chapter so these are the questions so what I'm going to do is I'm going to now I am sharing the screen once again full screen so that you can yeah so now there is a worksheet on your screen and let's start solving one by one okay so all of you have to answer here you can answer here itself so which of the following statement is statement is correct both both speed and velocity are same speed is a scalar and vector uh vector is a vector sorry velocity is a vector speed is a vector and velocity is a scalar and none of these what is the answer so you can type in your answer in the yeah you can type in all of you can type in your answer here in the great so you could solve the first one your answer is b go for the second one what is the slope of the body when it moves with uniform velocity question number so you'll have to write question number one b like that so question number two what is the answer answer question number two is so you'll have to write two a two b two c two d whatever so like that so that I can you know clearly see who is answering what to be what so uh slope what is slope of any slope of so question was question was what is the slope of the body when it moves with uniform velocity so actually when when we talk about slope it is uh you know uh velocity time graph so in in actually they have they should mention which graph you're talking about but let me explain this so let's say if it is uh velocity time graph then the slope the slope represents acceleration right so slope of of uh velocity time graph represents acceleration but if it is so what is slope basically the slope you know uh so this is the slope delta v and this one is delta t so slope or a was given as delta v upon delta t that was slope but if it is a distance time graph then the slope will give you loss if it is a displacement time graph then you'll get so let's say this is a displacement and this is time then the slope slope will give you see this is delta s this is delta t so here velocity or you know if it is displacement time graph it will give you velocity if it is distance time graph you'll give you speed so unless the s upon delta p okay next let's go to the next problem so which of the following is the position time graph for a body at rest so like this guys body is at rest yes so first one is the right answer why because if you let's let's analyze this and we'll talk about all the all the four cases so let's say so this was question number so this is the question now basically uh how do we find out find out so if this is so position is not changing with time so this is basically at rest this is at rest so this is uh the best what about these what type of what type of motion is it can you tell me 3 2 what is 3 2 what is this 2 what type of motion is it please tell me what type of motion does the graph 2 represent so is it clearly body is not at rest but is it moving uniformly uniformly or non non uniform motion what is it uniform motion why is it uniform motion because the slope is constant throughout the journey the slope is constant so delta s by so slope here in this graph slope is equal to delta s upon delta t delta t is constant so hence it is uniform okay what about this one third one what is this one third one what type of motion is it uniform non uniform third one third is uniform so why what is the difference between second and third then uniform towards zero what does it mean yes guys so why why are these in the third one it's moving in the opposite direction but which direction is it deceleration it's not deceleration got it's not deceleration clearly it's not deceleration but was it what is what is it towards actually towards origin is the right answer correct so in the third case in the third case the body is moving uniformly but it is moving towards the it is moving towards the opposite to the positive direction correct so it is in what is the case case is something like this let's say let's say there is a you know uniform or let's say the body is moving in this direction and this was the origin this is origin zero so in this case the body is trying to move or basically it is moving in this direction right so let's say it is moving in this direction uniformly so this when this happens then we say that it is uniform uniform motion uniform motion but towards the origin towards origin what about the fourth one try this what about the fourth one fourth one is positive acceleration so it is accelerating yeah so it is yeah in this case is accelerating isn't it if it was deceleration then what would it be like deceleration p now the loss in a position time graph position time graph so deceleration would look like something like this deceleration now let's go to the next question what does the area area velocity time graph give area in velocity time graph give question number four solve question number four what area what does area area of velocity time graph give four is c displacement correct correct now differentiate between distance and displacement four c you know all you all of you know that it's very what is difference distance and displacement so you can you can one by one you know tell me the distance difference between distance and displacement so let's say we have to differentiate between distance and so so let's say we have to differentiate let's say we have distance and displacement tell me the differences one by one distance and displacement let's enumerate all of them yeah great so first no before you go to scalar and vector differentiation what you can say first define it what is distance it's nothing but actual length length of the path traversed or traveled by we say traversed by our body right actual length of the path traversed by a body between between point a and b right so this is difference number one here you'll write the shortest shortest distance shortest distance between the origin and destination right so you you give a give example also so let's say you will give example like that you draw a picture let's say this is a this is b so distance is this is so distance is distance equals length of path a to b and displacement you can use a different color maybe and then you yeah so you can you can highlight label here itself so this is displacement okay and this one is distance to draw diagrams okay first first this thing second difference is clearly many of you have already highlighted it is a scalar quantity and here it is it is a vector quantity okay then third is third is it can be it it can only be positive there cannot be any negative distance and here it is it can have negative values it can be negative also it can be negative correct so in in this thing distance distance is always greater than equal to zero and displacement can be either less than zero greater than zero or even okay yeah so most of you know the difference the next question would be derive mathematically the first equation of the motion so let me take it here okay so what does it what is meant by or let's say if this question comes derive mathematically the first equation of motion that is equals to u plus k b so how will you do it so basically you have to start with the definition of acceleration so acceleration is equal to rate of change of rate of change of velocity with respect to time so hence a is equal to v minus u by t so this is let's say or in the intermediate step you can write in the intermediate step you can always write so it should be here what you can do is you can write it as delta v so hence and what will happen so it is nothing but v minus u is equal to 80 hence v equals u plus and you have to mention where u is equal to initial velocity v is final velocity a is acceleration and t is equal to time taken okay so this is this question now next is so question is calculate the acceleration of a body it starts from rest and travels 87.5 meters in five seconds I believe yeah so hence let me just copy this question and let's say this is a two-marked question let me go there and we will put it here yes so so this is the question so calculate the acceleration of a body which starts from rest and travels 87.5 meters in five seconds in such questions how do you solve solution in such question ideally you should draw a diagram just for your understanding so let's say this is a straight line path okay and let's say this is my a point a and this is point and let's say here it starts from rest so you can mention all the information here itself so u equals to zero and a is unknown that is what you have to find out s is given as 87.5 meters and t is given as five seconds so hence hence the three question out of the three question what is to be found out a right so if t is mentioned s is mentioned and u is mentioned then you have to you know that s is equal to ut plus half a t square and the question is why don't we use the other three questions the best way I have discussed in the class also write all of them so let's say v equals to u plus a t then second one is s is equal to ut plus half a t square you don't write this in the solution part you will just write it in some rough rough space so v square is equal to u square plus twice of a s these are the three equations of motion now clearly you know a is being demanded and there is no talk of v so ideally you know we should you know restrain from using this equation and this equation because you don't know v in this equation we know s also we know u also we know t so hence easily we can find out a so let's deploy the values s is 87.5 and before deploying the values please check that all the units all the quantities are in the same setup the same si unit u is zero so I don't really care here so let's say it is five seconds so this is gone so this is in meters and then half sorry plus half of a and t square so that is five square is it so let's not write the units because everything is in si units now what what what is this now so this implies this is 87.5 equals this becomes zero and this is 25 upon two into a yeah so this implies a equals 87.5 into two divided by 25 and the units will be meter per second square why because everything was in si units okay so this is nothing but so this is what 175 175 upon 25 which is seven meter so most of you have got it right ishani calculation 17.5 you are saying it's 7.5 sorry seven meters per second square check your calculation please next question define uniform velocity and uniform acceleration yeah so the question is define uniform velocity yeah so define uniform velocity and uniform acceleration so how do we define so uniform velocity is the velocity or you can define like that you can say is if a body moves in a straight line in one direction in one direction such that such that it covers it covers equal displacement mind the words displacement equal displacement in equal interval of times it is said undergoing uniform velocity or it is said to be undergoing uniform velocity or said to be traveling in traveling with uniform velocity yes and what about uniform acceleration uniform acceleration so you can say that if the rate of change of rate of change of velocity of an object rate of change of velocity upon object is constant with respect to time to time then object is said to be undergoing undergoing uniform acceleration okay so if object speed is increasing at a constant rate then it is uniform acceleration correct that's also correct so this is so this is the difference okay now next question is derive the second equation of motion graphically very good so let's find out so we have learned this also so we have to find what what is the question question is derive graphically the second equation of motion second equation motion and what is second equation of motion again whenever there is a talk of equations of motion I usually reiterate everything so first equation is v equals to u plus 80 second is s is equal to ut plus half 80 square and third one is v square equals to u square plus so we know how do we derive this graphically so we displacement is nothing but area under the curve in a vt graph so this is how we have done it last time so let's say this is the curve this is my v and this is my t the t graph and then we have to do what let's say this value was u and let's say this value was v and this value is time is zero and this corresponding value of time here is t okay and we have to find out the equation what do we do we have to find out the area of this curve area under this curve we have to find out this area to find out the displacement and how do we do that so either you can divide this particular geometry into two parts that is you can divide this into this or you can so basically let's name them so let's say this point is o this point is a let's say this point is b this point is c and this point is d so either you can do this that displacement displacement s is nothing but area under vt graph what is meant by under vt graph is like so this will be one boundary so this is the graph itself will be one boundary the y axis will become another one boundary the x axis will become another boundary and this vertical line a is another boundary so basically whatever is the area under this area of this trapezium so hence area is nothing but area area of trapezium trapezium which trapezium o a b c yeah or you can also say either you know the formula then you can use this or you can divide it into two areas you can say area of triangle c d b plus area of rectangle rectangle o a b c either of them you can use so i know the formula of area of trapezium and that is nothing but half into sum of parallel sides sum of parallel sides sum of parallel sides into distance between the parallel sides distance between the parallel sides either you use this or you do what i can do area of this the second off approach so area of this area of triangle c d b so what is the area of triangle c d b is nothing but half to base into height so it is nothing but half and if you see let me just you know take this picture down so that it is in front of our workout so let's say i am so let me copy this and paste it here here okay so what is the area of c b d so i am doing the second second second method so i am doing area of c b d so half into c d b is half into t so half into t or let me write the let me write the let me write in terms of the side so half into c d into d b plus half into o c into o a o c into o a so half what is c d if you see c d c d is nothing but this c d is nothing but time t which is equal to o a so hence we know what we know c d is equal to o a is equal to t now clearly clearly from the diagram if you see o c is equal to u and o b is equal to sorry not o b it's a b a b is equal to v so hence e b e b is equal to a b minus a d which is v minus q okay so let's deploy all this in in this equation so we get s is equal to half into c d this c d c d is how much t into d b d b is v minus u okay and then what plus a this will not be half sorry this is not half this is rectangle so o c into o a so o c is how much o c is u and again o a is t so it is t so hence if you simplify what will you get you will get okay now we can also do one thing we can use the first equation and first equation gives you v equals u plus a t so you now know v minus u is equal to a t yeah so you can use this in this place here so hence what will you get s is equal to half into t into a t plus u t so hence answer is u t plus half i hope this is clear to everyone yeah so next is let's move on to the next question so the question is the car moving with certain velocity comes to a halt so let me just quickly copy this so try this problem so car moving with a certain velocity comes to a halt if the retardation was five meter per second square find the initial velocity of the car try this so now let me solve it so basically so what is this question question is the car moves with a certain velocity comes to a halt if the retardation was five meter per second square find the initial velocity of the car so what is given what is being demanded certain so uh initial velocity is being demanded what is this so this is not known so v is sorry sorry u is not known okay and comes to a halt halt means final velocity must be zero final velocity this is final velocity and retardation now the moment it is said retardation that means acceleration will be negative so hence i know a is minus five meter per second square okay so u has to be found out okay and uh yeah so and there is no mention of time certain velocity comes to a halt if the retard only retardation is given find the initial velocity of the car so how do we find out so let's let's say that so we can find out the time and uh okay so we have to find out okay so u is not known yeah can anyone yeah so you guys try so again what what do we do in such cases we write all of them equations equals to u plus a t then we have s is equal to u p plus half a t square and then v square is equal to u square plus s okay so in this case let's apply you know because there are lots of so u is not known s is not known t is also not okay let us check if we can what is known only a is known and v is known a is known and v is known so hence okay uh yeah let me just check once the car move with a certain velocity comes to a halt if the retardation was five million times the initial velocity of the car yes so that means it will require a combination of so because not not a single one single equation will not be able to give you the solution okay so let's try so and let's try to utilize wherever v is there so v is there in the first equation so what can i do i can write zero is equal to u which is to be found out minus a minus five into t okay and from the third equation what do i get from the third equation you will get u square my minus two into five s okay so what do i get i get from here i get t is equal to u upon five and from here what do i get s is equal to u square upon ten yeah so i reduce t in form of in terms of u and then s in terms of yeah so u square by ten so now what we what will i do so i use two equations time is for the third equation so what is third equation s is equal to u times d plus half half a p square okay so what is s we just found out it is nothing but u square upon ten so substitute is equal to u what is u you let u be u and t was u u by five isn't it but uh no this this is not going to help you because at the end of the day the entire u will just go p square yeah this question you just a moment so there is some time information is missing i think so oh s so there here one information is missing 62.5 it's mentioned that was not not given here so this question is incomplete because there is one 62.5 data so that is missing question number ten one data is missing and that was s so if s was known then you'd have solved it right so hence another information which was missing missing here was s is equal to 62.5 so leave this equation now you know how to solve that okay now this could be a okay let's try this one 13th one this looks a good equation good question which will cover all the concepts so two boys a and b travel along the same path the displacement time graph for their journey is given in the following figure okay let's let's see if you can solve so spend two three minutes on this let's see if you can solve this so i i want all of you to solve this takes time and respond a is two meter houses is it two boys a and b travel along same path the displacement time graph for their journey is given in the following figure how far down the road has been traveled when a starts the journey how far he has traveled so b has already traveled when a starts the journey at fourth yeah so in if you if you're going to solve it through graph it will be little difficult because let me just because you'll have to do some calculations also just to check that the graph is scale okay let me just put it here so that all of you can also solve yeah it'll be difficult to you know measure it from but we'll try it let's see so how far down the road has b traveled when a starts the journey so when a starts the journey b has already you know so four seconds four minutes have elapsed so and then and then basically this is displacement so at if you have to draw a vertical line let's draw a vertical line so this is the vertical line so four at t equals to four this is somewhat near one so let me draw another another vertical line this is how you'll have to do so from this point of intersection you'll have to draw off it looks like one meter how far down the road has b traveled when a starts the journey so b is already b has already covered one meter next question without the calculation the speed of the speed state who's traveling faster a or b who is traveling faster a or b who is traveling faster and why is a traveling faster why is a traveling faster what is the reason why is a traveling faster guys greater slope correct a is faster because the slope of a is displacement time graph is more so this is a this is b so if you see slope of a this this inclination is more than slope of b this angle right so if this angle was theta and this angle was phi so clearly phi is greater than theta hence or otherwise you can also yeah so Aditya saying um a covers more distance in same amount of time yes you can do that as well how so basically you can so let's say if you see let's say there there's a common point here and in in this much time let's say we are taking this much time and this is my b's girl so b has traveled this much distance in the same time but a so a has traveled this much distance in the same amount of time isn't it so this is a's distance and this is b's distance in this time interval delta b so hence a is faster so what is the speed of a find out what is the speed of a what is the speed of a what is speed of a how do you find out speed of a so you have to get some known values of displacement and time you see so at four minutes it started its journey and let's say if you brought from this perpendicular this is eight so in eight eight from t equals to four to eight it has traveled two meters two meters so this is my delta s which is two meters from zero two and delta t is nothing but eight minus four minutes this gap is four minutes okay so what is the speed of a speed of a will be a is nothing but v a is delta s delta t which is how much two meters by four minutes that means zero point five meters per minute meters per minute yeah so you don't need to change the units you can write this unit also that's not a problem until unless specified you can leave it here if it if they are asking in s i units then you have to change what is speed of b find the speed of b quickly speed of b so speed of b is point two five per meter per minute how do i find out so again speed of b for speed of b you have to find out that yes so let's say if again this is eight this is eight so in for for b b b is equal to delta s upon delta t again so delta s let's say in zero two eight minutes it has traveled two meters isn't it right so this is two meters and time is eight minutes so hence it is two by eight meter per minute so zero point two five meters per minute yeah correct point two five meters per minute now are the speed of a and b uniform are the speed of a and b uniform yes or no so those who are writing only point two five please be careful if you don't write the units you will get no marks units units are very important yeah good so they are uniform because they have constant slopes and it is a displacement time graph hence what the what does point x on the graph represent what does point x on the graph represent what is the physical significance of point x please tell me your answer so it is it is the it is the point where a overtakes b because they are moving in the same same or you can say yeah a and b uh a and b meet so what what what happens is this is a straight line motion so b starts first so it's like uh or tortoise and a hair race so the first time so tortoise started faster uh initially and then here you know uh overtakes it after some time that's point x yeah so it is a straight line motion so meeting meeting could be in two dimension also but this is not not only meeting it is a specific case for example they there and these are the two bodies moving in these two directions they can meet here also but this is specific case what is happening is they are both they are let's say traveling on a straight road both of them only thing is b started first so this is the road so b started moving first but b speed was slow so after some time after some time let's say b was here b was here then a started its motion so a started its motion and after some time after some time again what will happen b is moving so b is moving like that and a just surpasses it so both of both of them this is the point of overtake okay next question what is the speed of speed of approach of a towards b what is speed of approach of a towards b that means if you are b how fast a would appear to be coming towards you so speed of approach of a towards b can can anyone tell the answer yeah so speed of approach is nothing but what is speed of approach so speed of approach of a towards b is velocity of a minus velocity of b this is what is called speed of approach so hence if let's say if you are this is our relative velocity also so let's say if if someone so b the point b or the b itself when it looks at a it would appear that a is moving towards b with this velocity how much is it so a's velocity was 0.5 meters per minute minus 0.25 meters per minute and the answer is 0.25 meters per minute so before they meet a would appear to be as if a is coming towards it at the rate of 0.25 meters per minute and similarly after they meet in this in this phase now what will happen is so you can you can you can do analysis like this also let's say in one minute let's say this is the point where they meet let's say this is the point where they meet right so what happens after one minute b travels a distance of 0.25 meters and a travels a distance of 0.5 meters 5 meters so how fast a is moving with respect to b is nothing but so if someone is sitting on b it will appear that a a in the same time has moved this much distance extra this much distance extra and that distance is nothing but 0.25 meters and how many how much time did it take to travel this extra distance nothing but one minute so this is the velocity of separation so every minute there will be 0.25 meters added to the separation between b and a after next next minute what will happen b would have come here so this is let's say after t equals to one minute this is after t equals to one minute after t equals to two t equals to two will be here for b it will again travel 0.25 meter but for t for a it is another 0.5 meters another 0.5 meters like that so this is t equals to two so at t equals to two the distance between a and b is nothing but 0.5 meters so every second 0.25 meters is added to the separation so hence the speed of separation will be or the question is speed of separation yeah speed of separation of a from b will be 0.25 meters per minute so I hope you know this gave you a good run through of the entire syllabus of motion let's go to laws of motion let's go to laws of motion yeah solve this what is the SI unit of momentum what was momentum momentum was defined as mass times the velocity of the object so what is SI unit of momentum be careful everyone is saying a look look carefully one c correct kg meter per second what is the numerical formula for force question number two question number two very good so you know force is mass times acceleration special case of newton's second law question number three if the initial velocity is zero then the force acting is the question is little wrongly framed but it is if the initial velocity is zero then the force would lead to or yeah it's okay start so the option number b should be accelerating not acceleration so force acting is a retarding b accelerating c both d none so it should be accelerating why because if the initial velocity is zero and later on it came to some velocity that means there is a net positive change in velocity that means positive acceleration okay what is the SI unit of force question of force SI unit of force yes so SI unit of force is kg meter per second square which is also called newton now here is the game now the subjective thing comes state newton's second law of motion so type in quickly type in newton's second law of motion and mention that in the comment quickly type in let's see who does it fastest state newton's second law of motion take 10 15 seconds of time is this answer correct the rate of change of momentum is directly proportional to the applied force equal to external force on a body some bit of modification is required think rate of change of new momentum is directly proportional to external force on the body repeat so right now when you're sitting on the table is there no external force acting on your body then why are you not accelerating net extreme where who's this who's busy place who's this guy or girl what's the name I don't know please mention your name so that I can call it out Varsha yes Varsha so see I made you recite it couple of times and it has rectified now so you are right you have to use the net external force just not any external force you have to either you write unbalanced external force or you write net external force is so the rate of change of momentum is directly proportional to the rate of change of linear momentum of a body is directly proportional to the net force acting net external force acting on the body no who's this anand sraddha right the force applied on a body is equal to product of mass and acceleration is a corollary to the second law it's not always true okay so you have to write second law is nothing but this is you should not write anything but this newton second law second law of motion newton's newton uncle second law so newton's second law of motion okay what is newton's second law of motion it says that the rate of rate of change of change of linear momentum momentum of anybody is directly proportional proportional to net or unbalanced whichever way you want to write unbalanced unbalanced force acting or not another body is external external force acting on the body okay so this is and then you can write mathematically also that f is directly proportional whichever way delta delta p upon this is newton's second law of motion that's it if you write even this after the job is done okay next go let's go to the sixth question what is the momentum of body of mass 200 grams moving in the velocity of 15 meters per second quickly do it nihar check your answer please again you are doing the same mistake you're not converting it into s i units first thing the moment you see any numeral value in any physics question the first thing which would strike your mind is is that number in requisite units if it is not then first thing you should do is convert that into s i unit if if if at all it is required so for example it is 200 grams so first of all you should convert it into kgs it is 0.2 kg moving the velocity of 15 meters per second so mass times velocity is momentum so 3 kg meter per second is the right answer okay so i'm not going into question number seven a typical question go to question number eight question number eight a force of 55 unit an axon of mass of 500 grams resting on a frictionless surface what is the acceleration produced yes 50 meter per second square correct go to question number nine great going great great going guys you're doing good you'll be able to solve all let's let's do question number nine let's do question number nine quickly all of you are doing good question number nine a force of 15 Newton acts for five seconds on a body of mass 5 kg which is initially at rest calculate final velocity of the body what is that tell me what is the value of acceleration how much is the acceleration you found out acceleration is the body accelerating first of all yes or no and if it is accelerating what is the value of acceleration aditya three will not help give me the units guys please understand numbers are of no value in physics you have to give units ritu it's not three three meter per second square you'll have to mention 15 upon five yeah check was that both of them are in SI unit so hence clearly 15 upon five is the acceleration yeah so what is the final velocity so a five second is initial initial velocity is zero acceleration is three meter per second square time is five seconds so final velocity will be final velocity is nothing but b is equal to u plus ad use the first equation of motion so basically how would I solve this question I would solve like this so it is given first of all I'll draw a representative diagram so this was my u equals to zero and let's say and a representative you know this thing so there is f so and this mass is given as five kgs and f is given as 25 Newton so a is nothing but 25 upon five which is I'm sorry hey f was 15 so hence it is three meter per second square so acceleration was this right now u is known v is unknown t is given as five seconds so v equals u plus ad so this implies v is equal to zero plus three into five meter per second that is 15 meter per second right and then they have asked about displacement displacement is ut plus half ad square which is zero into p plus half into a was three into t was five square this is meters in meters so how much is it so 25 75 by 2 is 37.5 meters great now quickly differentiate between mass and weight quickly one by one you mentioned that you know the points of differentiation between mass and weight what is mass what is weight so gurman never ever start only with you know the characteristic first define what is mass define start with definition you can write all this mass is constant means what even weight is constant under certain conditions yeah so mass is nothing but the amount of substance in a body may or amount of matter contained in a body is mass and weight is nothing but the force with which the center of the earth attracts or the earth attracts anybody yeah weight is correct then amount of mass is scalar perfect weight is weight is vector perfect mass is in kg weight is in Newton absolutely perfect see Shreya weight is not a downward force applied by a body it is a downward force applied experienced by a body not applied by a body so our body is not applying weight weight is being applied by earth on us weight is force of the earth Shraddha don't write like that weight is the force applied by earth on any given body on its surface that is yeah mass is constant universally constant so in classical physics we say that if the body is nowhere you know moving with the speed of light if it is the speed is not near the speed of light then mass stays constant for all practical purposes it doesn't vary with time and space right but weight changes even on earth and as you go up and as you go down weight can be zero yeah weight can be zero so weightlessness is that you know let's say when you are there in space in space station your weight is zero perfect so all these are absolutely personal you know this difference now let's go to question number 11 yeah 11 let's do 11 question number 11 a scooter is moving with the velocity of 20 meters per second check it is in SI unit when brakes are applied the mass of the scooter and the rider is 180 kg check again in SI unit the constant force applied by the brakes is 500 newtons check another SI unit so all are in SI units first of all you are certain that so you don't need to worry now let's go for the concept so how long should the brakes be applied to make the scooter come come to a halt solve it so again find out the deceleration in this case all of you tell me what is the deceleration experienced because of the braking force what is the decelerating force come on guys you can do it tell me you can you can express in fractions no need to convert into decimals if at all you are getting fractions so minus 7.2 meter per second square okay is it correct all of you agree ishani says minus 25 by 9 so 550 18 25 correct 25 by 9 appears to be correct ishani so it's not 7.2 then so i think kshiti has already solved it yeah so 25 by 9 negative is the decelerator you know so acceleration is negative 25 by 9 deceleration is 25 by 9 meters per second square how long should the brakes be applied to make the scooter come to a halt is nothing but v is given u is 0 sorry u is given 20 meters per second v is 0 so you have to find out the time so most of you have got it 7.2 seconds which looks like appears to be correct so 20 upon 25 by 9 so 24 by 5 into 9 7.2 is correct 7.2 second is correct how far does the scooter travel before it comes to rest so i think shudhi says 72 meters are all of you getting the same value so you have to apply what you can apply v square equals to u square minus 2 as in this case so don't go for t square thing you can use directly v square equals to u square minus 2 as right so v was 0 u was 20 square minus 2 into 25 by 9 right into s so this implies s equals 400 into 9 upon 15 8 so this is 72 meters perfect so let's solve this question now question number 13 stone is dropped from a height of 100 meters how long does it take to fall the first 50 meters and the second 50 meters good question solve find marker solve this so stone is dropped from a height of 100 meters how long does it take for it to fall for the first 50 meters and the second 50 meters okay which which do you think a and b would take more time so in which which of these two cases do you think more time will be required in a or b so a will take more time or b will take more time okay so at least all of you are clear on this that a will take more time now so find out in b more time will be required why we have in the second half it would have been it would already accelerate right it would have already accelerated to an extent so the to cross first 50 meter it will take more time than to cross the second 50 meters correct now all of you solve it find out the time taken for the first 50 meters you know s that is first 50s s you know initial velocity since it is dropped so initial velocity is zero t is to be found out and a already you know which is so you can take acceleration due to gravity as 10 meters per second square root of 10 or I get I am seeing two types of answers one is root 10 other is root 20 should be root 10 yeah so the first is root 10 what root 10 what kilometers root 10 seconds you'll have to write this thing is not going in your head that you have to write units if you write like that if you don't write units for b it is root 50 second okay let me see let others also yeah so let me solve this question okay so this is the case case is this is the tower beautiful tower it is okay so let's say tower has this post where someone is standing and he drops this thing it comes down let's say this is 100 meters so half of it is 50 meters so when it started this this direction u was zero right acceleration due to gravity was plus 10 meter per second square okay this is let's say s1 and this is s2 now s1 is equal to 50 meters and s2 also is 50 meters okay so so hence when you reach here first 50 meters is very easy so for the first first let's say the journey is a to b to c so let us analyze the journey from a to b let us write everything so initially u was zero acceleration during this is 10 meters per second square and v i don't know whatever it is but i know s1 s is 50 meters so i can write the second equation s is equal to u t plus half a t square so u was zero so s is half a t square so hence time taken will be nothing but 2 s upon a under root so what is deploy the value so 2 into 50 upon 10 which is root 10 seconds so it takes root 10 seconds root 10 seconds from here to here okay then i have to find out the time taken here let's say this time was t1 so what is t1 root 10 seconds let's say this time is t2 okay then what do i know so let's i know the full distance full distance was 100 so i can again write s1 plus s2 is equal to u times t1 plus t2 total time taken plus half into a into t1 plus t2 whole square right the total time from a to c now we are seeing what a to c from a to c total time was t1 plus t2 okay so now s1 plus s2 is clearly 100 meters and this was zero so hence if it becomes zero then only thing remains is half into 10 into t1 plus t2 whole square so this becomes 200 by 10 that is 20 is equal to t1 plus t2 whole square that means t1 plus t2 is equal to root of 20 seconds so total time to come down is nothing but root 20 seconds correct so hence t2 will be nothing but root 20 minus t1 which was root 10 so answer is root 20 minus root 10 seconds so this much amount of time will be taken yeah root 20 minus root 10 times seconds will be required for moving from b to c because that was what was t2 is it clear to everyone any question in this so you are mostly done yeah so the second chapter also if you are able to solve these questions that means you are mostly there i think we solved problems also and let us take a few problems in gravitation so that yeah let me see yes let's let's answer question number one even though stone also tracks earth towards itself earth does not move question number one another is we have done this in the class one answer is question number one answer is you don't have gravitation okay so aranar doesn't have gravitation never mind so it is only two three questions we'll see because why shreya why are you saying b because of the greater mass of earth is so heavy that for a small force small force the acceleration is too low for it to be appearing to be moving okay so answer is should be a the weight of an object is question number two question number two weight of an object is greater on earth and lesser on moon lesser on earth and greater on earth option b is written yes option is a correct third weight of an object has s i unit of question number three question number three guys weight of an object s i unit is k g oh my god no it's a newton weight is force masses masses s i unit will be k g please note we just did the difference between mass and weight question number four which of the statement is correct question number four so i think you know most of you were able to solve almost every question which we discussed today so here is my take on this there are few uh two and don'ts during the exam so and as i have already mentioned in the class also please divide your time such that in such a way that towards the end you must spare at least 10 minutes for the revision part especially in numericals please make sure that you know you take care of the units because if you don't take care of units you are most likely going to get very less marks so let me write it down as instruction so please follow these first is so lose and don'ts in exam lose and don't first one is do not panic you know everything you know everything because even if you panic nothing is going to change do not panic first of all second spare 10 minutes at least for revisiting for revision then third is put star marks star marks on the questions which you think you should revisit here now it's not working so i'm writing here okay this is three point number three point number four be careful in bold i'm writing careful be careful for or be careful for units first thing you should do is you should convert them into s i units so hence please no units no marks no marks remember this please make the life of the examiner easier he will make or she will make your life easier no units no marks in numericals in numericals try try try always to draw representative diagrams examiners love this so if you express everything in picture it is making their life easier so they will make your life easier so you try and it is also useful for you because you will have a clarity in your mind okay so i think you should be taking care of and you know especially in science subjects wherever required give ample amount of examples do not shy away from giving ample so for example whenever you are differentiating or you're explaining something action reaction whatever let's say if there is a if they only ask define newton second law and then let's say you're seeing that there are a couple of you know the marks attached is a two or three or more even it is always better to be examples but give examples only when you are very very sure that the example is correct otherwise it will backfire even let's say if you have right definition but wrong example it will just create more trouble than good so hence if you're sure that this is a this is a good example then please write otherwise do not enter that territory okay so i think you are good in case you are facing any other trouble any other trouble you can always you know send me the questions on whatsapp you can i am you know post it on to my individuals this thing or you can do it on groups as well no problem and i'll try to solve all your queries as much as possible sir can you share the question paper in the group which achai you're talking about these question papers nihar the sample papers which we were discussing i can share it no problems at all i'll share you i'll share with you just after this okay so i hope you guys do well there are the sample paper which i'm going to share even if you go through them and write all the answers religiously i am sure you are going to get more than 90 95 percent marks so hence guys you are all all set all the best for your exam do not panic because panicking anyways won't help so go there enjoy and share your experience in the next class or whenever whenever we meet so i will be logging off now so in case if you have any trouble you can always and yeah so it goes without saying please subscribe to our channel so that there are many more such sessions we have planned so you will get notification about those sessions and you can take advantage of them thanks a lot and have a nice evening all the best to you guys over and out