Yes good evening guys to all of you i welcome you all on this wonderful platform that is exam prep and guys as you know we were going for engineering mechanics we are already we have completed the trust topic and now we are going to start the next topic that is friction so you know that i already told you this is a common.
Subject for mechanical and civil and i also told you that from this topic every year one question is going to be there in both mechanical civil axi and also you can expect the questions from this in pi paper also so my dear one question will be there from friction.
If you are thinking of engineering services civil services isro bark drdo any examination friction will be there good evening nagaraju madhu abhishek yes good evening yes bhanush okay so guys we will be starting now.
So when we are talking about friction every person would be thinking about friction in some negative way people are thinking friction is something which is stopping us because we also sometimes using this term as a you can say like in a movie there is a hero there is a villain so we feel friction is a kind of villain in our.
Life but my dear you would also be shocked to know that we are capable of writing because of friction we are capable of running because of friction so friction is helping us also in many ways so my dear actually the understanding about the friction is given in the wrong way somewhere so because of that we get some wrong.
Conceptions about that so in this series in next three session we are going to learn friction in a wonderful manner we would be learning wonderful concept related to that so mother women she actually due to some reasons i was unable to take this session in the evening i was in journey actually at that time so i was unable to.
Take the session that's why we kept we kept this session at 11 pm so sorry for the delay and thanks for joining at that or timing also so we are starting the session guys now so my dear this is some detail about me and you know the subject going on is engineering mechanics.
Okay so the subject going on is engineering mechanics and my dear this is a an important announcement about the work show so there is a free gate workshop by rakesh sir and he is having 11 plus years of experience and mighty the timing for the workshop will be 12 30 p.m and this is on the fifth june so just note down the date so.
That fill the alarm of that day because it is going to be very important session for all of you okay uh sir will you start production yes yes yes yes yes yes abhishek so guys now whenever we are talking about friction first of all you must be clear that there are many types of forces are there so my dear friction will be coming into.
Picture when there will be a contact so what i want to say is friction will arise when there will be some contact if you want to see some example let us say there is a box which is there you want to move this box by some distance so because you want to move this box by distance you applied some force onto that and when you will be applying some.
Force onto that you know sometime when you start pushing the box just by applying the force you are unable to move it in the starting it is at rest so my dear when you are applying the force and you are increasing the magnitude of the force then after some time when the force reaches to some value then you feel that body starts.
Moving so body is not moving instantly we apply the force so my dear who is stopping to move that body that something is friction actually so when you are applying the force my dear you try to push this body you try to move this body in the starting you will be feeling some resistance coming because my dear friction is nothing but.
A contact force it is coming because of contact where it is acting what are the properties we will see one by one from very very basic point i am teaching you so my dear first thing about friction is it is coming because of contact so we will be saying friction is a kind of contact force my dear now the question arises do we.
Know about any other force which is the contact force so i will say yes there are total of four types of contact forces my dear like if we have a body present like this you know that this body will be having a weight.
And if you make the free body diagram mg would be acting like this and you make a normal reaction from the surface so you always make a normal reaction coming to this body from the earth so my dear normal reaction is coming because the body is in contact with the earth's surface so my dear first type of contact force is normal.
Reaction my dear second type of contact force is tension like if this block is attached with this string then my dear there will be a tension coming and that will be acting on this block and my dear that is why this is also a contact force the third type of contact force my dear.
Is the spring force because whenever you will be having a spring in connection with the block when you will be displacing this bloke the spring will be applying a spring force that is stiffness into displacement it is again a contact force now my dear we are talking about the fourth type of contact force that is friction force so.
My dear if the body is in contact with some surface when you will try to move this then my dear there will be some force coming by the surface that is said to be the friction force so my dear this is coming when spring is present this is coming when thread is present but these two forces requires.
Just contact with any surface for them there is no requirement of any spring there is no requirement of any string so my dear it means whenever two bodies are coming in contact we would be having two forces actually and one force is said to be the normal reaction and second force is said to be the friction force so my dear actually these are two forces which.
Would be acting so i would like to tell you one thing my dear if there is some body then you can see weight is acting downward no doubt about that other than weight what we can expect actually you will be having a normal reaction force that will be acting if you try to pull this body friction will resist it and as this force is coming from the surface we.
Must always show this force at the surface only so my dear first of all we have seen friction is a force second thing we have seen friction is coming from the contact is it clear and my dear generally it is it is a concept about friction that people says that friction always.
Stop motion friction always try to stop the motion but my dear i would like to tell you friction is not always going to stop the motion friction somewhere also generate the motion so if you are thinking that friction is always going to reduce or stop the.
Motion you are wrong my dear sometime friction also generate the motion you might want to see the application where friction will try to generate the motion and my dear i will be showing you the application like this if you can feel this.
You have seen this kind of picture at the time of marriage when dula raja is sitting on horse hella and my dear you know horse is actually moving horse is actually moving horse is moving and dula raja is sitting on the horse.
So dule raja is at rest he is not having his own motion but my dearest horse is having the motion so my dear if if this horse is going with some speed the same speed is there with that person also that person is also moving with the same speed ahead.
And where the horse will reach sorry guys where the horse will reach where the horse will reach in the same way that person who is on the horse will also reach why that person is not having its own motion actually there is friction between the surfaces that friction is.
Generating the motion for this person it is like when you are having a bloke on a surface like this and another bloke on this if you will try moving this block then my dear you will feel that this bloke is also moving and who is going to provide motion to this bloke that is nothing but the friction.
So my dear i will be showing you where is the magnitude where is the direction everything we will see first of all i want to say that friction does not always remove the motion eliminate the motion reduce the motion stop the motion friction also sometimes generates the motion.
And my dear as you have seen friction is a force you can see in this figure that friction when we are saying it always oppose the relative motion actually it always oppose relative motion friction cannot see the separation between two bodies like if i show you that i am having this.
Body a block this is another block then my dear if i am going to move this book then my dear what is going to happen when i am moving this book then my dear this mobile phone is there on this so relative motion means when these two bodies are trying to rotate like this.
But my dear friction is going to avoid this relative motion friction wounds bodies must travel together so i want to show you how the friction is acting that will clear your concept if you would be having if you would be having a block like this like this and you will try to move this my dear.
Friction is going to stop the relative motion friction will be acting like this why friction is going to stop the relative motion of yours with respect to the ground but if we are going to have one block like this and second block like this now when you will try to move this body.
With some force f then my dear obviously friction what friction will do if you are saying this is block one this is block two then my dear four block number one you want to move it here so friction will try to stop this motion because friction will try that block shall not move with respect to the contacting.
Surface that is the earth so my dear on the same at the same time on the surface friction will be acting in this way why because my dear when we are talking about this surface earth surface this block when there will be no relative motion between them when these bodies will be.
Moving together so friction tries to convince both the bodies friction will say to the surface let's move with the same velocity as block one is moving and it will be saying block one to stop because the surface is at rest so friction will try to convince both.
The peoples and if friction is going to be successful the bodies will be relatively at rest other will otherwise the body will start motion so my dear what i want to say is the friction will be acting on the surface in forward direction on this body in this way and my dear obviously to have no relative motion between these two.
Bodies on the block number two friction will be acting in forward direction so that these two bodies must travel together there should not be any kind of relative motion and might this is how friction is generating the motion friction is helping the motion so friction is not always going to stop.
The motion friction can sometime also generate the motion now we go to know friction is a contact force we got to know friction is not a force which is always going to stop the motion it can also generate the motion uh dear abhishek we will be coming on to that don't worry that point is far away.
Right now so we have seen one point that friction is contact force we have seen second point friction can also generate the motion then we have seen third point that friction opposes relative motion many students would be knowing what is relative motion some of you may not be knowing what is relative motion so my dear i would be telling you actually.
What is relative motion so my dear like let us say there is a person a he is moving with the velocity of 10 meter per second there is another person who is moving with the velocity of 5 meter per second.
This person is b and this is let us say a person c which is at rest so my dear you know that if we are talking about this person a this is having velocity 10 meter per second this is having 5 meter per second this is at rest but my dear you know when b will see a.
According to b according to b the velocity of a as per b will be 5 meter per second because you know that every person can see only the relative things means if i am having some velocity i would be able to see only the difference of the velocity of other person and mine yes static uh refers to okay great great.
Great you try to answer actually so my dear if you see this is 5 meter per second this is 10 meter per second so velocity of a as per b is 5 meter per second and my dear if you say that the person which is at rest it will be giving the absolute velocity of a and b when b will say a according to b the motion of a is with 5 meter per second.
In the forward direction if you have any doubt that how to see relative motion i am giving you a normal shortcut that if you are saying b is having 5 meter per second if you want to see the relative velocity of a with respect to b just do one thing my dear give this velocity of b to a.
In the opposite direction just give the velocity of b to a in opposite direction when you will be giving the velocity of b to a in opposite direction then my dear you can see for a the forward velocity is 10 backward below stage 5 net it will be left with 5 meter per second and if the b will be looking at c.
You know its velocity in the opposite direction will be given to c so my dear according to this b c will be moving with 5 meter per second in opposite direction so this is how the motions are going to be so this is said to be relative velocity so my deal there is a relative motion between a and b there is a relative motion between a and.
C the relative motion between b and c now let us say another person d is coming and that d is also moving with 5 meter per second absolute then my dear when d and b will look at each other they will find that we both are at rest so my dear at this position when both.
The bodies are moving with same velocity they both will be feeling that both are at rest so my dear this thing is said to be relative rest like there is relative motion there is relative rest also is it clear so my dear relative rest and relative motion.
Like if we have this block and this bloke they are not moving then this condition is said to be relative rest and my dear when both the blocks are moving with same velocity then also it is said to be relative rest.
Because when you will be giving this velocity in opposite direction to this body this body will be having zero relative velocity and when both the velocities are moving with different velocity this is 10 this is 5 this condition is a relative motion so my dear friction opposes relative.
Motion and any tendency of relative motion if anybody tries to have relative motion then also friction comes into picture what do you mean by try as whenever somebody is there you will be applying some force onto that body and initially you are trying to move it the body is having tendency friction will come and after that when the friction is.
Acting after some time you will be increasing this load and this body will start moving then also friction will be acting so friction is acting when there is motion relative or there is relative motion key tendency so friction does not always comes into picture at the time of relative motion it also comes at the time of relative rest.
When there is a tendency of motion tendency of motion means force is applied the body is about to move friction will come to stop the body friction will apply its complete power to stop the relative motion if it would be successful the condition.
Will remain relative rest if it is unsuccessful the condition will become relative motion is it clear for everyone or not please tell me that this was the detail about the friction we have seen now my dear we have just started it we have not even seen the types of friction because this session is related to old and modern.
View of friction in this session we are going to learn that from where friction is coming actually what is the reason behind the friction right now what we got to know if this is body one this is body two and if these two bodies have contact between each other if you see this contact with the.
Microscope there will be multiple points contact will be there at these multiple points of contact you will be having some force which is said to be contact force and these contact forces contact forces will be having two components one is horizontal other is.
Vertical one is horizontal other is vertical one is horizontal other is vertical one is horizontal other is vertical one is horizontal other is vertical one is horizontal other is vertical such kind of infinite point of contact will be there and my dear if i will be joining all these vertical component then summation.
Of all vertical component will give me normal reaction and summation of all horizontal force will give me the friction force so friction force is horizontal component of contact force this is the refined definition good evening bye ranjeet so now you tell me this definition is.
Fine or not definition of friction we are not discussed anything till now we were just going for definition first i said friction will come when contact is there then i said friction is a contact force like normal reaction like tension like spring force.
Then i said friction do not always stop the motion it also create the motion sometimes then i show you the example then i told you friction x when relative motion is there then i taught what is a relative rest when what is relative motion and my dear then i told you that actually friction is nothing but.
The horizontal component of the contact force okay so that is what we have discussed okay i am teaching you these things in a very very uh simple way and also slightly on a faster way my dear now we shall know that from.
Where this friction is coming actually what is the cause of friction actually and my dear when we are talking about the cause of friction after that we will see what is static what is kinetic and all the types and all the things that we will see at the later stage for sure but for the time we need to see what is the thinking of peoples about the.
Friction to define the friction there are two theoretical models given my dear one model is said to be old view purana logo search and second thinking of people is said to be modern view okay guys so we need to see what is old view what.
Is modern view obviously old view was unable to explain something because of that modern view comes into picture and my idea this is going to be very important for interviews also whatever right now we are going to discuss this is going to be an interview question also.
As i said this is first lecture we are talking only about some basic things so my dear when we are talking about old view previously what people were thinking actually as you already know whenever we see these two bodies it looks like a perfect connection we we feel it like this is a plane surface where both the bodies are connected this.
Is our feeling about these two bodies but reality is not this when you will be looking with the microscope what you will found that both the bodies are having actually the surface roughnesses this board is having surface roughness like this the surface is also having the surface roughness like this.
So my dear old view says that if you see zoom view zoom in facilities available when you will be going to zoom in you will see this and you can see my dear these surface irregularities these surface irregularities.
Are acting like interlockings between the two bodies like this so these two bodies are interlocked like this when you will try to move one body with respect to other these interlockings will be coming into picture so my dear you can see i can show you.
Another diagram of interlockings it is like that interlockings are like that okay guys so when you will try to move this body over another bodies these interlocking will not allow that to happen for that when you will be moving this body you need to break these interlockings.
Of these irregularities i am not saying that old view of friction says that that when one board is going to move over another it need to break these interlocking present between both the bodies and my dear when these interlocking would be broken yes achinthea friction is happening.
Because of relative motion but now i am telling you that okay relative motion is the condition when friction is coming but from where it is coming like if you are moving somewhere why friction is trying to stop you who is friction to stop you so i am telling you from where it is.
Coming actually just now we have seen the condition when it will be coming it is a vector quantity it is a force it will be having some magnitude direction okay but why it is coming whenever two bodies are in contact so my dear the reason explained by the old peoples in the old.
View was the interlocking present between the bodies and that interlocking is not allowing the body to move and if this body trying to move over the other body my dear interlocking is going to show some resistance and if you want to move the body you need to break the interlockings which will require some magnitude of force so when you will.
Apply that force interlocking would be broken and the body will be able to move so my idea this was the view of old people over the friction and according to them the reason for friction was the presence of irregularities on the body like if this phone i am saying that when i will be putting.
This phone on to other body and i will try to move it interlockings are going to be broken so my dear interlocking would be depending on the roughness if a body will be rough more interlockings if a body is smooth less interlocking if the surface is also smooth like this if the surface is like this another surface is also like this.
There might be there will be no interlocking so according to the old model old view interlockings are the reason for the friction so my dear when we see the old view there are two conclusions coming into the mind the first conclusion is coming if reason is interlocking then my dear.
More area more area means more interlockings and more interlockings means more friction so might this was the first conclusion of the old view of friction but when we have seen this conclusion.
That more area more interlocking more friction but later on when we see we found out that friction is independent of area later on we got friction as independent.
Of area we found that experiments show that then my dear second conclusion was okay if interlockings are the reason for the friction if i take two glass surfaces one over.
Other if i take two glass surfaces one over other you know that it is going to be highly dangerous to move one glass over another you know chances of damage will be very high and you will never try to do that when two glasses are like this they are sticking without any sticking agent without any.
Fevicol so when you will be having those two surface to be like this the motion of one over another when these are two glass smooth surfaces it is going to be the most difficult thing but why did you know when the surfaces are smooth friction should not be there but my dear it is going to be very very.
Difficult to move extra smooth surfaces so my dear it is going to be very very difficult to have motion on extra smooth surfaces extra smooth surface so my idea when the motion is going to be very very difficult on the extra.
Smooth surface that is going to be against this theory because my dear when irregularities are not their surfaces are smooth the friction should not be there but my dear it says it is very very difficult to have motion on extra smooth surface it is also going against the old view it means there are some problems with the old view so if someone.
Ask you guys whatever i am writing is visible to you or not i feel there is some issue is it visible or not yes sorry i feel there is problem of visibility.
I feel there is some problem with the color i feel with obs i was writing with white only okay once again i am saying one thing my dear as per the old view if area is more as per the old view if area is more irregularities will be more.
Is regularities will be more so when area is more irregularities will be more and my dear irregularities will be more it means friction should be more but what we found actually but we found with experiments friction is.
Independent of area so it is going very much against the old views theory old we were saying irregularity is the reason for the friction if irregularity is the reason area more more irregularities friction should be more but a friction was found to be independent of the area then my dear.
Second thing irregularities irregularities are less when they are less on extra smooth surface extra smooth surface so their friction should be less.
But it is found to be very very high so my dear irregularities would be the reason of friction then at extra smooth surfaces when we have glass over glass friction should be lesser motion should be easier which was not actually so mighty this was found to be the second challenge to the old view of.
Friction is it clear some idea these are the reasons which we were getting that friction reason which was explained as irregularities there is some problem into that so my dear when this view failed to explain why extra smooth surface have.
More friction and why friction is found to be independent of area then when this view was unable to explain that we need to leave that view and some modern people came and they have given some another view and my dear which is able to explain both the things both views are just the model of explaining the friction they are saying from where.
Friction is coming actually but my dear old view was unable to explain then modern view came into picture then my dear what modern view says will shock you more than you say that actually whenever you are having two bodies with irregularities this is true that bodies are having irregularities.
But bodies are not connected in the way of interlocking actually bodies are having connection like this like this so my dear when board is having having connections like this you can feel one thing that connection is there at some of the points so my dear according to you you were.
Thinking that this is the area of contact but actually area of contact were only some points when this board is coming in contact with this mobile my dear what we are thinking is we are thinking that this entire is the area of context but actually the contact area is just.
These points so when we have seen practically the actual area to the apparent area is found to be 1 by 10 is to 4 so actual area of contact was 10 000 times lesser than what it appears to be so when area of contact was very very.
Small you know that there will be some intermolecular forces of attraction between these two bodies there will be some intermolecular forces of attraction between these two bodies and if that force is divided by the area it will be giving you pressure so because area is very very less pressure will be very very high.
You all are mechanical or civil student you must be knowing that welding happens because of two reasons the first reason for welding to happen is heat the second reason for welding to happen is pressure so my dear because of this high pressure modern view says that this result into welding and as this welding is going on without heat they.
Have named it as cold welding and they said that at these joints some cold welds are going to be produced so when you will try to move this body along this these weld needs to be broken and everybody knows welded joints are stronger in nature so when you want to move this body over another body you need to break.
These welded joints these are made because of high pressure which is there because of less area and that is the process is cold welding so my these cold welds are producing resistance whose horizontal component is coming as friction force so whatever the force required to break these weights.
That is the friction force so they have said friction is not coming because of irregularities actually friction is coming because of the cold weld and when they have taken the concept of pressure pressure you know is force per unit area pressure is not depending on area.
Because it is per unit area so pressure is independent of area it explained the first point that friction is independent of area because pressure is independent of area so when pressure is there we are not talking about force we are not talking about area then my dear pressure means same at every point so whether the area is.
More or less friction is going to be independent of that first point was explained first challenge to old view was explained by modern view both these are theoretical modal ideas then my dear second problem was between two smooth surfaces why friction is more for that the reason was given as for the.
Smooth surfaces point of contact would be more and as well as the area of contact will increase means denominator is going to increase for sure but at the same time because flat surfaces have more area of contact because of that the intermolecular forces between smooth surfaces are also.
Very high and this increment is going to be quite quite times than that of this increment because of that this pressure is going to increase much which results into very high friction actually this was the explanation given by the modern view and my dear both these points were satisfying the problems or challenges come to the old view of.
Friction so my dear today what we have learned is friction is a contact force yes then we have learnt first we have seen friction is a contact force then we have seen friction not only stops the motion it also generates the motion then we have seen the examples of that.
Then we have seen there are four contact forces out of which friction is one and it is the horizontal component of the contact force actually so actually the resultant force of normal reaction and friction is that net contact force then we have seen it is coming to stop the relative motion then we have seen the concept of relative motion and.
Relative rest then we have seen both the views of friction what old view is saying what modern view is saying so guys please tell me how many of you understood both the views of friction old view says friction is coming because of what write down in the comment.
Section that according to old view friction is coming because of what according to old view friction is coming because of what and guys do like and share the session okay so that other people's can also reach here don't worry tomorrow timing will be 6 p.m only so today i was unable to take.
Session that's why we change the timing tomorrow session is going to be very important this was just for interview tomorrow session will help you in gate actually yes so my dear interlocuting is the reason for friction when we are talking about.
When we are talking about old view and what is the reason for friction as per modern view guys and i'm telling you these kind of things you will not be getting everywhere what is the modern view for the same according to modern view what is the reason for friction yes guys.
Yes guys so as i said old view says interlocking modern view says cold weights fine guys cold wed is the reason okay guys so my dear when we are talking.
About this was the old and modern view and modern view was able to explain all the things so my dear today we have understood very basic things and actually i want to teach more also as per the heading i was thinking to.
Teach you the types of friction also today that is static limiting and kinetic friction but time is less and i don't want to take it very fast i want to take it in the normal speed so as time is also going to 12 so my dear tomorrow 6 pm we will be continuing from the types of friction i will be telling.
You everything about static friction static friction is said to be self-adjusting force kinetic friction where it is coming we will see the condition for both static and kinetic friction because friction is a force we will see the magnitude and direction of the friction force also once again and we.
Will start the block problem which are majorly asked in gate paper so tomorrow we are going to see block problem then we will go for wedges and all screw jacks and all other things so goodbye guys for this session thanks for joining thanks for your love and support and before leaving uh do like and share the session.
Yes very good surya very good wonderful dear so you guys are now i feel learned a lot about this wonderful so guys thanks for joining and signing off with this we will be meeting tomorrow 6 pm and the session will be having blog problems.
As well as some detailed discussion about the types of friction so goodbye for the session i am sharing the pdf also which i have not shared yet i am sharing it