 So, in the previous session we understood normal force and we know that normal force is one type of contact force. Now, the another type of contact force is friction. It is very common term and we come across friction on a day to day basis. The wear and tear, the writing which you are doing on the paper right now or your soul of the shoe is going you know or getting torn or let us say when you are rubbing your hand against a wall then it gets rubbed up. So, you know whenever you talk about friction quite often. So, what is friction force is what is the point of discussion today. So, friction is a type of contact force which acts between two surfaces in contact right. So, you cannot imagine friction between two surfaces which are not in contact. For example, you know a wall on one side and the wall on the other side of your room if they are not in contact they are not rubbing against each other there is no friction. So, if you have two you know two hands in air separated there will not be friction. So, when you rub the hands against each other you feel that heat. So, hence only when the two palms come in contact when you apply some force and then the friction resist your movement of the palm and because of that resistance some heat is generated and you you know feel that warmth right. So, first thing is when two objects are in contact only then you can expect friction to be there. Now, it acts always along the surfaces in contact. So, we are going to discuss all of them one by one, but always remember friction is friction will be applied when two surfaces in contact acts along the surface and not that just because two objects are in contact they will experience friction in between them. No. So, for example, a book lying idle on a horizontal table will not experience any friction whatsoever. So, another criteria for the friction to exist is it must or it will be there only when the two surfaces are in contact or and then secondly it they must have a tendency to move relative to each other. What is related to each other? So, let us say if you are having a table of sorts table is there and there is something some object over there so the object must move with respect to the table that means someone sitting on the table must observe that the object is moving that is called relative motion you studied in the previous chapter motion. So, only when there is a relative motion between two surfaces in contact they will feel friction or they will experience friction. So, either they are moving relatively to each other or they have tendency of relative motion. So, what does tendency of relative motion mean? So, tendency of relative motion is that let us say you would you would see a ramp and there is a book kept over there and this is at an angle let us say theta with horizontal ok. So, there is a ramp inclined plane ok. So, there is an inclined plane on which something some book is kept and it is not slipping or it is not sliding down the ramp it is static over there. So, it does not mean that there is no friction in between the two objects just because they are stationary there is a great tendency of this book to go down like this. So, hence this is called tendency of relative motion. So, it is not moving but it given a chance it will start moving just like that. So, that is called tendency of relative motion. So, wherever there is motion relative motion or there is tendency of relative motion friction will appear over there. Now, it depends on the roughness of the surfaces in contact higher the roughness higher is the friction you know that right. So, you would have seen carpenters using sandpaper. So, different sandpapers are there with different coarseness. So, hence let us say more the rough surface harder friction it will be applying on the surface on which the sandpaper is applied ok. Now, it also depends on the normal force acting between the surfaces right. So, higher the normal force between the surface higher will be the friction what does it mean? So, this is the inclined plane this is the book over there we just learned that whenever there will be two surfaces in contact there will be forces. So, there is a normal force acting on the book by the let us say inclined plane then the friction acting here let us say this is the friction acting here this will be directly proportional to the normal force. So, higher the normal higher will be the friction ok. Now it is responsible for wear and tear of surfaces in contact and friction can support or oppose the motion both it is not necessary that friction always will oppose you know always oppose the motion. The only thing is friction always opposes a relative motion. So, you have to please you have to you know understand this very very subtly right. So, friction does not allow a does not resist all the motion in fact there will be many cases where friction actually supports the motion for example simple walking. So, we will show you through this diagram so you are seeing you know people are moving you know and hence what is happening is this toe here is trying to push the ground right it is trying to slip over ground like this. So, the movement of the leg is like that is it not you are trying to push the ground backward. So, hence what is happening here there is some kind of rubbing going on between the foot sole and the and the ground. So, the sole is moving in this direction with respect to the ground if you see this is let us say the ground. So, the sole is trying to move in this direction. So, what will happen the ground will try to oppose this relative motion right. So, friction always oppose this oppose this relative motion. So, hence it the force will be applied like this. So, ground is applying this force this is the force this is direction of force applied by ground on to your foot and hence my dear friends you are able to walk. So, you would see that on a slippery road when there is no friction then what will happen then what will happen when you will keep trying to push the ground or you know the sole is rubbing against the ground but then since there is no friction. So, your foot is not getting enough force back to let you move. So, hence in the slippery road you will fall down right. So, that is a common experience. Now, if you see friction is acting between the bottom surface of the block and the floor. So, in this case if you consider this block this block when you are trying to push right. So, when you are trying to push the bottom surface of the block is trying to move in this direction right. So, the ground underneath which is there the ground underneath which is there will try to oppose this motion of the upper block and hence the lower surface of the block which I am showing like this like this is going to experience a force in this direction and hence you would notice when you try to push a heavy trunk it becomes very difficult to move it why because there is tremendous amount of friction which is trying to resist the motion of the block towards the right. Now, let us understand few more things here. So, you can see you know this is where the boy is slipping down the ride right or this inclined plane. So, there is actual motion happening. So, the boy would have started slipping from here and it is and he is coming down like that. So, actual slipping is happening. So, there is actual relative motion happening. So, friction is trying to resist this boy from moving downwards. So, this is where we there is actual relative motion taking place. Let us take a case where motion is not there, but then there is a tendency of motion. So, you can see lots of water droplets sticking on to the glass pane. So, you would have seen that in a rainy season. So, what is happening here? If you see the all the drops have a tendency to move down and hence you can see the shape getting created accordingly right. So, they have a tendency to move down, but they are sticking on to that surface. So, what is stopping them from moving down or sliding down the surface? The friction force is there which is holding them from moving down. So, hence there is a great tendency of the tendency of the water droplets to move down, but the friction between them and the surface of the glass is holding the water drops from falling. And hence you see if you know if by any means the weight of the drop is more or the drop is of bigger size then even friction will not be able to hold it back and it will start dropping down. So, hence this is the case of tendency to move right. So, there is no movement actual movement, but there is tendency to movement here also friction will act. Now, let us go to the next top point and this is let us try this out with you. So, this is the question for all of you. If you are asked to slide over these surfaces, which one would you prefer and why? So, these are the three surfaces guys, this is one, second, third. Now, you decide which one you would prefer to slide on. Obviously, I know most of you would never go for these or these you will pick up this if at all you have to, it is made compulsory to slide on one of them. So, why is that? Because you can see this is highest roughness right. So, you can see the gravel which is there will give you tremendous amount of roughness and hence tremendous upon tremendous amount of friction. So, this is relative quantity. So, basically if you see this is also this will also give you some resistance because this is also rough. But as compared to option one and option two, this you can think of would be slightly better. Because the roughness here will be considerably lesser than that of a gravel filled road, is it not? And this my friend will be the highest or if I have to coin a term called smoothness coefficient then this will be the most smooth surface and you will pick this up you know if at all you have to slide on these right. So, what do we suggest from or what do we infer from this? We understand that higher the surface roughness higher will be the friction ok. Now next is it also depends on normal force. So, how why is it difficult to slide a heavier object on a rough surface as compared to a lighter object? So, let us take these examples right. So, what is this? So, you can see there is a huge this is the competition you know there is a person who is trying to pull you know air force aircraft and you can see the effort this guy is putting. So, why is it so difficult to push this object and this is relatively easier to pull is it not? This cart is easier to pull simply because the normal force which the ground is applying onto the onto the objects here is much higher in case of this aeroplane as compared to this cart. Why? Because you will later understand that normal force depends on how much is the weight also in these cases right. So, higher the weight higher will be the normal higher will be the normal higher will be the friction force. So, this guy is experiencing he has to apply tremendous amount of force as compared to this person. So, why? Because the friction offered by the ground to this object this aeroplane here is much, much higher much, much higher than in this case ok. So, hence again higher the normal higher will be the friction ok. So, understand this part as well now. So, responsible for wear and tear you all of you know so, you can see a tire surface totally worn out and you have to replace these tire every you know at a particular frequency you know that you know if not replace then it can lead to many accidents. So, these threads which are given here can you see this type of design is made on the tires to make it rough. So, that you know the contact surface between the tires and the road have, surfaces have significant roughness and because of that there will be significant friction and hence the vehicle will not skid. So, but then over a period of time it will get worn out because of that friction. Similarly here so, the sole of the shoe you have to replace many a times there will be some wear and tear some crack marks on the sole why because after continuous uses the friction will damage the sole right. These again these design are provided to make sure that enough roughness is there on the surface of the sole so that while walking you do not slip over. And this is something called you know this is brake pads so, you can see this is a brake pad used in cars to you know to up you know to whenever you want to stop the vehicle then you apply brakes right. So, what happens there again we use the concept of friction to you know where the brake pads rub against the wheel and because of that rubbing there will be a relative motion between the two and the brake pads will oppose the motion of the wheel and hence thereby stopping the vehicle right. So, but again after continuous use this gets worn out ok so, this is what we discussed about friction. So, what did we discuss again friction is a contact force it acts only when there is a relative motion between the two surfaces it depends on the surface roughness it depends on the normal acting between the two surfaces and it is responsible for wear and tear and friction does not always oppose motion it yes definitely oppose relative motion, but it actually also facilitate some motion right relative motion it will always oppose. But the motion otherwise there are cases where friction has facilitated that for we saw that example where walking is facilitated because of that friction only and similarly the vehicles move on the on the road because of friction only. So, these are these are the cases where friction actually facilitates motion, but if you see the relative motion of the tire or the sole with respect to the ground it is always oppose and because of that phenomena only we are able to walk or we are able to drive a vehicle right. So, we will discuss all that in a separate topic under friction because we have to also study something called coefficient of friction we have to also study something called variety of friction. So, there is something called static friction, kinetic friction and to an extent we will have to also study rolling friction. So, all these are different types of frictions which are there which we will have to study that will take up in a separate session.