 Hello friends and welcome to another session on physics. So guys, we are discussing something called Newton's laws of motion and we have discussed something called fundamental forces that exist in nature. Do you recall? So in the previous slide, we saw fundamental forces, fundamental forces, isn't it? And what all fundamental forces did we discuss guys? We discussed four, one was gravitation, right? Force of gravity. So we're just writing gravity. Then we had electromagnetic force, if you recall, electromagnetic force, isn't it? This is the second type of fundamental force. And third type was nuclear force. And in nuclear force, we had two varieties, nuclear strong and what nuclear weak forces, right? This is what we discussed so far, right? Now, the specialty about all these, what was the peculiar fact about all of these? These are all non-contact forces, non-contact. What do I mean? You don't need to establish a physical contact for the force to influence the object. What do I mean? Again, so let's say there is an earth, this is our earth, and an object is falling under the influence of earth's gravity. So when this happens, there is no necessity of the earth to be in physical contact with the object to pull it towards itself. Understood? So this is what is called a non-contact force. So there is no contact established between the agent of force, that is in this case earth, and the object which it is pulling towards itself. So there is no contact. Similarly, if you have a magnet, let's say if you have a horseshoe magnet, let me draw it. Let's say you have a horseshoe magnet very commonly available. So this is a horseshoe magnet and there is an iron nail. So let's say there is a nail, something like that. And if you place the magnet in closer to the nail, there will be, the nail is going to get attracted towards the magnet. Now again, if you see, you can attract the nail from a distance, right? So there is no contact again. Here again, no contact is there, no contact. So when, you know, these are no contact forces. But there are few forces which you will see, you, there has to be a physical connect for that force to act upon. For example, if there is a book lying on the table, right? So there is a book lying on the table, right? Now if you are studying on a table, you can see there is a book lying on the table. And you want to move this book from this point to let's say here. So in this day, what will you have to do? You'll have to just push this book, you know, and shift it towards the right. For that, you need to bring your hand and establish a connect between your hand and the book and push the book, you know, while you are holding the book, right? So this is an example of contact forces. Now there are many contact forces in nature. So when you are, you know, enjoying a ride on the swing or, you know, when you are moving on a bus, on a car, when you're riding a bicycle, when you're pushing the pedal, all of that is contact. There is a contact established and there will be, you know, the effort is being transferred to the object to a contact. So that's what we are going to study. Okay. Now there could be variety of contact forces. But we are going to restrict only on those forces, which we see commonly around us. What all are they? So we have categorized them as normal. Now, don't think that if there is a normal force, there will be an abnormal force. No, nothing like that. There is a normal force. Why normal? Because it has some connect in terms of mathematics. So in mathematics, normal is nothing but perpendicular. So if you have a line and if you're drawing a perpendicular onto it, then this line is normal on to this line or vice versa, right? So one line is normal to the other. That's how we are drawing this concept from mathematics. So normal means perpendicular. Just remember that. Then there is something called friction. We all know friction is very common phenomena around us. And there's something called viscous force also. So friction in fluids. So if you're talking about friction force in fluids, then we talk about, we don't call it friction as such. We call it viscous force, right? Viscosity. Now, there is something called tension. Now, this is not related to your exam tension, your performance tension and all that. No, nothing like that. So if you have ever seen suspension bridge, right, there, the cables are holding the bridge, you know, so that there is a force which the cables are applying on the bridge and that force is called tension, you know, simply put, if you have a object, you tie it in a rope and suspend it from ceiling, then the force which this string is applying on the object is called tension, right? So we are going to discuss about that. And then finally, there's something called spring force, which is very much in, you know, you also, if you see a stapler, maybe there is a spring in the jotter pen, there is a spring. In your bicycle, there is a spring which absorbs a lot of shock and doesn't, you know, doesn't let you feel uncomfortable while you are riding a bicycle on an uneven surface. So these are the four common forces which we are going to talk about one by one. So let's start with normal. So what's normal force? As I told you, normal force is nothing but, you know, some force which is acting perpendicular to some other line or surface, right? So let's say if there is a surface or there is a line and there's a force perpendicular to it, so that force typically is called normal force. And, you know, it comes into being because there is physical connect between two objects. Let us study a little bit more closely. Let us say there is a book which is lying on the table. So right now when you're studying on a table, the laptop is kept on the table, the book is kept on the table, maybe a water bottle is kept on the table. So the question is, why is the book kept on the table not falling on the ground? So what is stopping it from falling on the ground? So let's say, you know, what is that thing which is stopping it, right? And you can very easily say that the table is stopping it. But then, you know, looking at a very deeper detail, what exactly is happening at the juncture where the book is meeting the table? Okay, so let's focus on this area. Let's focus on the contact surface between the book and the table. So what's happening there? If you see, if you consider the lower surface of the book, let us call this as book B. And this is tabletop surface T. So I have just separated them, you know, so just to explain what is happening over there. So if you see, the tabletop pushes the book upwards, and the book is pushing the table downwards, right? So these are the two forces which are acting here at this and this interface. What is happening? So this force is the force applied by table, table, by table, on book, on book. And the other one is vice versa, that is the book is applying the force on the table. So these are the nomenclature. Now, the normal force applied by the table on the book doesn't let the book fall. So basically, if you consider only the book, let us say this is the book, okay? Let us say this is the book. What all forces are acting on it anyway? So one is the force of gravity. So there will be some weight of this book, which is pulling it downwards, but then the book is not falling down. Why? Because the table is countering it by what? So the table is applying a force upwards, right? This is nBT as I have shown. So n stands for normal and BT is subscript for book and table. So the normal force applied on book by table, right? That's what is the nomenclature. Did you understand? So basically, if you see the two forces here are countering each other and are not letting the book fall, right? If you remove this table, the moment you remove this table, what will happen? You are removing the normal. And the moment you remove the table, then this book is going to fall down, isn't it? This is what is normal. So what are the points to be noted? Please remember these, whenever two bodies come in contact, they apply normal force on each other, right? So if there are two, you know, two hands which are coming in contact, a book on the table, a cycle on the road, a car on the road, or a plate on the dining table, whatever. So whenever there will be any contact, it may not be a vertical contact. It can be horizontal contact when you are pushing against, pushing someone against the wall, or you are hammering anything, a nail, let's say. So whenever two things come in contact, there always be a normal force. And the other attribute is there will be two normal forces because there are two interface or two objects which are coming in contact, let's say, I am just showing this, this is one surface, this is another surface which are coming in contact. So one force is pushing the, let's say one force is applying a normal like that. And the other surface will be applying a force like that, right? So there will be two normal forces. And both will be equal in magnitude, but opposite in direction. And the two forces are being acted upon on two different objects. Now the normal force, and typically we only use normal, we don't attach the word force with the word normal. So we say the normal acts in a direction perpendicular to the surface in contact, right? So wherever there is a surface in contact, then the normal is going to act perpendicular to it. Always remember, this is always true. Now you can say what about two rough surfaces? Let's say there is one surface like that. And another one is something like that. And they are in contact with each other. So let's say this is one contact surface. And this is another contact surface. So what happens? So at each location, there will be one normal, right? So there will be a very fine or very microscopic view if you take, then you will see that there is a tangent. And on that surface, there will be normal acting like that on both sides, right? So wherever is contact being established. So let's say there is this contact. So at this location, if you magnify it, let's say the two surface will appear like that. And hence the forces will be acting perpendicular to that surface at that small, tiny location. Always perpendicular. So hence, if you are multiple points of contact happening, there will be different angles in which the normal will be acting. But then always this normal would be perpendicular to the surface in contact, okay? It will become much clearer in the next slide. Let's go to the third point. The electrostatic force between the nucleus and the electrons at the microscopic level gives rise to the normal force. So what does it mean? Let me explain. So let's say, let's take two surfaces and I am drawing the microscopic view of the two surface. So let's say our surface is something like that. One surface is this and this is appearing rough because any surface at a microscopic level will be something like that, okay? And let's say we have another surface here and the surface is something like that, okay? And it is this surface, the other surface is like that, okay? Now, when two surfaces are in contact, so there are few contact places. What all? Here, one, here, here, and here. And I am just giving you random examples. There will be infinitely many point of contact, but then just for the sake of simplicity, we are doing that. And these are empty spaces. What are empty spaces, guys? So there is air, let's say. This is air. They are not in contact. So this is air, right? So this is the practical scenario. Now, what happens is, this is such a close contact that what will happen is the electrons will be exchanged at this location, okay? And these locations, electrons are going to be exchanged. What does it mean? So electron from one object is going to jump on to the other, right? And whenever such thing happens, so let's say if one atom, the electrons are jumping from this atom on one object to another atom on the other object, understood? So let's say this is the interface which you are talking about, the contact. But they are so close that electrons revolving here jumps to this point, jumps to this point. So what will happen? There will be a positive charge here and a negative charge here, is it? So that the moment electron shifts from one object to the other, so it will leave the parent object positively charged. Why? Because electrons are negatively charged, they will go on to the other side, make the other side negatively charged. So hence, what will happen? There is a concentration of negative side, negative charge here, there is positive charge here. And now, we had learned in the previous session that if there is a positive and negative charge, they will attract each other, is it? So what happens? There is some kind of attraction at this location is happening. Similarly, some kind of attraction at this location, some kind of attraction at this location, some kind of attraction at this level. Now, these are typically or you can say in chemistry, we say these are some temporary bonds which are getting formed and the force acts in along the perpendicular to the surface. That is how the forces will appear, like that, like that at different, different location. Let me use the color, different color here to show you. So let us say this is the force direction here, this is the force direction, let us say here, let us say this is the force direction here, this is the force direction like that, it will randomly act there. Now, if you see, if you can understand by this particular thing that there will be, if you add all these forces or add all of the vectorially, there will be one component which is perpendicular to the surface as a whole and the other component is going to be along the plane of contact. So this perpendicular force, this perpendicular force is called normal. So where is this coming from? So basically, I was trying to explain this that if there is a charge separation because of nearness of two objects, so the electrons jump from one to the other, there is a temporary, you know, very weak, but there is some force of attraction at those local points and that only leads to something called normal force. So if you understand vectors, then you know that the resultant of all these forces, all these which I have drawn here, the resultant of all of them can be shown as two force, one towards the perpendicular of the plane and another one along the plane. So this perpendicular plane thing is called normal and you will later know that this along the surface thing of force is called friction. So these are the, this is the reason, basically electrostatic force of attraction is the reason for normal and friction. I hope you understood this part. Now let us talk a little bit more about types of contact and it will become much clearer to you. So you can see there are two spheres here which are in contact just like a billiard or a pool table, if you see all these spheres are in contact with each other. So this is called spherical point contact. So two spheres, if you place them side by side, then they touch exactly at one point here and here is where they will be, you know, applying normal onto each other. So you can see this is the normal applied to the left one by the right one and the normal here is by the left one on the right sphere. Now so these are the normal forces. Again, you can see they are perpendicular to the surface here. Other thing is called line contact. Let us say there is a roller, there are two cylinders. So two cylinders, you know, if you see here, they touch along one line. So there is one line, let me, so it is not visible from this angle, let us say, but I will try to show you. So this line here, they would be in contact with each other, isn't it? So this is line contact. Similarly, a cylinder and a plane will have a line contact just like a roller, which is now moving on the road. So it is always having a line contact and there could be a surface contact. For example, two books, here you can see there is, there are two surfaces, there are surfaces in contact here. So these are called surface contact. So what all type of contacts we can have, point contact, line contact, surface contact everywhere. You can see this is also perpendicular to the line of contact. This is perpendicular, this normal is perpendicular to surface of contact. So wherever normal is applied, it will be perpendicular to the surface is contact. Is that okay? Surface is contact, surface in contact, line contact or point contact. So this is about normal. So what is the, what do we, what did we learn here? We learned that normal forces are one type of contact forces. So contact, so they are contact forces. And we also learned that normal forces act perpendicular to, perpendicular to surface in contact. And we also learned that normal forces, what is the genesis? The reason is electro, electrostatic force is the reason for normal force to exist and different types of contacts we saw. So let's talk about friction in the next session.