 Hello guys and welcome to another session on laws of motion. Today we are going to take up a very important topic called inertia. Now inertia becomes the precursor for whatever we are going to study in Newton's laws. So it is very important to understand this particular concept. So let us begin. What does inertia mean? So you would have observed this that during winter season, you know, you don't really want to get up and your mom comes and wakes you up, the alarm goes off, but still you do not want to really leave the bed and come out and get ready for school. This is where you are showing a resistance to change or resistance to wake up in this particular case. Basically guys, you are displaying inertia. So you are displaying inertia. So you can now you would have figured out by now that something where you are resisting to change that that indicates inertia. So let's do an activity, which is a very famous one, you can do it just now. So you can get a glass or tumbler and put a card over it like this, right. And I have put a coin on top of the card. Now what you're supposed to do is you have to flick the card with force. So you can see a screen grab where the card is in air and just at that instant the card has been just displaced and the coin is flying in the air, right. Wherever the coin was, it's staying over there. Next, as the card moves away, the coin now starts falling down and eventually the coin falls down. So you can repeat this experiment for activity very easily. You might have done it already. So you can go and grab a glass tumbler from your kitchen, take a card. It could be some stiff card. For example, I have used a gaming coupon card here and you can use the old credit card of your parents or you know, but yes, whenever you're using it, do take a permission from them, then put a coin over it and then repeat this. So yeah, so basically what you're supposed to do is as I will explain once again, you have to put a coin over the card and then flick the card with reasonable force so that the coin falls down, right. So you'll observe that the coin doesn't move with the card instead it falls down in the glass below. Why does this coin not go along with the card? Because of inertia of the coin. The coin did not want to move from where it was, right, and hence it tends to stay wherever it was, but now the card has been removed from the, you know, from the low side and hence there is no support for the coin and hence it falls down. Okay, now let's say some more examples from day to day life so you can see whenever you shake a, you know, fruits and leaves fall, right, sacred tree. So let me show you this video where, you know, a machine is shaking the tree and it's basically some berries are being collected so this is how you do it and then a fast bowler in a cricket match takes a long run up. Why? Because he uses his inertia so he bowls like this, he runs and then the motion which he use it or in which he is helps him speed up or let's say throw the ball at a higher speed so he uses his speed and, you know, and the ball is with him, it is in the bowler's hand and hence the ball is also moving with a great speed so once he releases the ball it carries that inertia, right, so the inertia of motion and hence it goes with a very high speed to the batsman, right, this you would have observed. Another example is when, you know, you would have seen when you go for spreading the clothes for, you know, drying after washing typically we give a jerk to the clothes why we want to, you know, shed all the moisture. So why does it get, you know, the moisture gets driven away because when you jerk the cloth what happens is the moisture particles tend to stay wherever, wherever they were and since the cloth has moved out from its place the moisture content or the motor molecules stay wherever they were and hence they come out of the cloth, right. So the answer to all the above questions somewhere is linked to inertia so everyone is trying to resist any change, you know, so someone which is at rest is trying to be at rest, someone which is moving for example in this case the cricketer and the ball which is holding if it is moving at a particular direction at a particular speed it tends to stay like that only so that is some examples from day-to-day life. Now let us define inertia formally. So what is inertia guys? Inertia comes from a Latin word inners so you can see this is the word inners which means idle or sluggish, okay. Next let us go to the, you know, the definition which Newton gave in his book and what he defined inertia as so he defined inertia in this book philosophy naturalist principia matematica which states that the Inesita or Incita or innate force so he is not using the word inertia he is using the word innate force now he is using in Latin but it means innate force of matter is a power of resisting, right. So the very power through which it resists what? Resist what? Resisting by which everybody as much as it in in it lies endeavors to preserve its present state so everyone wants to stay as they are they have been right so no one wants to change its state so they want to continue with whatever state they were whether it be of rest that means something which is at rest tends to continue at rest this is the observation made by Newton and something which is moving uniformly forward in a straight line will tend to move that tend to do that continuously, right. So neither the body lying at rest would like to change its state of being at rest or a body which is uniformly moving in a straight line wants to change its state of motion, okay. Now that means this is what is the definition of inertia so hence what is inertia in a nutshell inertia is resistance to change but change of what? Change of state, state of what? Change of state of uniform motion so whenever now you can define rest also a special case of uniform motion whether body is totally at rest no movement at all and the other case of uniform motion is like when the body is moving in a straight line without much of a deviation moving continuously in straight line with the same speed that is uniform motion so hence Newton is suggesting inertia is that property by virtue of which any object which is at rest tends to continue at rest and any object which is at uniform motion tends to continue its state of uniform motion, right. So now next is if you look at it there is more information around inertia which if you look at it it says inertia was you know first introduced by whom? Johannes Kepler, right. So you can see that here inertia was just so inertia was introduced by Johannes Kepler the famous astronomer, isn't it? So he introduced the word inertia in his this book Epitome Astronomy Copper Nikkei, right. So I am really sorry if I am not able to pronounce it properly but this is what the book name is. However the meaning of Kepler's term the meaning of Kepler's term what did he mean? He meant that from the and you know this word is derived from the Latin word for idleness or laziness it is mentioned here the meaning was not quite the same as its modern interpretation. So right whatever we use for let us say explaining Newton's laws and Copper Nikkei's sorry Johannes Kepler gave this term but then he did not mean whatever you know the current meaning of the word is. Okay so now let us take some more examples. So what is inertia example? So in there are inertia of rest, you can see inertia of uniform speed and inertia of direction of motion, right. The three types of inertia we are going to discuss. So what is inertia of rest guys? So a body at rest resists anything which tries to set it in motion, right. So something some stone is lying somewhere in you know it will not move on its own. So hence in the coin over card activity when the card moves away the coin tends to continue at rest and hence falls in the tumbler because the card is not there underneath so hence the coin just falls down but the coin doesn't want to move from its place. If the card was there it would have stayed there isn't it. Now in inertia of uniform speed is a body moving at constant speed, right resists anything which tries to change its speed. For example if you have a you know a train moving at a very high speed right at let us say 100 kilometers per hour here you can see 100 kilometers per hour and it takes a huge breaking force to bring it to a halt isn't it. So you would have seen you know and it in the train the train actually holds after you know covering after a significant amount of distance. So hence this is the train doesn't want to change its inertia, right or state of speed constant speed. Similarly something is moving in one particular direction doesn't want to change its direction of motion, right. So you can see on a slippery road while negotiating a bend you would have experienced this your bicycle tends to move in a straight line and hence kids right. So let's say if you want to take a turn or in a circular path you want to go and you're moving at a very high speed typically on a very slippery road you will see that you know you have to decrease the speed from first of all to take a turn that will learn anyways in circular motion but you know typically if you don't really break you tend to go in the same direction yeah. So that's your inertia of direction you something is required to change the direction and until and unless that thing is available the bicycle will continue going in a straight line it will not change its direction right. So these are typical example of inertia now. So the thing is how do we measure inertia of any object now? So you will see inertia of an object is measured in terms of its mass. So that's now mass you would have learned by now that mass was nothing but the amount of matter continued but now mass is defined as how much inertia some object or any object is. So that is directly proportional to the mass of the object. So higher the mass higher the inertia higher the mass higher the inertia you can see here there's a man who is trying to push this rock huge mountain rock and obviously the mass is so high that it demonstrates a huge resistance to change its position. Obviously you know that entire effort of this man is going to go down the drain he will not be able to move that alone right. So because of its heavier mass while at the same time a small bubble soap bubble is blown away by just you know a small you know blow of air by even a child right. So it's so it's so light they are so light that you know even the blow of air from child pushes them away from or let's say set them in motion right or change their state. So that's what the difference is right. So heavier the mass heavier the object and hence you can now infer also that it takes humongous amount of effort to move or change the state of any object with higher mass whether it is a plane lifting of train stopping or a mountain being moved right. So hence higher the mass higher will be the inertia. So this is quite obvious now the rock with more mass has more inertia than a soap bubble. So this concept of inertia is directly related to Newton's first law of motion which we are going to see in the subsequent video. So once the concept of inertia is clear you will be able to appreciate Newton's first law of motion and hence we can build on our concepts of dynamics. So I hope you understood the concept of inertia. There are lots of lots of day-to-day events where inertia is involved. I would urge you all to you know go and see and observe in wherein you find something displaying the property or let's say its inertia to change its state of motion right that would be a good activity to do. So see you again in the next session next video where we will take up Newton's first law of motion. The journey is going to be exciting friends. So be there. See you in the next video. Bye bye.