 check out this cool demo all you need is a container some kind of a glass mug or steel mug a playing card on top of it i'm using my uno card and on top of that keep a coin and now we just flick that card and see what happens you can try this at home boom the card flicked away went away but the coin stayed and and fell into the mug why why didn't that coin also flick away to answer this question we need to understand something called inertia so let's do that what is inertia well inertia is an object's resistance to change its state of rest or motion what does that mean well let's take an example take a train which is at rest now if i go and push it will it move of course not why you would say it's extremely heavy well in physics we would say that it has a lot of inertia because of which it's resisting to change its state of rest see to change its state of rest you need to put a lot of force on it that means it has a lot of inertia but it's not just about rest now consider a moving train okay very fast moving train now can i go and make it stop again you'd say no ways no ways i can go and make it stop why you might say it's coming with a lot of speed but even if it was not coming with a lot of speed even if it was coming very slowly do you think i would be able to stop it no i wouldn't be able to stop it why because it has a lot of inertia again you can see because of its inertia it resists to change its state of motion this time so can you see inertia is basically the ability to you know stay at rest or to continue in motion basically the first law the first law is also called newton's first law is also called this law of inertia that's why so it's this tendency that objects have because of which it'll say no i will not change my state if an object is at rest because of its inertia it'll stay at rest it'll continue to stay at rest if an object is in motion because of its inertia it'll continue to stay in motion on the other hand let's look at a bicycle can if i go and push that bicycle will it start moving yeah of course and similarly if a bicycle is moving can i go and stop it yes very easily what does this mean does it mean it doesn't have inertia of course it does it does have inertia and an ant can't stop it right it does have inertia but its inertia is much smaller compared to the trains inertia so you can see we would say this has more inertia because it has more ability to resist changes and the bicycle has much smaller inertia much less inertia compared to the trains now why is that why do these trains have more inertia the bicycles have less inertia well you've guessed it they have more mass and this has less mass so you can see inertia is directly related to mass something has more mass will have a lot more inertia and something that has less mass has a much smaller inertia it's much easier to change its state of rest or motion now i'll show you an example of this a consequence of this that you can you might feel almost on a daily basis when you go in a train or a bus let's say you're standing in a bus and the bus suddenly starts what happens you fall backwards right or you feel a force that's like somebody's pushing you back but nobody's pushing you back why do you do why do you almost suddenly feel like you're being pushed backwards it's because of your inertia how does it work well let's say you're standing on the bus the bus is at rest let's say the bus suddenly starts moving when the bus suddenly starts moving the the floor pulls on your the friction between the floor and your feet starts pulling on your feet and as a result your feet starts accelerating forward but as of now there is no force acting on the rest of your body therefore the top part of your body will try to stay where it is and now you can see what's going to happen the bottom part of your body will start accelerating because there are forces acting on it the top top part of the body will try to stay as it is that's the reason why you feel like somebody is pushing you backward that's why you feel you fall backwards but notice it's not that somebody is pushing you back but the floor the friction is pulling your legs forward and that's why you fall back similarly now consider a case where you're in a moving bus what happens when the bus breaks suddenly you fall forward can you now pause the video and understand why that happens again it has something to do with inertia pause and try okay so here you have the bus which is moving you are also moving along with the bus so you're moving you're moving you're moving and suddenly the bus stops what happens well the bus has decelerated and stopped and when the bus stops you know there's frictional force that will pull back on your moving legs and makes your legs also stop but as of now there is no force acting on the top part of your of of your body therefore because of its inertia it'll continue to stay in motion right and therefore the top part of the body will keep going forward and that's the reason why you will fall again it's not that somebody is pushing you forward but this time notice the friction is pulling your legs backward and that's why you fall forward so this is again a beautiful example of inertia now let's see if we can use the idea of inertia to explain what we saw earlier so let's look at the same thing in a slow motion see what happened when i flick the card i put a force on that card and we learned that when you put a force on something it'll accelerate so it accelerated forward okay but what about the coin i didn't put the force on the coin there was some frictional force between the coin and the card but that card moved so quickly away that friction didn't have enough time that it didn't have enough time to put a force on that coin and so the coin was able to resist its changes in motion changes resist the change in the state its state of rest so it resisted it and it continued to stay at rest and that's why look the coin stays at rest for a small it does move a little bit because there is some force but it stayed at rest and therefore the coin did not move because of its inertia and of course once the card was gone now gravity starts pulling down on it and because of gravity it starts accelerating down and therefore it falls into the mug so there you go beautiful isn't it so remember inertia is an ability of an object to resist changes in its state of rest or motion and more mass more inertia