if i were to ask you how do you keep things in motion what would you say you might say we have to keep pushing or pulling on that object a push or a pull is called a force so in other words we need to keep applying a force on an object and if i asked you why do you think so you might say this is from your daily experience for example this chair is not moving right now if i have to make it move i have to keep on pulling on it or i have to keep pushing on it if i stop pushing or pulling on it look it stops moving and there you go a force is needed to keep this chair in motion without a force this chair naturally comes to a stop right well what if i told you that this was wrong would you believe me i'm guessing no because you just proved it with an experiment and so i guess the goal of this video is to convince you that this is wrong or at least to make you rethink about this statement and we'll do that by using the example of carom suppose we want to make this striker move on this carom board what should we do well again you might say we need to keep pushing it you need to push it to move it but let's say instead of pushing it gently we give it a strike you know what happens the striker will move some distance and it will come to a stop let's look at that again this time in slow motion what we see is when our finger touches that striker it pushes that striker making it move but as the striker loses contact from my finger i stop pushing it there's no longer a push anymore and what we are seeing after that is that the striker slows down slows down and eventually comes to a stop and at this point you might say ha i told you so you have to keep pushing it in order to keep it moving if you stop pushing it then it will come to a stop things have a natural tendency to come to a stop okay but let's do something let's add some power to the surface and repeat the experiment again you might know what happens in this case right now if i strike with pretty much the same force as before it goes much further before coming to a stop and if i ask why travel farther this time you might say well because we added powder the surface became smoother and things slide farther on smooth surfaces but what if i asked you why why do things slide further on smooth surfaces i mean if the natural tendency of an object is to come to rest why does that depend on how smooth the surface is huh think about that why does the surface matter this is where a man named galileo galilei came up with a crazy idea he thought that maybe this piece is not stopping because of its natural state he thought maybe this rough surface is forcing it to stop okay here's what i mean if we go back to before adding the powder once i strike this blue piece it gets it sets in motion and now galileo is thinking maybe the surface itself starts pushing this blue piece in the opposite direction opposing its motion and maybe it's this force that slows it down and eventually makes it stop this is just like how when there is an uncontrollable train moving superman comes in pushes the train in the opposite direction slows it down makes it stop and saves the day similarly galileo thought it is this force that's opposing its motion and makes it stop and you might even know the name of this force today we call it friction and if we understand how friction works maybe we can explain this entire scenario so let's quickly look at how friction works to figure out friction we need to look carefully at the surface where the striker meets the board you see although these surfaces look very smooth to our eyes at the microscopic levels they aren't smooth at all so if you could zoom in over here the surfaces might look somewhat like this these mountains and valleys are just too small for our eyes to make out and therefore it looks smooth to us but as this striker moves on the board notice because of the unevenness it causes obstruction and it is this obstruction which we call friction okay but what happens when we add powder you see powder particles are so small they can fill in these gaps and smoothen it out ah now because the surfaces are much smoother than before the striker can slide with much lesser obstruction and that means friction reduces so if we come back to our board according to galileo when i strike this coin it is friction that is opposing the motion and stopping it since friction is the culprit for stopping this coin when we add powder to the surface the surface gets smoother and it's the friction that decreases that's the change that's the effect of smoothing the surfaces and since the friction decreases the opposing force decreases this means now it is harder to stop the coin and as a result the coin travels farther before coming to a stop so this means logically as i make the surface smoother and smoother our coin will travel farther and farther before stopping given that our cutting board is say big enough and now this is the big moment this is the big big galileo moment what if we made this surface perfectly smooth what happens then imagine we somehow made this board perfectly smooth and gave it a tap what happens now well now that our board is perfectly smooth that coin will never stop it will keep moving forever of course provided that our board is super super big and now galio would look at this and say look look things in motion stay in motion and so to keep things in motion you don't need to push or pull on them they have a natural tendency to stay in motion and why don't we see this in our daily life because of friction because friction always acts in the opposite direction of the motion and makes it stop so it's the friction that's the culprit that makes everything stop even the chair example that we saw earlier same case if there was no friction and if you stop pushing on it that chair would keep moving forever but it's the friction that opposes things and makes everything come to a stop so the natural state of moving things is to keep moving now you may want to defend this and you might say wait wait wait wait wait but initially the striker was at rest and you had to put a force on it to start making it move right so that way force is needed to start motion yeah but now i can also say a force is needed to stop the motion and the only reason why things around us are at always most of the times are at rest is because of friction because friction opposes motion and makes them come to a stop in the absence of friction there is no reason for things to be at rest things might as well be in motion and so the natural state of things in the absence of forces is either rest or in motion and if you find this hard to digest and if you find this a little confusing then you are not alone my friend because humanity took thousands of years to figure this out so please take your time and i'll tell you what what helps me is to think about celestial things like the planets or the stars or the galaxies they are in perpetual motion yes their motion is a little complicated because there are forces acting on them but they are always moving why don't they stop well because things in motion tend to stay in motion and a final question that we might have is what does a force do i mean we just saw a force doesn't keep things in motion then what does it do well like we saw a force can start motion in other words if force can speed up things and like in the case of friction force can also slow down things and stop the motion turns out that force can also change direction of motion but don't worry too much about that so in general we can say force can accelerate things that's right in the absence of forces if objects are moving they will move with a constant speed in a straight line but if you want to accelerate that body then you need to put a force on them and so what did we learn in this video we saw that a man named galileo looked at simple experiments well not karen maybe but other things he looked at them carefully and came up with a revolutionary idea that things in motion have a natural tendency to stay in motion you don't need to keep pushing or pulling on them to keep them in motion and what does a push or pull do what does a force do well a force in general speeds up or slows down things or in general it accelerates things you