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Course: Class 11 Physics (India)>Unit 13

Lesson 1: Stress, strain, and modulus of elasticity

Elastic and non elastic materials

Is steel more elastic than rubber? We all have some intuition for elastic and non-elastic materials. But how do we define elasticity? How do we say whether something is more or less elastic? In this video, we will explore the true meaning of elasticity. Created by Mahesh Shenoy.

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• When we take steel wire and rubber wire, and if we apply force on the wires [with our hands], won't the steel wire be permanently deformed [won't the steel wire be bent if we apply force on both the sides?]
Similarly with rubber, the rubber wire would snap back to its original shape
(7 votes)
• Maybe the difference is that the force you apply to do that(bend a steel wire by applying a force on both sides)is greater that that required to do the same with the rubber band. And that is what elasticity essentially is.
(4 votes)
• Do perfectly elastic or non elastic bodies exist?
(3 votes)
• I am not sure about perfectly non elastic but there are no perfectly elastic as in this video also he explained EVERY ELASTIC BODY HAS A LIMIT beyond that it will permanently deform
and Non Elastic bodies are elastic bodies whose youngs modulus is almost zero or negligible
(7 votes)
• why is it that while both spoons and knives are made of steel when we bend a spoon it deforms permanently while bending knives deforms it only temporarily

I think it has to do with the type of steel
can someone explain
(1 vote)
• Well, I think if you apply the same amount of force to both of these things (with equal force), they'll deform.
(1 vote)
• Are clay perfectly non elastic?
(0 votes)
• Yeah, they are almost perfectly plastic. Since they have a very low elastic limit(even for a very small deforming force, they do not tend or have the ability to regain their shape).
(2 votes)
• He explained springs with springs
(0 votes)

Video transcript

let's talk about elasticity elasticity now we may already have some some idea about what it is but in this video we will try to concretize the exact meaning of this particular term all right so let's start with some examples we all know what elastic materials are and what non elastic materials are for example we may have played with say rubber bands or Springs so they are elastic in nature so let's write that down let's write down examples of elastic materials so elastic we know that things like a rubber band so rubber or maybe a spring so let's say spring these are elastic in nature and things which are not elastic well you probably know them as well for example let's say a clay clay is pretty non elastic mud these are some of the examples of non elastic materials these are non elastic non elastic and now the big question is what is the meaning of elasticity I want you to think about this why do we call these things as elastic and these things as non elastic well the main idea is that if you take a rubber band let's say and if you stretch it you deform it and then if you let go of the d-forming force then it snaps back to its original shape same is the case with the spring however if you take say something like clay or mud and if you were to deform that by putting some force and then you remove that d-forming force well it doesn't snap back to its original shape its shape has changed permanently and that's what allows us to make beautiful dolls like this using clay so we could now say elasticity is a property is a property due to which when you remove a d-forming force so you remove the d-forming force the material regains its original shape so regains original shape so rubber bands and springs do that clays and mud balls don't do that perfect but here's the big question what about something like steel where do you put that is that elastic or is it non elastic think about this look at the definition of elasticity and I want you to pause the video think about this for a while where would you place steel all right so let's start with the common answer the most common answer for this is that steel is not elastic and if I ask why then one may say well that's because well you can't be from steel right a steel is rigid body doesn't undergo deformation now does it well the answer is it doesn't ago deformation you can definitely say bend spoons right oh then you could say oh okay the reason why it's not elastic is because if you bend steel it doesn't snap back to its original shape that's why it's not elastic well turns out not really and in fact you can test this at home so take a knife which is made of steel and then you bend it a little bit and notice it snaps back try it one more time ah it snaps back so guess what right in your kitchen you can just convince yourself that steel is in fact elastic another thing to note would be is that most of the springs that we may have played it is indeed made of steel so if the springs are elastic it's because steel itself is elastic so from this we could actually say that yeah steel is actually elastic in nature another example could be born human bone is pretty elastic for example a human bone undergoes deformations so when you're carrying very heavy weights for example and the bone of your legs does undergo deformations but there are very tiny deformations but they're there and when you get rid of the d-forming force when maybe you get rid of those weights then they snap back to their original shape just like a rubber band the only difference is with the rubber band you can see that deformation so we feel that yeah these are elastic and we'd steal another material the deformations are microscopic and we can't see them but they're there and when you remove the deforming force they snap back to their original shape and therefore they're also pretty elastic but now here comes the big question what makes things elastic or not lasting what's the reason for this property well to understand that we need to dig deeper into the material so for example if we were to look into rubber band what makes a rubber band so elastic well if we look deep into it this is a mic magnifying glass what would we see well we would see a bunch of atoms so imagine you're zooming in and we're seeing a bunch of atoms I want to draw a couple of atoms literally just two and what we'll do is we have to understand the nature of the force between them that's going to help us figure this out so it turns out that these atoms can attract each other or they can even repel each other when the rubber band is in the relaxed state the atoms are pretty much in equilibrium they don't repel they don't attract they are happy there but if you were to stretch the rubber band then the atoms are pulled far apart they don't like that they want to come back to their original original position equilibrium position so in that case what they do is they end up pulling on each other and similarly if you were to if you were to compress this rubber band somehow so if you had to push these atoms close to each other again they don't like that and now they start pushing each other away they start pushing each other and so because of this kind of force the rubber band always tries to come back to its original shape so it's as if the atoms are being tied by a spring I mean they're not really done they're not really tied like this but it feels like that right so when you stretch a spring it you know comes back and when you compress a spring again it snaps back so we can visualize that the atoms of the rubber band are all connected by Springs and that's why when you stretch them or when you compress it it tends to snap back to its original shape and same is the case with steel and bone but just like with any spring if you were to pull it with a huge deforming force then it loses his elastic property it now undergoes permanent deformation it doesn't snap back anymore well similarly if you were to pull these atoms too far apart then even they would lose their elastic property even they would stop stop attracting each other and there will be permanent deformation so elasticity has limit right so we would say it has has limit so if the deforming force is too high then the material undergo permanent deformation it will not regain its original shape and we would say it has lost his elastic property but a good question would be well how much is too much well that depends on the material for example something like if you take clay or or mud then it turns out that that limit is very low so these things have very low limit so even with very tiny forces the forces we put with our fingers they lose their elastic property very quickly and it's for that reason we call them as non elastic we also call them as plastic materials they're the same things non elastic or plastic but with stuff like say a rubber or steel then that limit is a little bit high so it has a little bit higher limit so high limit so when you put tiny amount of forces with our fingers we can actually see them deforming and we can see their elastic properties but they do have a limit for example if you were to take say a rubber band and stretch it too much then it will also have some permanent deformation so they do have a limit all right so the last thing we'll do is think about how do we compare electricity of two materials how do we check whether something is more elastic or less elastic well to do that we'll go back to the definition of elasticity well if something can withstand a lot of d-forming force and still regain its original shape then we'll say it is more elastic on the other hand if something even even for small deforming forces it permanently gets deformed then we will say it is less elastic make sense right so based on that let's now tackle the most classic question when it comes to the last which one is more elastic rubber or steel think about this I want you to pause the video and based on the definition of elasticity and based on what we just discussed think about which one do you think is more elastic in nature and try to come up with your reason all right the most common answer again I get for this is rubber and if asked why well one may say that well that's because rubber stretches a lot and still snaps back but steel I can't imagine steel being stretched as much as rubber right it will just break so maybe rubber is more elastic right well remember when we are thinking about elasticity we don't care how much the material deforms that's irrelevant what we only care about is how much d-forming force it can withstand and still snap back to its original shape so the fact that rubber stretches a lot is not a good way to think about elasticity a better way to think about that could be something like this what we could do is we could have a wire of Steel so imagine this is steel wire steel wire and then we could have a similar similar sized let's say same length and same thickness rubber wire and what we will do is will deform them by hanging weights suppose we hang say 50 kilograms of weights for both of these what would happen to steel well steel would hardly get any deformation the deformation in steel would be microscopic and when you get rid of that 50 kilogram if you remove that 50 kilogram well Steve would just snap back so it is elastic but with rubber if you were to hang 50 kilograms of weight and I'm sure you would agree with me that it would stretch a lot it would stretch a lot like this here is the 50 kilogram but that's not important what's important now is that if you were to remove that 50 kilogram a rubber will not snap back to its original shape it would have had some permanent deformation again you can try that with the rubber band at home stretch it a lot get rid of it and then you will see the rubberband is a little bit longer so steel can easily withstand 50 kilogram without having any permanent deformation whereas rubber can't in fact guess what steel as we will study later can withstand both thousand kilograms about thousand kilograms of weight without undergoing any permanent deformation but with rubber few tens of kilograms and it will undergo permanent deformation so if you think of it that way which one is more elastic well its steel because it can withstand a heart of d-forming force without undergoing permanent deformation alright to summarize usually we think that if you can stretch something a lot it is more elastic but that's not how you should think about elasticity think of it as if it's something can withstand a lot of d-forming force and still snap back then that is more elastic