in today's video we're going to look at the different types of elasticity explain the terms at spring constant and hooked law and finally look at some force extension graphs when you apply a force to an object you could cause it to compress to stretch or to bend this is easiest to see with a spring but the same concept applies to other objects too like a ball or a phone these objects are just less elastic so it's harder to notice any kind of change in shape whatever the object is though we always have to apply more than one force if we wanted to stay still otherwise the object will just move as we pull or push it even if you squish something against the floor and so you only apply one force the floor itself is applying a force upwards so there are still two forces acting on the object now when an object changes shape we say that it's been deformed but importantly there are two different types of deformation that you need to know elastic and inelastic if an object returns back to its original shape after the forces have been removed then we call it elastic deformation because it's able to spring back like an elastic band however if the object doesn't quite return just normal shape and stays deformed in some way then we call it inelastic deformation or sometimes plastic deformation because it keeps its shape like plastic the next concept we need to look at is extension which is the increasing length of a spring when it's stretched for example if we hang a spring from a solid support then we can measure how the spring's length changes as we add downwards force to the bottom of the spring now even before we've had any chance to add force ourselves the spring's own mass will be exerting a force downwards in the form of weight and this means that the natural length will be a bit shorter than the spring itself because there will already be some extension this extra length is generally pretty small though so we tend to ignore it and think of the entire length as the natural length if we then add a mass to the bottom of the spring the weight of that mass will pull on the spring and so increase its length which we can then measure as the extension one thing to point out here is that the solid support will also be exerting an equal but opposite force upwards and this is why the spring doesn't fall down when we add any mass it's being perfectly balanced by the support as we increase the force on the spring for example we add more mass the extension increases proportionally which we can write as f is proportional to e because f is the force and e is a symbol for extension although you might sometimes see it represented as an x instead exactly how much the spring extends though for a given force depends on the particular object's spring constant which is denoted by letter k and if we add this into our force extension equation that we wrote in the bottom left we get f equals ke with k being measured in newtons per meter and extension in meters the spring constant tells us how many newtons it would take to stretch the particular object like this spring by one meter so the higher the spring constant the stiffer the material because it requires more force to stretch it we can show this relationship by plotting a graph of force against extension as the force increases so does the extension and because it's a straight line that passes through the origin we can tell that force and extension are directly proportional we call this relationship hooke's law and importantly all of this deformation will be elastic deformation meaning that once the force is removed the object will return to its original shape there is a limit to this relationship though at some point our line will start to curve and we call this point the elastic limits or the limits of proportionality after this point hooke's law no longer applies and the object won't necessarily go back to its original shape meaning that it would have been inelastically deformed anyway that's everything for this video so hope you found it useful and we'll see you soon