[Music] hi and welcome back to freiz lessons. co.uk by the end of this video you should be able to State the difference between elastic and inelastic deformation you should then be able to calculate the force needed to stretch or compress an object and finally you should be able to describe the energy transfers taking place when an elastic object under goes stretching compression or bending I'm showing you here a selection of objects we've got a slinky rubber bands rubber gloves an ER razor a playground surface and a tennis ball all of these are examples of elastic materials so the question is what exactly is an elastic material this shows an elastic material and we're applying a force at either end these stretching forces are equal in magnitude but opposite in direction in other words these forces are balanced now these forces cause the object to stretch like this but when the forces are released the elastic object returns back to its original length so the key fact is that elastic materials will always return to their original length or shape if we take away the forces acting on them this shows the same elastic material but this time we're applying squeezing forces in this case the material compresses and if we take away the forces it returns to its original length now in this this case we're applying three forces to the elastic material this causes the object to bend but once again if we remove the forces then it returns to its original shape and length scientists call changes like this elastic deformation now there is one important idea here in order to change an object's length or shape then we have to apply more than one force if we applied only one force to a stationary object then the forces are no longer balanced in this case the object would simply move rather than changing length or shape I'm showing you here an inelastic material when we stretch inelastic materials they do not return to their original length when the forces are removed and a good example of inelastic materials include certain polymers scientists call changes like this in elastic deformation now we can calculate the force needed to stretch an elastic object by using this equation the force in Newtons equals the spring constant in Newtons per meter multiplied by the extension in meters and you're not given this equation in the exam so you do need to learn it here's a typical question calculate the force required to extend a spring by .04 m the spring constant is 200 Newtons per meter so pause the video now and try this yourself okay to calculate the force we need to multiply the spring constant by the extension the spring constant is 200 Newt per me meter and the extension is .04 M multiplying these together gives us a force required of 8 Newtons now we can use the same equation to calculate the compression when an elastic object is squeezed in this case e stands for compression and not extension when we stretch or compress an elastic object we're using a force to do work in these cases elastic potential energy is stored in the object the work done is equal to the elastic potential energy however that's only true if the object is not inelastically deformed and we're looking at that in the next video now I should point out that we've already seen elastic potential energy in the energy topic and I'd recommend that you watch that video now in the next video we're going to look at the required practical on stretching a spring remember you'll find plenty of questions on forces and elasticity in my vision workbook and you can get that by clicking on the link above [Music]