[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to describe what's meant by elastic potential energy you should then be able to calculate the elastic potential energy stored in a stretched spring now there are a couple of tricky Concepts in this video but stick with it and you'll get the idea okay I'm showing you here a windup toy and a clock the question is what do these both have in common well the answer is that both of these get their energy from the energy stored in a spring when we stretch a spring we're applying a force to change the length of the spring scientists call applying a force like this doing work and we're going to look at work in later videos now we're putting energy in to stretch the spring the stretch spring is storing this energy and we call that elastic potential energy so we're going to learn how to calculate elastic potential energy but first we need to take a closer look at Springs this shows a spring with no force acting if we apply a force the spring will stretch and we call this stretch the extension so here a force of 10 Newtons is acting on the spring and you can see that the spring has extended the extension is given the symbol lowercase e if we plot the extension of the spring against the force applied then we get a straight line passing through zero and that tells tells us that the extension is directly proportional to the force now there is a problem here because if we apply too greater force then the extension is no longer directly proportional to the force and the graph looks like this at this point the spring has been stretched beyond the limit of proportionality now it will not return back to its original length if we take away the force so as we've said a stretched spring is a store of elastic potential energy we're going to look at how we calculate this here's the equation and you are given this in your exam so you don't need to learn it the elastic potential energy in Jews is No.5 multiplied by the spring constant multiplied by the extension squared there are a couple of points about this equation firstly the extension must be in meters so if it's in centimeters then you'll need to convert to meters secondly the value of the spring constant depends on the spring so here's a question for you to try a spring has an extension of 20 cm calculate the elastic potential energy stored in the spring and the value of the spring constant is 100 Newtons per meter pause a video now and try this yourself okay so here's the equation and remember you don't need to learn this one the elastic potential energy is .5 multiplied by the spring constant multiplied by the extension squared the spring constant in this case is 100 Newtons per meter and the extension is 20 cm now the first thing we need to do is convert the extension from cm to met and to do that we divide by 100 20 cm is 0.2 M putting these numbers into the equation gives us an elastic potential energy of two Jew remember you'll find plenty of questions on elastic potential energy in my or Vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe what's meant by elastic potential energy you should then be able to calculate the elastic potential energy stored in a stretched spring [Music]