[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to describe how to investigate the relationship between force and extension for a spring and this is a required practical so it's really important that you learn the details in the last video we looked at the idea of elastic and inelastic objects in this video we're going to look at what happens when we apply a force to a spring so let's start by looking at the equipment we've got a clamp stand two bosses and two clamps we now place a heavy weight on the Clum stand to stop it falling over next we attach a meter Rule and a spring the top of the spring must be at the zero point on the meter rule it's really important that the meter Ru is vertical otherwise the readings will be inaccurate the bottom of the spring has a wooden splint attached as a pointer and this pointer must be horizontal or again the readings will be inaccurate We Now read the position of this pointer on the meter rule this is the unstretched length of the spring in other words the length with no Force attached next we hang a one newton weight on the spring like this we now read the new position of the pointer on the meter rule next we continue adding one newton weight to the spring and reading the position of the pointer I'm showing you a closeup now of the meter rule we now need to work out the extent extension produced by adding each weight to do that we subtract the length of the unstretched spring from each reading so the extension produced by the 1 Newton weight is 4 cm and the extension produced by the 2 Newton weight is 8 cm the extension produced by the 3 Newton weight is 12 CM and the extension produced by the 4 Newton weight is 16 cm we now need to plot the extension against the weight and we end it with a graph like this now in the required particle you're meant to use the spring to work out the weight of a mystery object for example a stone to do that we need to measure the extension of the spring when the stone is hanging from it we then read the weight of the Stone from the graph like this now there are several points about this graph that you need to understand firstly the graph is a straight line going through the origin in other words through zero this tells us that the extension is directly proportional to the weight scientists say that there was a li relationship between force and extension and that's because we get a straight line graph the word linear means straight line this shows the same experiment but using a rubber band as you can see in the case of a rubber band we do not get a straight line This is called a nonlinear relationship going back to the graph for the spring we can see that the spring is elastic that's because if we remove the weight the extension returns to zero now there is a problem here if we had too much weight to the spring then we get a graph that looks like this as you can see the graph is now nonlinear in this case we've overstretched the spring in other words if we took all the weight away the spring would still show an extension scientists call this inelastic deformation by overstretching the spring we've exceeded the limit of proportionality and we can see that on the graph there is one final idea that we need to look at in the last last video we looked at how to calculate the force required to extend a spring we used this equation the force equals the spring constant multiplied by the extension we can use the linear part of the graph to determine the spring constant at any point the spring constants found by dividing the force by the extension and remember that the spring constant will be the same for any part of the graph as long as we don't exceed the limit of proportionality we remember you'll find plenty of questions on this required practical in my vision workbook and you can get that by clicking on the link above [Music]