Thursday, October 10, 2019

Newton Weights Essay

As it is known, different materials have different properties. They act differently under different circumstances. There are a number of properties of matter which can be explained in terms of molecular behaviour. Among these properties is elasticity. Intermolecular forces: these are electromagnetic forces between molecules. The strength and direction of these forces differ in accordance to the separation of the molecules. Materials are often subjected to different forces. Forces can be distorting, that is they can alter the shape of a body. Two distorting forces I shall look at are tension and compression. Tension/tensile stress, more generally referred to as stretch, happens when external forces (larger red arrows) act on a body so that different parts of that body are pulled to go in different directions. In most materials, the intermolecular force (smaller aqua arrows) of attraction shows resistance to these external forces, so that once the external forces have abated, the body resumes its original shape/length. Compression/compressive stress, more generally referred to as squashing, happens when external forces act on a body of material so that different parts of that body are pushed in towards the centre of the body. In most materials, the intermolecular force of repulsion acts against these external forces, so that when the distorting force is removed, the molecules return to their original arrangement and spacing. Materials that do this are known to have the property of elasticity. In short, elasticity is the ability of a material to return to its original shape and size after distorting forces (i.e. tension and/or compression) have been removed. Materials which have this ability are elastic; those which do not have this ability are considered plastic. This always happens when the distorting force is below a certain size (which is different for each material). This point where the body will no longer return to its original shape/size (due to the distorting force becoming too large) is known as the elastic limit (which differs from material to material). As long as the distorting force is below this size, the body that is under the external forces will always return to its original shape. As the body is put under more and more stress (distorting force), the body strains (deforms, extends) more and more. Right up to the elastic limit, the body will continue straining, in accordance to the size of the stress. This is where Hooke’s Law comes in. Hooke’s Law states that, when a distorting force is applied to an object, the strain is proportional to the stress. For example, if the load/stress is doubled, then the extension/strain would also double. However, there is a limit of proportionality (which is often also the elastic limit), only up to which Hooke’s Law is true. Since the strain is proportional to the stress for different materials where Hooke’s Law is true, then there should be a fixed ratio of stress to strain for a given elastic material. This ratio is known as its Young’s Modulus. Young’s Modulus can be calculated from the stress and the strain of an object under tensile/compressive stress. e = change in length/extension of object, in cm p = original length of object, in cm a = cross-sectional area of object, in cm2 f = size of force applied, in newtons For example, the Young’s Modulus of Mild Steel = 2 x 1011 N m-2 Copper = 11 x 1010 N m-2 Hooke’s Law and Young’s Modulus apply to most elastic materials, with the exceptions. A special shape which material can be bent into to in order to optimize use of the elasticity of a material is a spring. Springs are used by us everywhere: in seats, mattresses, cars, toys, and all other sorts of necessary objects and items we encounter in our daily lives. They are normally made from metal, though they can come from plastics, rubber or even glass. When compressive stress is applied to a spring, the spring noticeably ‘shortens’, though the actual length of the body material shortens very little. It is due to this special shape of springs that let it do this. The same occurs when tensile stress is applied. When a spring is being extended or pulled on, it may seem the spring is changing length dramatically, but in actual fact the spring’s body material relatively doesn’t change shape at all, but rather the shape of the body is more spaced out. AIM My objective in this experiment is to find out how a spring varies in length with added load. I also want to witness Hooke’s Law in action, and I want to observe the behaviour of the spring/s even after the load added causes the stress in the spring to exceed the elastic limit. PLAN My experiment is fairly straight forward to set up and carry out. In my experiment the data that I intend to assemble is the extension of the spring each time new/extra load is added to it. It is necessary that I use the most appropriate equipment for my experiment, hence I have chosen to use a retort stand which will hold up the spring and its weights up, a second retort stand from which a meter rule will be suspended. The metre rule will be right up against the spring, so as to ensure an accurate reading. There is no evidence that I can take before hand, other than the material of the spring. This entire experiment has to be as accurate, fair, precise and reliable as can practically be, but it is only possible to make it so to a certain extent. For instance, I cannot be absolutely sure that that all Newton weights weigh exactly 1000 grams, nor is it practical to find a ruler that is absolutely accurate. Hence I am forced to settle for the metre rule, which is accurate to about 1 millimetre, and I will be aware that the Newton weights will be within an accuracy of about i 20 grams. These factors will not really be in my control; however I can reasonably account for them when I construct a graph from my table by using error bars for each point plotted. Another measure I am taking is that I shall not be the only one to take readings from the metre rule; I shall have two other peers who will also be reading off the same metre rule. From these 3 readings I shall draw up averages of level of weight applied to the spring. To be practical and observing at the same time, I must choose an appropriate extent and range, as well as appropriate integers, for the data that I intend to collect. I will be going to take the first measurement as the length of the spring when there is no mass attached to it. The last measurement shall be right up to when the spring can no longer hold on to the weights. I have a rough idea of the spring that I shall use, and I am assuming now that the spring shouldn’t be able to hold much more than 13 kg. I shall be adding the weights one at a time (one Newton/kilogram at a time), and I shall be taking measurements at each of these intervals. The measurements that I shall take of the length of the spring will be in millimetres. So basically, once I have set up the entire apparatus, I shall start off taking the measurement of the spring when it is free of load, then let my peers take theirs. Then I shall add a Newton weight one at a time, taking measurements with my helpers each time I add one. Of course we’ll be wearing our goggles, because I don’t want to take any risks. 1) Collect equipment. 2) Prepare apparatus as shown in diagram. 3) Record the length of the spring when it is load-free, to cm, in the prepared table for results. 4) Add a weight/mass of 1 kg or 1 N, and then take the new length of the spring. Record in the prepared table for results. 5) Continue adding on weights/masses of 1 kg, recording the length of the spring each time in the prepared table for results. This should be carried on until the weights can no longer be attached to the hanging spring. APPARATUS. The apparatus that I shall need set up for my experiment consist of the following items: 1. Retort Stands (x 2) 2. Boss and Clamps (x 2) 3. Metre Rule 4. Spring (length: 50 mm) 5. Newton Weights (x 15 approximately) Other items I shall need are three pairs of goggles. SAFETY I must consider my safety when working in the laboratory. It is common when this type of experiment is carried out that when a weight or anything for that matter is suspended from something as unstable as a hanging spring, the item in being suspended is prone to fall.

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