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For this investigation I will be testing various materials to determine how much force is required for them to snap.

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A2 Physics investigation

For this investigation I will be testing various materials to determine how much force is required for them to snap. Figure 1 below shows the forces that will be acting upon the test material.


The set-up above will increase the force applied to the test material until a crack, started in the surface in tension, propagates to cause the material to break. To experimentally determine the level of stress that is required to break an object, I need to be-able to exert a force onto an object and be able to quantitatively measure and record that force. This, I found, is not as simple as it sounds due not only to the limited equipment available to me, but also to what I could use practically and safely. I came up with many methods of measuring the force exerted as well as many ways of creating that force. A summary of the major methods and their associated problems is included below.

Method 1: Creating force using my own strength.

This has several obvious advantages including ease of use. My own strength requires no preparation to use, is easy to store when I’m not using it and can create a very broad range of force i.e. up to 800N. Unfortunately for all the benefits, there are several disadvantages that make this method untenable. The greatest concern is safety, namely my own safety.

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Method 6: Creating the force using weights.

This method of producing force has the added benefit of measuring the force as it is produced. The force produced is quantitative, steady and the maximum level reached is easily found. There are disadvantages however; the maximum force that can be produced is dependant on the maximum strength of the string that attaches the masses to the test medium. The larger the area of contact, the less pressure is produced. This means that for the test to be fair, the string used has to be the same in each instance. This method is the one that I finally decided to use.

My choice of method means that a small alteration is required to my original force diagram; see figure 2.


Fair Test

In order to perform this experiment fairly I will need a means of supporting the material and ensuring the struts stay in the correct place because the force I exert will be laterally transferred to them. I considered several ways of supporting the material and how to prevent the supports shifting during testing. Figure 3 shows the forces involved.

Vertical force applied to material, causing it to flex and become compressed and under tension.


The flex of the material means that the edges of the material are closer the centre of the material and that the material is therefore shorter horizontally despite being slightly longer in length.

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The first thing that I thought would be a safety concern in the experiment was the force involved in breaking the materials. Creating large forces would mean large amounts of mass being used and this is a hazard. To minimise any risk involved; I selected materials that would not require huge forces to break and I used only small samples of them so as to further reduce the force required e.g. the wood samples I used were cut to a thickness of 2mm.

Another concern was that of what the material would do when the stress point was reached. Some of the materials I used could shatter or splinter e.g. plastic and wood. To cut the risk of personal injury to myself and those around I erected Perspex safety screens, wore protective goggles and performed the experiment in a closed classroom. I also used shatterproof plastic in tests instead to normal plastic as I considered this too dangerous.

Due to the method used in generating the force i.e. using suspended masses, I had to consider where the masses were going to land once the material gave way. I placed a carpet tile where the masses were going to fall so as to prevent damage to the floor and furthermore I suspended the masses as close to the floor as possible so as to reduce the velocity at which the impacted the floor.

With the preparation of the experiment sorted and a few preliminary test runs completed, I decided to start collating results from the various materials I had collected.

...read more.

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