An investigation into the stretching of materials

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Kate Graham 10 Tees Moor

An investigation into the stretching of materials

PLANNING

I am examining the stretching of materials.  I decided that research on Hooke’s law, (The law named after Robert Hooke who formulated the law governing elasticity), and more particularly the way elastic bands reacted to stress would help me to do this, so I found as much material as I could, relevant to my investigation, that would help me.  This material is at the back of this document.

Elasticity is defined as the property or substance in a material that causes the material to resume its previous size and shape after having been stretched (or compressed) by a force, and this is what I have decided to investigate.

According to my research below, Hooke’s law states that a force applied to an object creates a tension within that object and the stress can cause the material to ‘deform’.  Stress is directly proportional to strain, defined as the quantity that expresses the amount of deformation.  The strain is therefore linked to the tension created.

   Hooke’s law is defined as a law expressing the proportionality of strain to the stress causing it.

Apparatus

The appropriate pieces of equipment I have chosen to use in my experiment are;

·        Elastic bands of three different lengths, 2cm, 4cm and 6cm,

·        A clamp and stand,

·         A metre ruler,

·         Six  weights each weighing 1 Newton.

Diagram

Method

I am going to set up the experiment as shown above, and position the elastic band in the clamp.   I will then control the number of weights I add and using the ruler I will record the extended length of the elastic band after each weight is added.  

Variables

 There are many things I could change to alter the outcome of my experiment.  A list of variables I could change which may affect the results of my experiment are;

·        Length of the elastic band,

·        Width of the band,

·        Elasticity of band,

·        Previous stress and strain the band has undergone,

·        Weight of band,

  • Weight added to band,

·        Colour of band,

·        Temperature of band and room at time of experiment.

I have deemed these last two factors insignificant as I feel they will not have an actual effect on the outcome of my experiment.  I chose to change the length of the band only for my experiment and to make the test results reliable, I decided I would not change any of the other variables, except the weight I add to the band, so they would be kept constant.  I then decided I would measure the extended length of each of the three elastic bands differing in length and compare this to the original length of the bands to find how far each individual band had extended and retracted, after each newton weight was added.

EXTENDED LENGTH (CM)  - ORIGINAL LENGTH (CM)  EXTENSION LENGTH (CM)

Fair Test

To ensure my experiment is a fair test I must ensure that the other significant variables I am not purposefully changing  remain the same or my results cannot be relied upon.  The same force must also be applied to each elastic band.  To be certain the results I have obtained are correct I have decided to repeat the experiment, and then compare the two sets of results.  

To make the experiment fair, the elasticity limits of the band must not be reached.  If the force is too strong, my research below tells me that Hooke’s law no longer applies.  The elastic limit is the maximum amount of stress a material can stand before it becomes permanently deformed.

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  For example, when small weights are suspended from a spring, doubling the weight doubles the extension (Hooke’s law) and the system is termed linear.  If too much weight is added, such a simple law does not govern the extension.

Safety Precautions

In order to safely perform my experiment I need to make sure that I stand at all times, in case the weights accidentally fall or the elastic band snaps and the weights fall on my foot.  This is not a particularly dangerous experiment and therefore there are no other precautions I need to take in order ...

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