# Investigation on whether Rubber obeys Hooke's Rule

Physics AT1 – Hooke’s Rule

Investigation on whether Rubber obeys Hooke’s Rule

Plan

Introduction

Hooke's Rule states that extension of a material is proportional to the tension force applied to it unless the elastic limit is reached, which is the point at which the material no longer obeys Hooke's Rule. There are only a few materials that obey this rule. In this investigation, we will find out whether rubber obeys Hooke’s Rule. We will measure in detail the way in which the extension of a rubber band depends on the tension in the band. This will be done by applying various amounts of weights, as it is a continual variation.

Hooke’s Rule = F = ke

• F = Force in Newtons
• k = Spring constant
• e = Extension in Centimetres

Rubber is a natural polymer which is made up of long chains of molecules which are bent back and forth with weak forces acting between them. As the rubber band is stretched, molecules straighten out and allow the rubber band to become larger. Eventually, as the molecules become fully stretched, the long chains will become parallel to each other and can stretch up to ten times its original length. Extra force will make the rubber band break. If the rubber is not stretched to breaking, once the force is removed the molecules tend to curl back again into their original position because of the attraction and cross-links between adjacent molecules. The return is elastic.

Hypothesis

I predict that the greater the weight applied to the rubber band, the further the rubber band will stretch. This is because, according to the findings of Hooke’s Rule, the extension is directly proportional to the load (i.e. tension force). So if the load increases (i.e. the tension increases), then the extension increases. Furthermore, I think that doubling the load (doubling the tension force), will double the extension. However, the rubber band will reach its elastic limit where the extension will continue to increase, but not increase in proportion to the load due to unusual temperature changes and weaker forces of attraction.

The rubber band has a large elastic limit i.e. retains elasticity and returns to its original shape and not suffer permanent distortion.

After a large force is applied, the gradient will begin to decrease because the molecules within rubber will not be able to be stretched any further. A larger force will be required to produce an extension.

Extension = New length – Original length

This is a predicted graph showing how the extension of the material depends on the tension force.

Extension /cm

Elastic

Behaviour

Force /N

Scientific Knowledge

Rubber is a ductile material which ...

#### Here's what a star student thought of this essay

Their quality of written communication is good, with no obvious spelling or grammatical mistakes. However, they could present their work better by including more clear headings to break up the text and show to the marker that they can do an Ã¢â‚¬ËœanalysisÃ¢â‚¬â„¢ of their Ã¢â‚¬ËœresultsÃ¢â‚¬â„¢ and come to a Ã¢â‚¬ËœconclusionÃ¢â‚¬â„¢. They also occasionally use more colloquial language such as Ã¢â‚¬Ëœloads of varied massesÃ¢â‚¬â„¢, which could be better expressed as Ã¢â‚¬Ëœa variety of masses ranging from 100g to 1000g, increasing in 100g amountsÃ¢â‚¬â„¢ Ã¢â‚¬â€œ this is a bit clearer and less informal. However, the majority of their report is very well done, with only minor problems in the layout.

The author has analysed their results and compared them to the expected trend predicted in the hypothesis. However, there were many opportunities to extend their analysis slightly, for example using the gradient of their graph to calculate the Ã¢â‚¬Ëœspring constantÃ¢â‚¬â„¢ in the equation given in the hypothesis. They could also have calculated the uncertainties in their measurements and plotted uncertainty bars on the graph, and, if they had repeated their experiment more times, any anomalies in their results. These may be essential to gain high marks, and provide the opportunity to show good mathematical skills. Occasionally they have not shown a thorough understanding of the physics behind the experiment Ã¢â‚¬â€œ for example their explanation of the energy transfers when rubber is stretched is flawed. They need to think more logically about the molecules, in order to realise that the rubber gains elastic potential energy as the intermolecular bonds/ cross-links are stretched. The energy is not used to increase the temperature (although a slight increase in temperature could be due to friction as the molecules slide past each other), and the work done is only to pull the molecules in order to stretch the bonds. Less force could be required to produce a relatively larger extension due to the intermolecular bonds breaking/ weakening after a certain stress has been applied. They have also conflated the terms Ã¢â‚¬Ëœelastic limitÃ¢â‚¬â„¢ and Ã¢â‚¬Ëœlimit of proportionalityÃ¢â‚¬â„¢ Ã¢â‚¬â€œ the latter is the point after which the material stops obeying HookeÃ¢â‚¬â„¢s Law, whereas the elastic limit is the point beyond which the material stops behaving elastically i.e. undergoes permanent deformation under stress. Despite these small mistakes, they have analysed their results well and used them to support their conclusion.

The author has provided a good response to the question, as they have carried out a successful experiment to investigate the relationship between the stress applied to the rubber band and the extension, showing that the rubber band appears to be following HookeÃ¢â‚¬â„¢s Law. They have also considered the science behind the experiment, including a good explanation of elasticity.