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Elasticity 1

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Changing shapes Forces can make objects change their speed or their direction when they are in motion. They can also make objects change shape. Stretching forces can change the shape of objects, making them longer and thinner. Photo credits: resistance band © Demid Borodin; crushed can © pixelman, both 2012 Shutterstock.com Compression forces can squash objects, making them smaller.

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Elastic or plastic? In physics, materials are described as having either elastic or plastic properties. Most materials display both properties, depending on how much they are deformed. Plastic materials change shape permanently when they are deformed by an external force. Photo credit: modelling clay © ra3rn; elastic bands © Irina Nartova, both 2012 Shutterstock.com Elastic materials return to their original shape once the external force is removed.

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**Elastic potential energy**

A stretched or compressed material, like the spring in a jack-in-the-box when the lid is closed, has elastic potential energy (EPE) stored in it. EPE is the energy stored in a body due to a load causing a deformation. According to the law of conservation of energy, no energy is created or destroyed when a spring is compressed. Therefore the work done in compressing the spring is equal to the EPE stored in it, plus any energy released as heat and sound. Teacher notes Students may need to be reminded that the law of conservation of energy states that energy cannot be created or destroyed, just changed in form. This means that: energy never just ‘disappears’ the total amount of energy always stays the same, i.e. total input energy = total output energy in most energy transfers the energy is transferred to several different forms that may or may not be useful energy that is transferred to unwanted forms of energy is wasted. See the Potential and Kinetic Energy presentation for more information on the law of conservation of energy. Photo credit: © ZTS, 2012 Shutterstock.com

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**Slingshot science Teacher notes**

This virtual experiment illustrates the relationship between EPE and KE. It could be used as a precursor to running the practical in the lab, or as a revision exercise.

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**Investigating springs**

Teacher notes This animation could be used as a revision activity, or as a precursor to running a practical experiment in the classroom. Students could design and carry out an experiment to find out k based on this and the next slide. A spring can be suspended from the edge of a desk and its length measured. Weights can be added to it to increase the force on it, and its new length measured for each weight added. Extension for each weight can be calculated by subtracting the original length from these values. A graph of weight (force) against extension can be plotted. The gradient of this graph is k. Other materials such as a rubber band or a polythene strip could also be tested to see if they obey Hooke’s law (discussed in further detail on slide 9), i.e. if they have a constant gradient for their force-extension graphs. Issues to discuss surrounding this experiment can include: Accuracy in measuring the length of the spring. Students should measure from the same point each time (the end of the coils as shown above, or the last coil). Safety – the spring could snap if overloaded, so safety goggles should be worn and a box should be placed underneath the weights to catch them if they fall. Have enough points been plotted on the graph to ensure as accurate as possible a line of best fit and therefore gradient?

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**limit of proportionality**

Results plastic region force (N) break point limit of proportionality elastic limit elastic region extension (cm) Teacher notes It should be pointed out that the limit of proportionality is at the point where the graph is no longer a straight line. Students should be able to make sense of this since a straight line graph represents direct proportionality. The link between the definitions of elasticity and elastic limit could be highlighted – the elastic limit is the extent to which a material displays the property of elasticity. If a spring is stretched far enough, it reaches the limit of proportionality and then the elastic limit. The elastic limit is a point beyond which the spring will no longer return to its original shape when the force is removed.

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Hooke’s law The limit of proportionality is a point beyond which behaviour of an elastic material no longer conforms to Hooke’s law: The extension of a spring is directly proportional to the force applied, provided its limit of proportionality is not exceeded. F µ e or F = ke where k is a constant. original length x Teacher notes F is the force in newtons, N; k is the spring constant in newtons per metre, N/m; e is the extension in metres, m. The stiffer a spring is, the greater its spring constant (k). F

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**Elasticity calculations**

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**Glossary Teacher notes**

compression force – A force that can squash objects, making them smaller. conservation of energy – The law that states that energy cannot be created or destroyed, just transferred into different forms. elastic – A material that can be returned to its original shape when deformed once the external force is removed. elastic limit – The point at which any larger force will cause permanent deformation of an object. elastic potential energy (EPE) – The energy stored in a object when stretched or squashed. energy transfer – A process by which one form of energy is transferred into another form. Hooke’s law – For elastic materials, force and extension are proportional provided the elastic limit is not exceeded. kinetic energy – The energy an object has because it is moving. limit of proportionality – A point beyond which the behaviour of an elastic material no longer conforms to Hooke’s law. plastic – A material that changes shape permanently when deformed by an external force. 12

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Anagrams

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**Multiple-choice quiz Teacher notes**

This multiple-choice quiz could be used as a plenary activity to assess students’ understanding of elasticity. The questions can be skipped through without answering by pressing the forward arrow. Students could be asked to complete the questions in their books and the activity could be concluded by completion on the IWB. 14

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