# OCR - P6: Matter – models and explanations

 P6.3 How does the particle model relate to material under stress? Background to the topic What you should be able to do: When more than one force is applied to a solid material it may be compressed, stretched or twisted. When the forces are removed it may return to its original shape or become permanently deformed. These effects can be explained using ideas about particles in the solid state. A substance in the solid state is a fixed shape due to the forces between the particles. Compressing or stretching the material changes the separation of the particles, and the forces between the particles. Elastic materials spring back to their original shape. If the forces are too large the material becomes plastic and is permanently distorted. For some materials, the extension is proportional to the applied force, but in other systems, such as rubber bands, the relationship is not linear, even though they are elastic. When work is done by a force to compress or stretch a spring or other simple system, energy is stored, this energy can be recovered when the force is removed. 1. Explain, with examples, that to stretch, bend or compress an object, more than one force has to be applied. In your practical work you will investigate the force-extension properties of a variety of materials, identifying those that obey Hooke's law, those that behave elastically, and those that show plastic deformation. 2. Describe and use the particle model to explain the difference between elastic and plastic deformation caused by stretching forces 3. a) describe the relationship between force and extension for a spring and other simple systems 4. Describe the difference between the force-extension relationship for linear systems and for non-linear systems. 5. Recall and apply the relationship between force, extension and spring constant for systems where the force-extension relationship is linear force exerted by a spring (N) = extension (m) × spring constant (N/m) 6. a) calculate the work done in stretching a spring or other simple system, by calculating the appropriate area on the force/extension graph b) describe how to safely use apparatus to determine the work done in stretching a spring 7. Select and apply the relationship between energy stored, spring constant and extension for a linear system: energy stored in a stretched spring (J) = ½ × spring constant (N/m) × (extension (m))2