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A material shows elastic behaviour if it returns to its original length when any deforming forces have been removed. During elastic behaviour, the particles in the material are pulled apart a little, so they return to their original positions when the forces are removed. A material shows plastic behaviour if it remains deformed when a load is removed. During plastic behaviour the particles slide past each other and the structure of the material is changed permanently.
◁◁Fig. 1.42 Hooke’s law is shown by a straight line on a force-extension graph, for any object being stretched.
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HOOKE’S LAW On page 50 you saw that, for a wire, there is a section of the f orce– extension graph which is linear. Like a wire, when a spring stretches, the extension of the spring is proportional to the force stretching it, provided the elastic limit (the point where the spring returns to its original length after the load is removed) of the spring is not exceeded. This is Hooke’s law and is shown by a straight line on a force–extension graph.
The gradient of the line is a measure of the stiffness of the spring.
measure the extension of the spring against the ruler
weights provide the stretching force
∆∆Fig. 1.43 Apparatus to investigate Hooke’s law.
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An experiment to investigate how extension varies with applied force for a spring: 1. Assemble the apparatus (left) and allow the spring to hang down. Measure the starting position on the ruler. 2. Take the first mass, which consists of the hook and base plate, typically of mass 100 g (a weight of 1 N), and hang it on the spring. Measure the new position on the ruler. The difference in the readings is the extension of the spring. 3. Add masses one by one to the first one. Typically each mass is C‑shaped, and adds an additional 100 g. Add the masses carefully so that the spring stretches slowly. 4. You should reverse the experiment to see what happens as the masses are removed.
235978 Edexcel International GCSE Physics_P008_P075.indd 51
20/03/17 4:30 PM