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11 - Materials (11.2 Springs (Hooke's law (States that the force…
11 - Materials
11.1 Density
Density of a substance is defined as its mass per unit volume , p=m/v
Measurements
Regular solid: measure mass using a top pan, measure dimensions using vernier calipers or micrometer and calculate volume (v=4/3 x π x r.cubed)
Liquid: Measure mass of empty cylinder, measure mass of cylinder with liquid and find difference
Irregular shape: Measure mass, immerse object in liquid in cylinder and observe increase in liquid level
Density of alloys
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density, p = (p1V1/V)+(p2V2/V)
11.2 Springs
Hooke's law
States that the force needed to stretch a spring is directly proportional to the extension of the spring from its natural length
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Greater k= stiffer spring, graph F against ΔL is a straight line with gradient k, If spring stretched beyond elastic limit it does not regain initial length
Springs in parallel
Weight supported by 2 springs, k = k1 + k2
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11.4 Stress and Strain
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Strain energy
Area under line of force-extension graph= work done to stretch wire. Work done to deform object is strain energy
Metal wire or spring
Provided limit of proportionality is not exceeded, the work done=1/2TΔL. As elastic limit is not reached, work done is stored as elastic energy in wire: elastic energy stored in stretched wire = 1/2TΔL
Rubber band
Area between unloading and loading represents the difference between energy stored in rubber band when stretched and useful energy recovered when its unstretched. Difference occurs as some energy stored becomes internal energy of molecules
Polythene
As it does not regain initial length, area between loading and unloading represents work done to deform material permanently as well as internal energy retained when it unstretches