Questions about Hooke's Law graphs.

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vector12
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1) On a force extension graph, if the gradient is a straight line but it doesn't go through the origin, does it still Obey Hooke's law or not? I've seen some that are a straight line gradient but the line starts at y>0 on the force axis. (y axis)

2) If a graph has a straight line gradient up to a certain point, it is obeying Hooke's Law until the point where it reaches the limit of proportionality. After that, it starts to curve which is the elastic region. How can you tell where the plastic deformation region starts?
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Physics Enemy
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(Original post by vector12)
F vs ext graph; if line doesn't go through O, does object obey Hooke's Law?
Depends whether F represents applied or total force (usually the former). If applied, line goes through O for Hooke's Law, as an applied force must cause ext. If total, initial force = object's weight; graph's linearity implies Hooke's Law.

(Original post by vector12)
If graph is a straight line up to limit of prop, Hooke's Law obeyed. Graph then curves; how can you tell where plastic deformation starts?
After limit of prop (P) we get elastic limit (E), then yield point (Y); points usually close together on a graph. Object behaves elastically up to E then plastically after Y. So it will return to original length/shape up to E, deformed after Y.

The small gaps 1) P -> E, 2) E -> Y mean the object 1) behaves elastically for a small extra load whilst F =/= ke 2) small extra load between the end of elasticity, start of plasticity. Thus object likely deforms a short way into the curved region.
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