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?