Does stress cause strain or vice versa?
Stress is defined as force per unit area, while strain is the change in dimension divided by the whole dimension, in other words, the relative change in dimension. If we bend a piece of balsa wood or we stretch a piece of elastic material, we are the cause of the force, which creates the stress, and as a result, the object changes its shape. So is it obvious that stress causes strain? No. We must be very clear as to what we mean by the statements made above. We are using words to describe the changes to a physical object. The words are not the physical reality: even a mathematical formula is only a metaphor. Nobody believes that when we pull something, it somehow thinks it has suddenly to obey Hooke’s “Law”, and stretch according to the force. Let us analyse what happens when we stretch an object. At some moment we start to pull our hands apart, and the effect of this cause is that the object starts to elongate. This is cause and effect as we normally understand them. But if we ask abo
