Difference between strength stiffness

Answer 1

Strength and stiffness are not the same thing. Here's an analogy: A rubber band is stretched to failure. The rubber band failed at five pounds of force, but it stretched more than double its length before failure. The rubber band was not very stiff. In fact, it was elastic. Next, we stretch a kite string and find that it also fails at five pounds. It only stretched five percent before failure. It is very stiff. Both the rubber band and the kite string have the same ultimate strength. However, one is very stiff and the other is very flexible. This should demonstrate that strength and stiffness are not the same thing, and they are dependent upon the chosen material. Furthermore, the shape of the material also determines its stiffness without affecting its ultimate strength. For instance, if we take a plastic ruler that is 1/8" thick and 1" wide and bend it in the flat direction it is obvious that it is flexible. However, if we try to bend across the 1" thickness we find that it is very stiff. This demonstrates that the shape of the material causes the stiffness to change. We can take a piece of metal with a given weight and length and change its stiffness by making it narrower and thicker. Conversely, we can make it more flexible by making it wider and thinner.