Hello to all:

I recently finished a wonderful book that I am sure would appeal to the members of this forum. It is "Cat's Paws and Catapults, Mechanical Worlds of Nature and People", by Steven Vogel, ISBN 0-393-31990-3. It is about the way living and made things work. It is an enlightening comparison of natural and human "technologies" and their underlying principles. Highly recommended.

I wish to offer this contribution after reading (yet another) discussion on the differences among the terms strength, stiffness, toughness, etc. Here is an excerpt that discusses these terms.

Material Properties that Matter, and examples of each that exhibit low and high values for each property.

Strength: Stress (force per area) needed to break a material. Low--jelly; High--steel cable.

Extensibility: Strain (relative stretch) needed to break a material. Low--brick; High--rubber band.

Stiffness: Stress divided by strain as a material is stretched. Low--soft plastic; High--brick.

Work of extension, Toughness: Energy to stretch a material to breaking. Low--cast iron; High--spring steel.

Resilience: Energy output upon release divided by input during stretch. Low--taffy; High--rubber band.

Other interesting topics include:

The Virtues of Flexibility, How Nonmetallic Materials Can Avoid Cracks, The Stiff and the Soft, Swimming at the Surface and many others. I found it educational and my understanding of why my kayak (mis)behaved the way it does was enhanced.

I have no desire or illusions that this will end the debates, but I hope it could clarify them.

Ed Valley