Most of you will have eaten walnuts at some time or other and possibly associate them with the Christmas season. But have you ever tried to crack one without a nutcracker just by squeezing it in your hand? Unless you find a particularly weak nut, it is very difficult indeed, if not impossible. However, if you take two nuts and squeeze them together as shown in Fig. 2.1, you will probably have little difficulty in cracking them. Can you explain this rather surprising effect? Why do you think drawing pins have sharp points, and knives with sharp blades cut better than knives with blunt blades, In order to answer these and similar questions, we must try to understand the idea and the meaning of pressure.
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