Everyone knows that a sharp knife cuts better than a dull one, but do you know why, exactly?

 

　　The purpose of the cutting edge of a knife is to concentrate the force that’s being applied at the handle.

 

　　When you push down on the handle of a knife you exert a force. The force on the handle and the edge are the same, but the areas in which the force is concentrated are quite different.

 

　　At the handle your hand’s force is spread over a wide area, but on the cutting edge it is concentrated in a very small area, so therefore the pressure on the edge is much greater.

 

　　You can easily see how this works with a piece of modeling clay, a nail with a wide head, and a nail with a small head. First hold the nail with the wide head by its shaft and push it head-first into the ball of clay. Then do the same with the small-headed nail.

 

　　The small-headed nail goes into the clay much more easily than the wide-headed nail. Since the head of the small nail has less surface area than the big one, the force of your push is concentrated in a smaller area, exerts more pressure on the clay, and so the nail goes in more easily. The same principle causes pointed high-heeled shoes to sink into a soft surface more easily than flat shoes.

 

　　By applying this principle to the knife you can see that with a keener edge there’s a greater concentration of the pressure on the edge, so it takes much less force to make the knife cut.

 

 

　　利刃特别在何处？

 

　　大家都知道锋利的刀刃的切割性能胜过钝刀，但您知道确切原因吗？

 

　　刀刃的作用就是把人们用在刀把上的力集中起来。

 

　　当你按下刀把时，你施加了一种力。这种力无论是在刀把还是刀刃上都是等量的，但力的聚集区域不同，所产生的效果十分迥异。

 

　　在你手中的刀把上，力会在一个较广阔的区域传播，但在刀刃上，力会聚集在一个非常小的区域，因此刀刃上的压力会剧增。

 

　　只要有一块橡皮泥、一枚宽头钉和一枚窄头钉，你很容易就能看到这一原理是如何运作的。首先将宽头钉的杆竖起，将其头朝下推入橡皮泥球。然后对窄头钉做同样动作。

 

　　窄头钉比宽头钉更容易嵌入橡皮泥。由于窄头钉的头部面积比宽头钉小，你的推力将集中在一个更小的区域，对橡皮泥产生更大的压力，因此钉子更容易嵌入。同理，尖高跟鞋比平跟鞋更易陷入表面柔软的物体。

 

　　将这一原理运用在刀上，你能看到，在锋利的刀刃上，压力能更好的聚集，因此切割起来更省力。