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Name: Irja
Status: student
Grade: N/A
Location: N/A
Country: N/A
Date: 11/16/2005


Question:
Why does a sharp knife cut better than a dull knife?


Replies:
Irja,

It is a question of pressure versus force.

When you push on a surface with the blade of a dull knife, a fairly wide piece of material makes contact with the blade. That whole piece of material works to keep the knife from passing through. The force that your hand exerts is distributed over a very large number of molecules.

When you push with a sharp knife, you apply the same amount of force. Now, however, a much thinner piece of material makes contact with the blade. Since there are fewer molecules working to keep the blade from cutting, each molecule must work much harder. If the blade is sharp enough, the molecules will not be strong enough to keep the blade out. The sharp blade will tear the molecules apart.

This is in some ways like men in ancient Egypt working together to move a very large stone. They had no machines or trucks. They just dragged the huge stones across the sand. Five men working together could not do it. Fifty men working together could not do it. Five hundred men pulling together on ropes could move the stone. The more dull the blade, the more molecules can work together against the knife.

Dr. Mellendorf



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