All right guys, maybe you know the answer, but maybe not. So, what makes something sharp and something blunt? Did you even stop and think about it? Here I will try to explain, why are knives, but also needles and other objects sharp. And does the material they are made from play role in it?
Let’s find out!
In definition sharpness is the cutting ability of an edge. So, basically the sharper something is the better it is at cutting. And cutting is defined as an act of removing a part of something from the whole thing. Great definitions, right? Do I have a clue what sharpness is at this point? No.
Let’s put these definition to a real-life situation. I want to cut an onion. I take a butter knife and press against the onion. Well, surprisingly, nothing really happens. Yeah, I am strong, and I could cut the onion even with butter knife if I really wanted to. I think. But I would have to use a lot more force. So, I take a kitchen knife and cut the onion. With ease. Hmm. What observations did we get from this experiment?
Well, first of all butter knife and kitchen knife are different. In what? Apart from shape and size the main difference lies in their blade. Kitchen knife has much thinner blade. Another thing – we could probably cut the onion with both knives. But. We would need to use much higher force in the case of butter knife. Here we get to basic physics:
The p stands for pressure, F for force and A for area. And the definition for pressure goes like this: It is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. What does this mean in real life. Well imagine these two already mentioned knives – butter knife and kitchen knife. Imagine they are almost the same size. Now imagine that you apply the same force to both, meaning that you press against the poor onion with equal force. In the case of butter knife nothing happens, but in the case of kitchen knife you cut the onion. Why is that? The answer – even though force is constant, area of the blade is not, thus when you decrease this area the pressure exerted by the blade has to increase! And it is this high pressure that allows the knife to cut through a material (such as onion) by breaking the bonds between crystals, fibres or molecules in the material. And the same principle applies to needles, swords, guillotines and I don’t know what else. Thin means sharp!
But. Even though this is important, there is also another thing or two to consider. Material of the cutting edge. The blade has to be strong enough to resist breaking before the other material gives away. But hardness is not always the best way to go. Why you ask? Increase in hardness usually means increase in brittleness as well. If you compare for example steel knives and ceramic knives, then ceramic knives win in the contest of sharpness. Steel axe is great at chopping wood, because it is much harder than the wood, but also tough enough to withstand the dynamic impact. Ceramic knives on the other hand are harder than steel, but also more brittle – so do not drop one on the floor. Basically, the answer lies in the application of the blade. So, if you want to have precisely cut steaks use ceramic knife, but please, do not take ceramic sword for duelling – you would probably die. Another thing is the shape of the blade, that is also the reason why we do not chop wood with kitchen knives and vice versa. Rapiers are good at fast stabbing, whereas lacking strength and cutting ability. On the other hand, khopesh or machete are great for cutting. Saw or bread knife represent another example- they have serrated blade and thus are great at cutting soft or fibrous material (wood, bread). I am really no expert in blades or physics, but yeah this makes sense, right? :D
Hope you did all learn something new today. Even though things like this are something common, we usually don’t even stop to think about the mechanisms that underline the application of such basal technologies. I did get a bit out of my biological comfort zone, so hopefully I said everything as it is and will not be pursued by physics-hunters.
Stay sharp!
Blade – Wikipedia article
And my own knowledge and experience.. :D
