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Why do Sharp Things Prick?

Vishwas Purohit Oct 03, 2020
'Don't go near that...It's sharp'. All of us have heard it before. But what is about sharp things that we are scared of? 'It can cut and prick'. Sure, but why? A scientific explanation.
Have you ever wondered why a needle so easily pierces things? Why is it so easy to drive a needle through a piece of cloth or cardboard, and so hard to do the same thing with a blunt nail? Doesn't the same force act in both cases?
The force is the same, but the pressure isn't. In the case of the needle, the entire force is concentrated on its point; in the case of the nail the same amount of force is distributed over the larger area of the blunt end. So, though we exert the same force, the needle gives a much greater pressure than the blunt nail.
We know that a twenty-tooth harrow loosens the soil more than a sixty-tooth one of the same weight. Why? Because the load on each tooth of the first harrow is more than that on each tooth of the second.
When we speak of pressure, we must always take into consideration, beside forces, also the area upon which this forces act. For example, when we are told that a worker is paid 20 dollars, we don't know whether this is for a whole year or for one month or just for a week.
Similarly, does the action of a force depend on whether it is distributed over a square centimeter or concentrated on the hundredth of a millimeter?
Skies easily take us across fresh snow; without them we fall through. Why? On skies the weight of your body is distributed over a much greater area. Supposing the surface of our skies is 20 times more than the surface of our soles, on skies we would exert on the snow a pressure which is only a twentieth of the pressure we exert when we don't have skies on.
Fresh snow will bear you when you are on skies, but will treacherously let you down when you're without them.
For the same reason horses used in marshlands are shod in a special fashion giving them a wider supporting area and lessening the pressure exerted per square centimeter. For the same reason people take similar precautions when they want to cross a bog or thin ice, often crawling to distribute their weight over a greater area.
Finally, tanks and caterpillar tractors don't get stuck in loose ground, although they are very heavy, again because their weight is distributed over a rather great supporting area. An eight-ton tractor exerts a pressure of only 600 grams per square centimeter.
The caterpillars, which exert a pressure of only 160 grams per cm square, despite a two-ton load, which makes for the easy crossing of peat bogs and sand-beaches. Here, the large supporting area gives the advantage, whereas in the case of the needle, it is the other way round.
This all shows that a sharpened edge pierces things only because it has a very minute area for the force to act upon.
This is why a sharp knife cuts better than a blunt one; the force is concentrated on a smaller area of the knife edge. To sum up: sharp objects prick and cut well, because more pressure is concentrated on their points and edges.