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Pressure Pressure is defined as force exerted over a certain area, and is given by the equation:  Because it is force over area, the units would be Newtons over square meters, which has been given the name Pascals (After the French mathematician and physicist Blaise Pascal), which can be expressed as Pa. Question One: What is the relationship between pressure and area? It is obvious by now that the pressure has an inverse relationship with area, and subsequently a direct straight-line relationship with force. The relationship with force is quite obvious, as one could surmise the larger the force the larger the pressure would be. The relationship with area is the one that helps explain several apparent tricks.
If you take your index finger and press hard agaisnt your chest, it can cause some pain. The reason: the area of contact is small, so the pressure is greater. If you consider stepping on one nail that is sticking through the floor, you can imagine that it would cause even more pain. Again, the area of contact with the nail is even smaller, so the pressure is even greater. So great in fact, it can puncture the skin more easily. So, we can express all of this with the following concept equation:
 Now consider pressing your whole hand agaisnt your chest with the same force you used with the single finger. It doesn't hurt at all. Why? The contact area is much larger, so the pressure is a lot less. This is exactly why people are able to lie on a bed of nails without getting hurt. One nail causes considerable pain, but a bed of nails consists of several. With each nail added, the contact area gets increases thereby decreasing the pressure. Therefore, if you have enough nails on your bed, you can decrease the pressure enough to stop the nails from puncturing your skin. However, I'm not going to say a bed of nails doesn't hurt at all. If you watch someone get up from one, you can see the indentions each nail caused in the skin, which probably does cause some discomfort. And of course, the less nails used, the more pain it will cause. The point is, the bed of nails is not a trick, but really a physics lesson.  Atmospheric Pressure |