Calculating these numbers gave me 1,394 nails-not that many actually. A 40 x 40 square has 1,600 nails, which exceeds the number required to prevent skin punctures.
So, what if you replace nails with a pile of broken glass? This is really the same thing. Of course, glass may be sharper than nails, but it also has some flat parts. As long as the contact area is large enough, the glass will not hurt anyone.
So this is the secret: it does not need hard skin, only some physical knowledge.
Crushing rocks, mass and acceleration
Now, let’s continue the part of the demonstration, when he was lying on the bed of nails, a soldier smashed a stone on his chest. The key physics lesson here involves Newton’s second law. This is the resultant force acting on the object (Fnet), the mass of the object (m) and the acceleration of the object (a). If the object is restricted to move in only one dimension (making things easier), then we can express it as the following equation:
The acceleration of an object tells you how the speed of the object changes. Therefore, if the object just stays still, then the speed will always be zero, that is, zero acceleration. However, even if an object is moving, as long as its speed does not change, its acceleration can be zero. If the speed of the object is increasing, then its acceleration will be positive. This means that when an object decelerates, it has a negative acceleration. (Note: This assumes one-dimensional motion.)
This is an example: suppose two people are each standing on a skateboard. (These are zero-friction skateboards-you can find them in physics stores.) One board is an adult weighing 80 kg, and the other is a child weighing 40 kg. If I push an adult with a force of 80 Newtons, it will provide 1 meter per second (1 m/s2). If I push the child with the same force, the acceleration will be twice (2 m/s2), because the weight of a child is half that of an adult.