TECHNOLOGY

# Want to lie on a bed of nails? Physics your back

Moving the numbers gives me 1,394 nails – which is actually not that many. A 40″ x 40″ square grid is 1,600 nails, that’s more than you need to prevent leather punctures.

So, what if you replaced the nails with a set of broken glass? It’s really the same thing. Sure, the 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.

Here’s the secret: It doesn’t take tough skin, just some physics.

Smashing rocks, mass and acceleration

Now, let’s move on to part of the demonstration when a soldier smashed a stone into the man’s chest as he lay on a bed of nails. The basic physics lesson here includes Newton’s second law. This is a relationship between the total force on the body (FClear), body mass (m), and body acceleration (a). If the object is constrained to move in only one dimension (to make things easier), then we can represent this as the following equation:

The acceleration of an object tells you how the velocity of that object is changing. So, if the thing remains constant, the velocity will constantly be zero, which will be zero acceleration. However, even if an object is moving, it can have zero acceleration as long as its velocity does not change. If the velocity of the body is increasing, it will have a positive value of acceleration. This means that when the body is decelerating, it has a negative acceleration. (Note: This assumes movement in one dimension.)

Here’s an example: Suppose everyone is standing on a skateboard. (These are friction-free skateboards – you can find them in the physics store.) On one board there is an adult with a mass of 80 kilograms, and on the other plate a child with a mass of 40 kilograms. If you put pressure on the adult with a force of 80 newtons, it will provide an acceleration of one meter per second per second (1 m/s)2). If you press the same force on the child, the acceleration will be twice that (2 m/s2), because the mass of a child is half that of an adult.

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