# Want to Lie on a Bed with Nails? Physics Has Your Back

Cranking the numbers gave me 1,394 nails — which is actually not very many. The 40-by-40 square grid is 1,600 nails, and that’s more than you need to avoid skin piercing.

So, what if you replace the nails with a set of broken glass? It’s exactly the same. Of course, glass can be sharper than nails, but it also has some flat edges. As long as the contact area is large enough, the glass will not hurt anyone.

That’s the secret: No need for hard skin, just some physics.

Crushing Rocks, Mass, and Hastening

Now, let’s move on to the demonstration part when a soldier hits a rock on the dude’s chest as he lies on the bed with nails. The key physics lesson here includes Newton’s second law. It is a relationship between the net force of an object (Fnet), the mass of the object (m), and the velocity of the object (a). If the object is forced to move only in one dimension (to make things easier), then we can represent it as the following equation:

Speeding up an object tells you how the speed of the object changes. That is, if the object remains non -stop, then the speed is always zero, which becomes a zero acceleration. However, even if the object is moving, it can have zero acceleration as long as its speed does not change. If the object increases in speed, then it has a positive value for acceleration. That means that when something slows down, it has a negative acceleration. (Note: This claims to move in one dimension.)

Here’s an example: Suppose two people are each standing on a skateboard. (These are zero-friction skateboards — you can find them at the physics store.) On one board there is an adult who weighs 80 pounds, and on the other a child who weighs 40 kilo. If I push the adult with a speed of 80 newtons, it will give a speed of 1 meter per second per second (1 m / s.2). If I push with the same force as the child, the acceleration will be twice as large (2 m/s2), because the mass of the child is half the mass of the adult. 