Cranking the numbers provides me 1,394 nails—which is definitely not that many. A 40-by-40 sq. grid is 1,600 nails, and that is greater than it’s worthwhile to forestall a pores and skin puncture.
So, what in case you change the nails with a bunch of damaged glass? It’s actually the identical factor. Positive, glass could be sharper than nails, but it surely additionally has some flat components. So long as the contact space is giant sufficient, the glass received’t damage anybody.
In order that’s the key: It would not take robust pores and skin, just a few physics.
Smashing Rocks, Mass, and Acceleration
Now, let’s transfer on to the a part of the demonstration when a soldier smashes a rock on prime of the dude’s chest as he lies on the mattress of nails. The important thing physics lesson right here entails Newton’s second legislation. It is a relationship between the online power on an object (Fweb), the mass of the thing (m), and the thing’s acceleration (a). If the thing is constrained to solely transfer in a single dimension (to make issues simpler), then we will symbolize this as the next equation:
The acceleration of an object tells you the way that object’s velocity modifications. So, if the factor is simply staying nonetheless, then the speed will continuously be zero, which might be a zero acceleration. Nevertheless, even when the thing is shifting, it could actually have a zero acceleration so long as its velocity would not change. If the thing is rising in velocity, then it will have a optimistic worth for the acceleration. That signifies that when an object slows down, it has a unfavourable acceleration. (Be aware: This assumes movement in a single dimension.)
Right here’s an instance: Suppose two individuals are every standing on a skateboard. (These are zero-friction skateboards—yow will discover them on the physics retailer.) On one board is an grownup with a mass of 80 kilograms, and on the opposite is a baby with a mass of 40 kilograms. If I push on the grownup with a power of 80 newtons, it’ll present an acceleration of 1 meter per second per second (1 m/s2). If I push with this identical power on the kid, the acceleration might be twice as a lot (2 m/s2), for the reason that child’s mass is half that of the grownup.