This caller video from North Korean authorities tv showing soldiers performing feats of strength was impressive; it included hand-to-hand combat, breaking boards, smashing done bricks, and it besides demonstrated immoderate physics. Although I don't cognize overmuch astir martial arts, I cognize thing astir physics, truthful I'm going to explicate however that portion of it worked.
The objection goes similar this: First, a dude lays down connected a furniture of nails. After that, his person places a ample chromatic connected his thorax and smashes it with a sledgehammer. I'm going to exemplify it with my ain diagram:
The objection really has immoderate variations. Another mentation replaces the nails with a clump of breached solid connected the ground. But don't worry, the basal physics are each the same. Let maine commencement with the furniture of nails.
Bed of Nails, Pressure, and Force
Suppose you usage your unfastened manus to propulsion connected an interior partition successful your house. (You know, the benignant that’s made with sheetrock.) Most likely, thing volition happen. No substance however hard you push, you’re not going to poke your manus done the wall. But if you repetition this aforesaid propulsion utilizing the extremity of a metallic nail alternatively of your hand, it's very apt that the nail volition penetrate the wall. In some cases, the magnitude of unit is the same, but the unit is different.
We specify unit arsenic the magnitude of unit per area.
If the unit is measured successful units of newtons and the country successful quadrate meters, past the unit would beryllium successful newtons/meter2. We besides telephone that 1 pascal.
Of course, immoderate radical similar different units, specified that 1 N/m2 is adjacent to 0.000145 pounds per quadrate inch (psi.)
Let's spot if we tin estimation the quality successful unit connected the partition erstwhile pushing with a manus versus with a nail. I measured the size of my hand, and it's astir a rectangle of astir 8 by 15 centimeters. That puts the country astatine 0.012 quadrate meters. Now, let's presume I tin propulsion with a unit of 100 newtons. (That would beryllium 22 pounds.) This means that the unit from my propulsion connected the partition would beryllium 8,333 pascals.
The pointy extremity of a nail is beauteous small. By my estimation, it has a diameter that's little than 1 millimeter—so let's conscionable circular up and accidental it's 1 mm. That means it has a interaction country adjacent to the country of a ellipse with a diameter of 1 mm, oregon 7.85 x 10-7 m2. (Yes, that's tiny.) Pushing connected this country with the aforesaid 100-newton unit gives a unit of 1.27 x 108 pascals. That's way higher than the unit from pushing with your hand—and that's wherefore the nail pokes done the wall.
But what astir the furniture of nails? If you sat connected a azygous nail, you cognize what would happen. One nail has a precise tiny country and produces a ample pressure, which means that it would conscionable interruption close done your skin. However, if you person a whole bunch of nails you volition beryllium fine. Having galore nails efficaciously increases the area, which reduces the pressure.
So however galore nails would you request to prevarication connected successful bid to beryllium safe?
It turns retired that it takes a unit of astir 100 psi (689,000 pascals) to puncture quality skin. Let’s accidental you person a quality with a value of 750 newtons. (This is the unit that volition beryllium pushing down connected the furniture of nails.) You request that unit to beryllium dispersed retired implicit an country truthful that the unit connected immoderate idiosyncratic nail is ne'er much than 100 psi.
With this minimum aboveground country of 1.1 x 10-3 m3 and my estimation of the country for 1 nail, I tin cipher the fig of nails required for the bed. (I'm going to usage unit successful units of pascals.)
Cranking the numbers gives maine 1,394 nails—which is really not that many. A 40-by-40 quadrate grid is 1,600 nails, and that's much than you request to forestall a tegument puncture.
So, what if you regenerate the nails with a clump of breached glass? It’s truly the aforesaid thing. Sure, solid mightiness beryllium sharper than nails, but it besides has immoderate level parts. As agelong arsenic the interaction country is ample enough, the solid won’t wounded anyone.
So that’s the secret: It doesn't instrumentality pugnacious skin, conscionable immoderate physics.
Smashing Rocks, Mass, and Acceleration
Now, let's determination connected to the portion of the objection erstwhile a worker smashes a stone connected apical of the dude's thorax arsenic helium lies connected the furniture of nails. The cardinal physics acquisition present involves Newton's 2nd law. This is simply a narration betwixt the nett unit connected an entity (Fnet), the wide of the entity (m), and the object's acceleration (a). If the entity is constrained to lone determination successful 1 magnitude (to marque things easier), past we tin correspond this arsenic the pursuing equation:
The acceleration of an entity tells you however that object's velocity changes. So, if the happening is conscionable staying still, past the velocity volition perpetually beryllium zero, which would beryllium a zero acceleration. However, adjacent if the entity is moving, it tin person a zero acceleration arsenic agelong arsenic its velocity doesn't change. If the entity is expanding successful speed, past it would person a affirmative worth for the acceleration. That means that erstwhile an entity slows down, it has a antagonistic acceleration. (Note: This assumes question successful 1 dimension.)
Here’s an example: Suppose 2 radical are each lasting connected a skateboard. (These are zero-friction skateboards—you tin find them astatine the physics store.) On 1 committee is an big with a wide of 80 kilograms, and connected the different is simply a kid with a wide of 40 kilograms. If I propulsion connected the big with a unit of 80 newtons, it volition supply an acceleration of 1 metre per 2nd per 2nd (1 m/s2). If I propulsion with this aforesaid unit connected the child, the acceleration volition beryllium doubly arsenic overmuch (2 m/s2), since the kid’s wide is fractional that of the adult.
But what if I propulsion connected thing with a ace ample force, similar the benignant that you would get from the interaction of a sledgehammer? It turns out, it doesn't substance however ample the unit is: Higher wide objects volition ever person a smaller acceleration (with the aforesaid applied force).
Here is simply a amusive physics demo that you tin effort astatine location (and it's overmuch safer than utilizing a sledgehammer). Get an entity with immoderate important mass—something similar a afloat vessel of h2o should work. Now get 2 pieces of drawstring oregon thread, thing that is lightweight capable that you could interruption it. Use 1 drawstring to bent the object. Attach the different to the bottommost of the entity and permission it hanging escaped below, similar this:
Now, propulsion connected the bottommost drawstring that’s hanging down. (Remember, these are anemic strings.) Which drawstring volition interruption during the pull?
The reply is: Either 1 tin break, depending connected however accelerated you propulsion it.
If you propulsion the bottommost drawstring precise slowly, the apical drawstring volition break, since it has to enactment the value of the hanging entity and the unit of your downward propulsion connected the bottommost string.
However, if you propulsion the bottommost drawstring with a speedy jerking motion, it breaks. This is due to the fact that the hanging entity has a mass—and callback that it’s hard to accelerate masses. This means that the wide doesn’t really determination close away, and the apical drawstring doesn’t get stretched into breaking. I cognize it seems crazy, but it’s true. It's astir magical.
Here is what that looks similar successful existent life, on with much explanation:
This doesn't conscionable enactment for hanging objects—it besides works for ample chromatic blocks sitting connected apical of North Korean soldiers. You tin astir apt ideate that a nonstop stroke from a sledgehammer to the man’s thorax would not extremity well. But erstwhile the chromatic is added connected apical of his chest, it's imaginable to deed the chromatic precise hard—hard capable to interruption the object—without harming the worker due to the fact that the stone’s ample wide prevents it from accelerating down and smashing into him. (In fact, if the worker dilatory pushed down connected the chromatic with the hammer, oregon if possibly helium stood connected it, the feline who’s lying down would beryllium injured. It would beryllium conscionable similar the dilatory propulsion connected the bottommost drawstring of the bottle.) I'll admit, it looks precise impressive.
You cognize what would beryllium adjacent much impressive? If they smashed a tiny stone connected apical of the guy. That would genuinely beryllium a feat of superhuman strength.
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