📜 ⬆️ ⬇️

Killer drop of kefir

For every sage is quite simplicity.
A. N. Ostrovsky

Sometimes when reading articles or comments on Habré, I would like to exclaim: Sir, if you could realize how much stupidity was written, you would die of laughter!
People sometimes put reality upside down and, surprisingly, for this they get the “pluses” of encouraging other users of Habr.
To the article about the dangers of emissions of mobile phones, WiFi, etc. there was such a comment, which the public appreciated very highly (+6 at the time of writing these words):
Maximum radiated power (watts):
GSM-1800 - 1.
GSM-900 - 2.
CDMA2000 - 0.25.
WiFi - 0.1.
And the energy of a bullet is about 1000 J (0.2 kcal, as in a drop of kefir)
But this does not prevent her from killing ...
Power is not yet an indicator ...

Let's see if power is an indicator or not by solving the following problem.
The bullet, the kinetic energy of which is 1000 J, hits the person and kills him. Assess what the average power of the bullet when interacting with the human body. For simplicity: the mass of the bullet is 8 grams, the bullet penetrates the body by 10 cm and stops. Answer options:
The power of the bullet in these conditions is comparable to the power of the motor ...
A. Coffee grinders (20 W)
B. Electric Drills (300 W)
C. Moped (4000 W)
D. Hummer H2 car with a 6.2 liter engine (300,000 watts)
E. Car BelAZ 75600, the largest truck produced in the CIS: own weight of 240 tons, payload of 320 tons, total weight of 560 tons (2.6 million W)

Let's count. What is “average power”? This is energy divided by time. Energy is known to us: 1000 J. According to the condition, the bullet completely stops in the body, i.e. it loses all its kinetic energy. It remains to determine the time during which the bullet stops in the body. Someone else did not have time to forget that the kinetic energy is equal to half the body mass multiplied by the square of speed:

E = ½ · m · v 2
From here we find the speed of the bullet:

v = (2 · E / m) ½ = (2 · 1000 J / 0.008 kg) ½ = 500 m / s

The bullet flew at an initial speed of 500 m / s and, after passing 10 cm, stopped. How long did she move? For simplicity, we consider the motion of the bullet uniformly accelerated. Then, perhaps, not all had time to forget that with uniformly accelerated motion, the distance traveled s can be expressed through acceleration a and time t like this:

s = ½ · a · t 2

But we do not know the acceleration either. How to be? Express acceleration through speed and time:

a = v / t


s = ½ · (v / t) · t 2

or, after shortening t on the right, multiplying both parts by 2 and dividing by v:

t = 2 · s / v = 2 · 0.1 m / 500 m / s = 0.0004 s

The bullet makes a fatal hole in the body in 1 / 2500th of a second!
Find the final answer, i.e. average bullet capacity:

P = E / t = 1000 J / 0.0004 c = 2500000 W (!!)

Two and a half megawatts! This is only slightly less than the capacity of the most powerful BelAZ (2.6 MW), carrying, besides its own 240 tons, another 320 tons of cargo. This is more power of medium-large datatsentr (2 MW).

The bullet, in fact, has enormous kinetic energy for such a tiny object, and consumes this energy at an incredible speed: in just 1 / 2500th of a second. That is why it has such an impressive power, which is more than a million times higher than the radiation power of a GSM phone and, as a result, it kills. So, paraphrasing the well-known saying, “power matters.”

Why not kill a drop of kefir? First, a drop (1/12 gram) of kefir has an energy value much less than 1000 J. It is, rather, the caloric content of a drop of honey. Well, well, why doesn't a drop of honey kill? Do not tear apart the poorest hamster, which breaks a drop of nicotine?
I have almost no doubt, dear reader, if you have read this far, you know the answer ...

Source: https://habr.com/ru/post/100684/

All Articles