# Raindrops That Might Kill Like A Bullet…

A few days ago, in the early morning I listened in to the  weather forecast telling us that we would have a hot sunny day for the whole day long, actually indeed we were all scorched by the relentless blazing  sun until about 4pm when, suddenly the skies turned grey and the masses of cloud gathered  up. All of sudden, not within 10 minutes, the lightnings began to flash away, followed by roaring thunders. In no time rain started to fall and soon it bucketed down.

As I looked out through the window (I was still in the office) I was thinking to myself (Heck I have no idea why every time it rains while I’m not having anything else to do, my so-called creative thinking always rushes readily in to my head) that led to this writing. I was thinking to myself, what if there is no atmosphere to protect us from the falling droplets? What would happen? Readers might think “are you kidding me??” What is so dangerous about tiny raindrops?? They can’t hurt us no matter how hard it’s tipping down!

Okay, don’t readily jump to the conclusion like that, remember my article on the falling cat? (Sorry, it’s written in Bahasa Indonesia for those who are English readers) The atmosphere acts on the falling cat by preventing it from falling with  ever-increasing speed. It acts like a buoy (more or less)  on the cat. And of course if it acts on the cat to “slow down” the fall speed, the atmosphere also acts on the raindrops in the same manner. So that’s why in the absence of the atmosphere raindrops will fall down with ever-increasing speed! As we know, the speedier thing that hits us, the more force would work on us no matter how small it is.

To understand the nitty-gritty of this case, assume this simplified scenario that we can live on the planet where there is no atmosphere to breathe in, and we survive cosmic rays battering us. Suppose, clouds form in the absence of the atmosphere (which is practically impossible). The droplets of rain fall from 4000 metres overhead. Let’s assume that a droplet of rain weighs about one gram. Let’s see what is the speed of the raindrop when it hits our heads? Using a formula of the falling body problem we can determine the raindrop’s speed when it hits our heads, it then would be: $v = \sqrt{2gh}$ or when we substitute the data given for the variables it’s gonna be $v = \sqrt{2 \times 9.8 \times 4000}$ or 280 metres per second that it approxiamtes to 1000 kilometres per hour! (g is the earth’s gravity constant which is 9.8 m/s2. Now to see how much potential ‘destructive’ work it has on us, we have to find the energy kinetic of the raindrop: $E_{k} = \frac{1}{2} m v^{2}$ , Ek denotes the kinetic energy, 1 gram of droplet equals 0,001 kilogram. So, if we put the data into the equation we’re gonna have $E_{k} = \frac{1}{2} \times 0.001 \times 280^{2}$ that approxiamtes to 40 joules.

Forty joules? That’s it? Yeah…. but remember it only comes from a single raindrop. If you are battered by a thousand raindrops, roughly calculated that the force that would work on you will be 40,000 joules! To see how monstrous 40,000 joules is, let’s compare it to the kinetic energy possessed by a 150 kg motorbike that runs at 80 km per hour *).  It has a kinetic energy of $E_{k} = \frac{1}{2} \times 150 \times 22^{2}$ or 36,300 joules. So, a thousand raindrops would have more kinetic energy than a 150 kg motorbike running at 80 kph does if there was no atmosphere to buoy the falling raindrops!

That explains whenever it rains why should we be very grateful to be saved by the atmosphere from the falling droplets of rain, whilst in the absence of the atmosphere  the tiny droplets would potentially kill us like a bullet, or at least they  could hurt us very badly!

P.S.

I do miss my high-school physics class now 😦

*) 80 kph equals 22 metres per second.

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1. Hi Yari,