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Physics of Rain

Knowledge I’ve gained after spending Friday and Saturday learning about policy debate and Sub-Saharan Africa: “The Malaria epidemic is like loading up seven Boeing 747 airliners each day, then deliberately crashing them into Mt. Kilimanjaro.”  : (  How can such a statistic not be depressing?

As for physics…it started pouring on my way home on Saturday.  (Which was actually quite a liberating experience.) 

Let’s take the hypothetical average raindrop to be a few millimeters in diameter.  With only gravity, the drop would accelerate towards the Earth at 9.8 m/s².  That’s pretty scary considering that it falls from thousands of feet above sea level.  Luckily for us, there’s air resistance, an upward force which opposes motion in air.  Drag will increase as the velocity of the raindrop increases.  When that force is equal to the downward force of gravity, the raindrop reaches its maximum or terminal velocity.  Since a raindrop is so small, it reaches its terminal velocity rather quickly and therefore won’t cause too much physical harm to those out in the rain.

Furthermore, we could even calculate the change in potential energy as rain (it’s probably too difficult to find the mass in kg of a single droplet) falls to the ground with the handy equation below! 

ΔPE = mgΔh

But we would expect the actual kinetic energy released from rainfall to be a much smaller number due to the opposing drag force. 

Anyways, throughout this whole experience I kept remembering the famous question, “Do you get wetter from walking or running in the rain?”  I’m sure the answer is physics-related… maybe Doc could answer this. : )

Rain.jpg Rain picture by xtaintedwatersx

FYI: In the time it’s taken you to read this post, 4 more children have died from malaria. 

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