Why doesn’t rain kill you ?

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Presentation transcript:

Why doesn’t rain kill you ? Or at least hurt when it hits you?

Scrap paper question Why doesn’t rain kill you? How fast is a raindrop falling when it hits you? Was it in ‘freefall’? Why or why not?

If a raindrop was in freefall, what info do we need to estimate its final velocity? The time it takes to fall? The vertical distance it falls? Simplified equations y = 5t2, v = 10 t

Since we can’t time it, …. Get info on vertical height of rainclouds The bottoms of rainclouds are generally below 2000 m or 6500 ft Let’s be conservative and say that the rain falls from a height of 1500 ft or 460 m

Calculate the time it takes to fall to the ground and the speed at that time using y = 5 t2 and v = 10 t 460 = 5 t2 460/5 = t2 t =9.6 sec V = 10 (9.6) = 96 m/s , 316 ft/s , 214 mph!!!! Is this reasonable??

So…why doesn’t rain kill you? Thoughts? Air drag slows it down!

How does drag work? It’s complicated but it largely depends on The square of the object’s speed http://www.grc.nasa.gov/WWW/K-12/airplane/drag1.html When the drop falls fast enough, the drag is large enough to balance the force of the earth pulling on drop The drop reaches its ‘terminal velocity’ and stops speeding up or accelerating!!

What is the terminal velocity of a typical raindrop? At sea level, a large raindrop about 5 millimeters across (house-fly size) reaches a terminal velocity of 9 meters per second (20 miles per hour). That’s more like it! Kingda ka, raining, raindrops tore the test dummy’s skin??, original speed was faster than 128 mph??