Pop quizOctober 2, 2007
You need to refill your water bottle, so you go to a nearby pair of drinking fountains, one of which is closer to the ground to permit easier access by children, people in wheelchairs, et al. To fill your bottle as completely as possible, you should use…
A. the higher fountain
B. the lower fountain
C. whichever one has a better stream, which you can’t know until you’ve tried both
Until recently, I had never thought carefully about this question. That must be why I’ve gotten it wrong hundreds of times over the past 15 years or so. Almost invariably, I’d activate the higher fountain, be disappointed by its sad little trickle of water, try the lower fountain, and receive a stronger, higher-arching stream that could be more easily directed into the bottle. I never learned anything from this experience because I always attributed the better flow of the lower fountain to “luck.” Such a scientific mind, eh?
Then last week it occurred to me that pressure is needed to drive water through the fountains, and that the higher the water climbs against gravity, the more the pressure will dissipate. The water is under higher pressure when it exits the lower fountain, so it’s ejected more forcefully.
I don’t know why it took me so long to realize such a simple truth, but the reason probably relates to the fact that I’m a biologist rather than a physicist.