Rain - Run or Walk?
Date: 08/05/2003 at 23:53:58 From: Simon Wood Subject: Rain Is there an optimum speed at which to walk through the rain to minimise the number of drops you get hit by? I'm not very good at calculus and I think it is necessary to work it out. Say you want to travel a distance in the rain. If you stand still (velocity=0), you get an infinite amount of rain on the surface of your head. As your velocity increases, the number of drops on your head decreases while the number of drops on the front of your body increases. If you run really fast (velocity = infinite), you get a fixed number of drops on the front of your body based on the density of the rain. I believe there is an optimum speed based on the proportion of the areas of the front of the body to the head, and the speed of travel versus the speed of the rain which minimises the number of drops that hit the body, but I can't prove it. I've thought about this for about 10 years, but am no closer to an answer. Help, please!
Date: 08/06/2003 at 09:56:20 From: Doctor Edwin Subject: Re: Rain Dear Simon, I'm so glad you asked, because this problem has been niggling at me for years and I have never taken the time to think it through. Here's what it looks like to me. There are, as you pointed out, two sources of wetness - the rain that runs into you (on your head and shoulders), and the rain that you run into (on your front). The rain that runs into you is a constant amount per unit time. The longer you're out there, the wetter you get. If you're out 30% longer, you get 30% wetter. But the rain that you run into is a constant amount per unit distance travelled. I know it seems like you're getting more water if you run into it. But if you don't move at all you get zero water on your front. If you move very slowly, you get almost no water on your front per minute, but you're out there for many minutes. In other words, the amount of water on your front depends on entirely on how far you go, not on how fast. Here's another way to look at it. You're going to pass through a volume of watery air, and you're going to absorb all the water that's in the air that you pass through. The amount of water per unit volume doesn't change over time - the number of drops falling out of that volume are the same as the number falling in - so it doesn't matter when you get to a particular piece of that volume or how fast you're going when you get there. Imagine if we could stop gravity for a few minutes so the raindrops would just stay where they are. Would it matter how fast you went? So running faster means you run into just as much rain, but you're out there less time, so less rain runs into you. Here are some more things to think about: - What if the wind is moving in the same direction you are? If you can run at the same speed as the wind, you don't run into any rain. But if it's blowing slowly, then the amount that falls on your head during your slow walk might be greater than the amount you ran into if you went faster than the wind. The best case would be if the wind is moving along with you, exactly as fast as you can run. If it's going slower than that, then you have to bite the bullet and do the calculus problem. - What if you're going fast enough that you can lean your body into the wind? If you were leaning at exactly the right angle, you'd get no rain on your front or back, but you'd take more rain on your head. Would you get more wet or less? - What if you'd just rather get the water on your head and shoulders than down your whole front? I just did a Web search and came up with this link: To stay drier, do you walk or run in rain? If you walk, researchers say, you're all wet - Eric Sorensen, Seattle Times http://seattletimes.nwsource.com/html/localnews/134370003_rain23m.html Apparently someone actually did this as a physical experiment. - Doctor Edwin, The Math Forum http://mathforum.org/dr.math/
Search the Dr. Math Library:
Ask Dr. MathTM
© 1994- The Math Forum at NCTM. All rights reserved.