Will you be dryer if you walk or run in the rain, and what effects would the intensity and the angle of the rain falling have on this theory? Logic to me would suggest if the rain falling perfectly downwards then it would make no difference if you were to run or walk due to being hit by an increased volume of rain when running.
Imagine running infinitely quickly. You'll cut a path through the raindrops in the shape of your silhouette, but no drops will fall on you. Slowing down allows drops to fall on your head in addition to hitting you horizontally, therefore it will make you wetter.
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Dryer over a certain period of time, or dryer over a certain distance? Are you interested in a theoretical account (like Derakon answered) or also in practical effects, like splashing puddles or additional rain swept through flailing your arms?
There's a mythbusters episode about it, where they did some actual tests.
There are actually 2 Mythbusters episodes, with conflicting results.
Tub hits the nail in the head with his first two questions: Derakon's answer is for the theoretical case, with uniform rain distribution and considering your silhouette to be uniform as you walk/run through the rain.
The answer is quite different if you are talking about a certain period of time (in which case it is better to stand still) or over a certain distance (in which case running is better); and effects from changing the uniform rain density, wind speed and silhouette can likewise make the answer swing back and forth between running and walking as the better choice.
Imagine running infinitely quickly. You'll cut a path through the raindrops in the shape of your silhouette, but no drops will fall on you. Slowing down allows drops to fall on your head in addition to hitting you horizontally, therefore it will make you wetter.
On the other hand, water that hits the body is not immediately distributed over your whole surface, so where it hits matters, not just the total amount of water encountered.
If you are only hit from above, your hair and shoulder clothes can absorb a fair amount of water, and even when saturated, some water will drip from there directly to the ground. At least for a short exposure, you can think of the upper surface of your body as an umbrella (or sponge) for the lower ones. That part will get wet, but the rest is shielded (for a while).
By running, you expose the whole front of your body (and especially the upper surface of the legs) to the rain, which could lead to a larger surface area of your body getting somewhat wet, instead of a small part getting very wet.
I think the question of whether to run or not in the rain is much more complicated that people give it credit for. The answer will depend on what sort of wetness, what clothes are worn, how long one's hair is, how heavily it is raining, the size of the drops, wind, how far one needs to go, etc.. But I think, for reasonable assumptions about the above, the answer will be "walk" for short distances and "run" for longer ones.
There's a mythbusters episode about it, where they did some actual tests.
I had a look at that exert on YT yesterday, and I noticed that their experiment is flawed, because they should be weighing themselves, and not their overalls.
Imagine running infinitely quickly. You'll cut a path through the raindrops in the shape of your silhouette, but no drops will fall on you. Slowing down allows drops to fall on your head in addition to hitting you horizontally, therefore it will make you wetter.
Because I just noticed, I have to disagree with this.
a) Running at an infinite (or very high) speed, you'll sweep up an infinite (or very high) volume of rain-filled air with your front. Running slower, you'll sweep up less, which is good.
b) Even when running at high speeds, you still get the same amount of rain on your head as when standing still. Rain doesn't stop falling just because you're running! You would need relativistic effects or supersonic shockwaves for that to change.
Neither makes running a good idea.
To determine how much rain you get over time, all you care about are your exposed surfaces, their area and speed relative to the raindrops.
Let's assume you're a block of 2x1x0.5 meters, and you can run at 5 meters per second. Raindrops fall at about 10m/s. The rain is falling straight down and is uniformly distributed.
Standing still for a second, you'll be hit by all the raindrops that were up to 10 meters above you when we started measuring, and your exposed surface area is 0.5m², so you get 5 m³ worth of rain on your head.
In other words: the top surface of the block moves with 10m/s relative to the rain, so 10m/s * 0.5m² = 5m³ per second of rain.
Running at full speed for a second, you'll sweep up all the rain in front of you, which is 5m/s * 2m² = 10m³ per second. Clearly wetter. Additionally, you still get the full 5 m³ of rain on your head - the surface area is still 0.5m² and the relative speed to the rain is still 10m/s!
So over time, running will always make you wetter than standing still. You minimize your exposed surface area by standing upright parallel to the rainfall. If there's wind, tilt slightly against it.
Running will not only increase your surface area due to flailing limbs around, but will also pick up more rain due to the increased speed.
But it's rare to encounter a scenario where this is relevant. You're stranded in an infinite, empty parking lot and need to stay as dry as possible until the rain stops? Yeah, right. Doesn't matter, you'll be soaked.
The usual scenario is that you need to cross the street until the next cover, or you need to run from the car to your home's door. Is the additional wetness from running worth the savings from a shorter exposure? And then there's wind, often changing. And puddles. And everything. And the volume of water accumulated doesn't directly translate into being wet, much less into feeling wet and cold and miserable or into ruining your expensive leather purse. And that's where things get complicated.
Mitjitsu wrote:
because they should be weighing themselves, and not their overalls.
Unless they swallow the rain, that shouldn't matter much. The experiment is set up in such a way that most of the rain will be caught in the overalls. They were dry underneath.
But yes, like most experiments on an entertainment show, there are flaws to be found, and not all of their assumptions and ideas were explained well.
So over time, running will always make you wetter than standing still. You minimize your exposed surface area by standing upright parallel to the rainfall. If there's wind, tilt slightly against it.
I would have thought if there's wind involved. You'll want to run at the same speed and direction as it is blowing.
I would have thought if there's wind involved. You'll want to run at the same speed and direction as it is blowing.
For the simplified block, that's true. Either tilting or moving means that no rain will hit the sides, but moving reduces the relative speed of the rain against the top surface, so you'll get a bit less rain.
For a human, anything but standing upright with legs closed and both arms straight next to your torso increases your surface area, so any form of walking or running is a bad idea unless you need to get somewhere.
But while we're at impractical theoretical movements: try free falling.
Granted, tilting parallel to the rainfall without falling over might require the ability to adjust your body's density at will so that the wind can support your stance. But if you can't do that, you deserve to be wet, right?
If you were running at a nearly infinite speed, then you would be compressing air in front of you so much that it would actually start fusing, causing a thermonuclear explosion.
Even though your body couldn't take such a high-speed collision with air, and would be pulverized, I think that the explosion would start before you are dead. Thus, I think, you would be atomized by the explosion quicker than what the collision with air would.
Luckily, though, this all would happen in an infinitesimally small fraction of a second, so your brain would be physically unable to be aware of any of it. You would disappear before you would notice anything.
Also, luckily, this would be a pure fusion explosion, which means that there would be little to no radiation. But don't do it near any buildings, or they would get badly destroyed.
If you were running at a nearly infinite speed, then you would be compressing air in front of you so much that it would actually start fusing, causing a thermonuclear explosion.
Even though your body couldn't take such a high-speed collision with air, and would be pulverized, I think that the explosion would start before you are dead. Thus, I think, you would be atomized by the explosion quicker than what the collision with air would.
Luckily, though, this all would happen in an infinitesimally small fraction of a second, so your brain would be physically unable to be aware of any of it. You would disappear before you would notice anything.
Also, luckily, this would be a pure fusion explosion, which means that there would be little to no radiation. But don't do it near any buildings, or they would get badly destroyed.
A much fuller, and clearer explanation
Link to video
a) Running at an infinite (or very high) speed, you'll sweep up an infinite (or very high) volume of rain-filled air with your front. Running slower, you'll sweep up less, which is good.
b) Even when running at high speeds, you still get the same amount of rain on your head as when standing still. Rain doesn't stop falling just because you're running! You would need relativistic effects or supersonic shockwaves for that to change.
Isn't the premise of the question that you're running to get somewhere, so that the faster you move, the shorter your exposure to the rain? Otherwise the question isn't very interesting, nor very relevant for real-world situations. The running vs. walking from A to B situation is surely what Derakon was referring to, and also what I thought of in my answer. In this case, moving infinitely fast would prevent rain from falling on your head because it wouldn't have time to do so before you arrive at your destination - no need for relativity or shockwaves here.
That doesn't mean that I support the conclusion that running is always better, though - see my previous post.
I have been pondering: What are the most fundamental properties of this universe? The most basic phenomena that are not just a composition or a form of another, more fundamental element. The most "atomic" features of the universe, if you will.
I'm thinking: Spacetime, energy and the fundamental interactions.
Are those (as far as we know) the most fundamental properties of the universe, and is there something missing from the list?
Can everything be explained in terms of those things? (For example, can the behavior of subatomic particles, such as the double-slit experiment, be described with those fundamental properties? Or is there yet another fundamental property that's needed to explain it?)
if, a zonktillion years after the heat death of our universe, another big bang occured, would the laws of physics be the same? Or might they be utterly different if the fundamental particles generated by that bang are different than ours?
http://forums.xkcd.com/viewtopic.php?f=18&t=107636
Tchebu suggested locality and lorentz invariance, with the unspoken premise that the universe is free of infinite or negative infinite energy. Just read the full thread.
Stop me if you've heard this one before. It is answerable with secondary-school physics but has stumped the three physics PhDs I have asked:
In an experiment there are two spheres, one resting on a flat surface and one suspended from a string. The two spheres are completely identical in every other way. The two spheres start out at the same temperature, and each is supplied with the same quantity of thermal energy. The spheres are completely thermally isolated thereafter. Which gets hotter?
Answer:
The ball hanging from the string gets hotter. As materials heat up they expand. Thus in the case of a suspended ball its centre of mass drops and it loses some gravitational potential energy, which is converted into extra thermal energy. The reverse is true for the ball placed on a flat surface.
thatguy: see, I would have given the same answer, but for different reasons (viz. that the sphere resting on a flat surface is less thermally isolated because it can dump heat into the ground). Of course the properties of the surface it is resting on, and of the string the other sphere is suspended from, are not specified.
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thatguy: see, I would have given the same answer, but for different reasons (viz. that the sphere resting on a flat surface is less thermally isolated because it can dump heat into the ground). Of course the properties of the surface it is resting on, and of the string the other sphere is suspended from, are not specified.
I said the one on the table remains hotter because the table will re-radiate back some of the lost energy. I think the question is ambiguous.
(Though I would be curious to see if we could determine which of the two effects dominates for "typical" materials and an ideal blackbody table.)
Whoops, I read that as "the spheres are thermally isolated from each other" and mentally went "well, duh." Go go reading comprehension. :)
I still think it's a pretty silly question, but oh well.
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Speaking of potential energy... I know this is basic high school physics, but could someone explain what it exactly means?
I always thought that it was just an abstract concept used to aid figuring out the math, rather than it being an actual physical thing that actually exists. If it really physically exists, and it's a form of energy, it feels like a rather... esoteric form of energy. Where exactly does it exist? Can you point out to some point in space and say "there are X joules of potential energy right here"? Can you measure it?
Speaking of potential energy... I know this is basic high school physics, but could someone explain what it exactly means?
I always thought that it was just an abstract concept used to aid figuring out the math, rather than it being an actual physical thing that actually exists. If it really physically exists, and it's a form of energy, it feels like a rather... esoteric form of energy. Where exactly does it exist? Can you point out to some point in space and say "there are X joules of potential energy right here"? Can you measure it?
Your notion of potential energy is built right into its phrase. As I understand it, early physicists believed that energy is observed only in the motion of objects. They later discovered that by adding some terms for "bookkeeping", you could conserve energy for many more physical situations than just ideal elastic collisions. The manifest energy of motion became "kinetic" energy ("kinema" is Greek for "motion") and the "bookkeeping" energy became "potential" energy, as in the potential for real energy.
As for where potential energy resides, I'm no expert on the subject, but I believe you could build up a theory based on the fundamental particles that mediate forces. For example, if your system has electric potential energy, there must be photons being exchanged. My research advisor would probably go one step further and say that it's all somewhere in the quantum fields, but I don't have a good grasp on what those are so I won't offer it as my official explanation.
Gravitational potential energy is easy to measure; it's just mass * height * force of gravity. So a 1kg ball 1m above the surface of the earth has 1kg * 1m * 9.8m/s^2 = 9.8J of potential energy. This is equal to the kinetic energy the ball will have right before it hits the ground.
More generally, you ought to be able to quantify the kinetic potential energy of any object held in a vector field (e.g. magnetic field, wind, flowing water) by the ultimate amount of kinetic energy that the object will attain when it is released in that field.
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