Main Page
Welcome to the explain xkcd wiki!
We have an explanation for all 3305 xkcd comics, the 21 extra comics, and everything xkcd-related.Only 51 explanations (1.5%) are incomplete. Help us finish them!
Thank you for your tireless documentation effort :)
Latest comic
| Ground Effect |
 Title text: Runners looking for aerodynamic advantage typically wear sneakers because some fancy dress shoes can create wingtip vortices. |
Explanation
Ground effect is an aerodynamic phenomenon, which reduces lift-induced drag when an airfoil is close enough to the ground (usually within 1 wingspan) that circulation is significantly affected. Some vehicles make use of this effect to increase their efficiency of operation.
This comic uses a different interpretation of the term 'ground effect'. Runners depend on their feet making contact with the ground in order to push against it and generate forward motion. In order to do this, all of them must be within one (of their own) body length of the ground. If they were not, their feet would not touch the ground and they would not go anywhere. This could be considered to be reducing their 'efficiency' to zero. This process has little to do with aerodynamic effects.
The title text claims that runners avoid wearing fancy dress shoes because some dress shoes can create wingtip vortices. Wingtip vortices are longitudinal vortices (horizontal tornadoes parallel to airplane fuselage) that a wing creates near the tip (where lift ends) when they generate lift. These create unhelpful drag (without adding lift), so modern airplanes generally have winglets at the end of the wings to make these vortices smaller. There is a style of dress shoe called wingtips with small holes in the upper leather in a pattern that somewhat resembles a bird's wings. In reality, runners would not wear dress shoes because they would be uncomfortable and restrictive to run in, and typically have soles that would not provide sufficient grip on the running surface.
Detailed discussion of ground effect
An airfoil can be modeled as a wedge with flat bottom, vertical leading edge, and sloping top. Undisturbed air has molecules (mostly nitrogen) moving in all directions, some as fast as the speed of sound. (Air molecule motion conveys sound and sets its speed.) A hand placed in a stream creates a depression in the water behind the hand, as the flowing water takes time to go under the hand. Likewise the leading edge pushes forward air molecules moving forward slower than the wing. The air molecules left to hit the sloping top are those moving down and faster forward than the wing. The flat bottom pushes down the half of the air molecules moving up. Every action has an equal and opposite reaction. Pushing air forward, drags the airfoil backward, Pushing air down below (more than above), lifts the airfoil. Circulating the air forward and down, rotates the leading edge of the airfoil upward. (An airplane's empennage or tail counters the airplane wing's tendency to nose up.)
An anemometer (airspeed meter) sees slower airspeed above the wing and faster airspeed below the wing. The airstream above the wing takes more time to reach the trailing edge than the airstream below. Aviation engineers call this circulation. Assuming the air pressure is only changed about 1%, this means for an airplane moving left to right over the ground, the wing encounters a right to left airflow, and adds a clockwise vortex (circular wind) to the otherwise undisturbed right to left airflow. In the wake of the airplane, downwash (i.e. air thrown downwards) compresses air below before bouncing back up. Trailing geese and airplanes in close formation take advantage of this downwash bounce back to fly with less effort or fuel. If the relative humidity is high enough, the compression will condense water vapor into a contrail. ("When contrails trace across the sky, expect that rain might be nigh.")
When the airfoil is close to the ground, this circulation is reduced by friction with and interruption by the ground. The circulation partially becomes downwash from the leading edge, and bounces back like a cushion to counter the loss of form-factor lift. This is why air hockey pucks slide almost without friction, ground effect vehicles slide across rivers and beaches, and light aircraft land gently.
Ground effect is very relevant for aircraft: excess airspeed when landing makes the aircraft float along the runway, potentially causing a late and dangerous touchdown. When taking off, the ground effect reduces form factor lift. Without enough airspeed to create non-ground effect lift, the airplane will stall as soon as it climbs out of the ground effect layer, and loses the downwash cushion. When taking off from short or soft (grass) runways, airplanes will use ground effect to float like a ground effect vehicle and accelerate more quickly to flying speed before initiating climb.
Incidentally, a reverse effect is known in ships. When a ship accelerates in shallow water, it is sucked towards the floor, and its draft increases.
Transcript
- [Five Cueball-like people are shown in the panel. At the left, two of them are floating in the air, flailing their arms and legs as they float at least 12 feet off the ground. At the right, three others are running to the right in a normal way. A dotted line is shown above the tops of these three runners' heads, and a regular line below them indicates the ground. An arrow points down toward the dotted line and another arrow points up toward the regular line behind the runners.]
- [Caption below the panel:]
- The "ground effect" explains why sprinters can run most efficiently if they stay within roughly one body length of the surface.
File:Codex icon speechBubbles color-progressive (v2.6).svg View comic discussion
.svg){kind=link}
This Week's Explanation for Improvement
As part of the Explanation Improvement Project, each week the community selects one older, stagnant, or otherwise neglected explanation to expand and polish together. This week's pick is comic 172: Skateboarding is Not a Crime, you can help us by editing its explanation!
To get involved with the project, you can also nominate an explanation that you think should be improved, or vote on current nominations!
| Skateboarding is Not a Crime |
 Title text: 'Arrest me, I'm a skateboarder' is an even more obnoxious variant. |
Explanation
|
Welcome to This Week's Explanation for Improvement! Anyone is encouraged to help explain comic 172: Skateboarding is Not a Crime; click here to get started! If you are new to wiki editing, you can read our editing FAQ for guidance. Here are some suggestions for improving this explanation: .svg){kind=icon} Better explain the controversy around skateboarding, the stickers, and similar occurrences. (You can learn more about the protest on Wikipedia's page for Skateboarding is not a crime)
.svg){kind=icon} The last parts of the explanation read more like a transcript, so improve the explanation of the caption & title text instead of just describing what they say.
|
Skateboarding culture emerged in the 1980s, especially among young people (high schoolers, college students) as a countercultural movement initially associated with the "punk" movement. In addition, because skateboarding can take place in public locations with railings and benches, many cities have adopted specific measures to either increase the difficulty of skateboarding (Skatestopper), or by trying to pass laws to prohibit the act of skateboarding in public parks. In addition to the stated public safety concerns of fast-moving skateboarders among pedestrians, many cities unfairly associate skateboarders with those who drink, do drugs, or engage in acts of juvenile delinquency, such as truancy or underage drinking.
As a result, the skateboarding subculture has taken up the slogan "skateboarding is not a crime" as a protest against these restrictions from cities. This slogan began in 1987, and since then, several skateboard manufacturing companies have adopted it. At one point, the slogan was a countercultural push against those who would consider skateboarding to be a crime, but as the phrase became more popular (by 2006, the time at which the comic was posted), there was likely a backlash against the slogan. Perhaps some people, both within the community of skateboarders and outside, would consider this sticker to be obnoxious, or to appear to have a persecution complex. For instance, in this comic, the slogan is the *only* sticker displayed on these lockers, perhaps put there as a way of the locker owner trying to stand out, in a "not like the other girls" manner.
Randall apparently really does not like these stickers and states that when/if he becomes president (a common expression that is equivalent to "If I were in charge, ..."), any and all displays of stickers bearing the phrase (like the one on the locker in the comic) will be considered a felony, though skateboarding itself will still be legal. Actually implementing this would be against the first amendment and would be unlikely to ever become a real law. Even if it did, it is so trivial and disproportionate that it is unlikely to be enforced. Still, some schools and other establishments ban or restrict stickers on lockers, and may use a similar rule.
The title text comments on another protest phrase, "arrest me, I'm a skateboarder", that Randall appears to hate even more than "skateboarding is not a crime", potentially for the more standoffish tone of the message. It is unsurprising that he hates this phrase, as it has a similar message and tone.
Transcript
- [A set of lockers, that are dark pink, blue, yellow, dark pink then blue. The yellow locker at center has a white sticker attached to it at an angle reading "Skateboarding is not a crime".]
- [Outside of the Panel] When I'm president, skateboarding will still be legal, but display of those stupid stickers will be a felony.
File:Codex icon speechBubbles color-success (v2.6).svg View comic discussion
.svg){kind=link}
New here?
You can read a brief introduction about this wiki at explain xkcd. Feel free to create an account and contribute to the wiki! We need explanations for xkcd comics, characters, what if? articles, and everything in between. If it is referenced in an xkcd comic, it should be here.
- See our editing FAQ for specific guidance to this wiki, and the more general help on how to edit wiki pages. There's also a handy wikicode cheatsheet.
- Discussion about the wiki itself happens at the Community portal.
- You can browse the comics from the list of all comics or by navigating the category tree at Category:Comics.
- The incomplete explanations are listed here. Feel free to help out by expanding them!
Rules
Don't be a jerk!
There are a lot of comics that don't have set-in-stone explanations; feel free to put multiple interpretations in the wiki page for each comic.
If you want to talk about a specific comic, use its discussion page.
Please only submit material directly related to xkcd and, of course, only submit material that can legally be posted and freely edited. Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.
If you need assistance from an administrator, post a message to the Admin requests community portal.
