1038: Fountain

Explain xkcd: It's 'cause you're dumb.
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Fountain
Implausible, did you say? Sorry, couldn't quite hear you from all the way up heeeeeeeeere!
Title text: Implausible, did you say? Sorry, couldn't quite hear you from all the way up heeeeeeeeere!

Explanation[edit]

This comic is about how it's considered implausible to "fly" by positioning an umbrella over a water jet.

From the first panel (and assuming that Cueball is of average height) - it looks like the center fountain is about 10m high. By comparison with the size of his head in the second panel, the jet appears to be about 10cm in diameter. The velocity of the water exiting the nozzle has to be about 14 meters/second in order to reach 10m against gravity. If we approximate the nozzle as being a 10cm x 10cm square - that translates to 140 liters/second - or 140kg/s of water. That produces an upward force of almost 2,000 newtons! If we presume that Cueball weighs 100kg (~1,000N)- he should be experiencing a net upward force of about 1,000N. Which means that he'll accelerate at about 1g! Holding onto the umbrella against a force of 1g is very different than hanging by your hands from a horizontal bar, since you would actually experience two gravities of force, due to gravity being added. Some people could still manage this, but you would probably need to be in good shape physically to pull it off.

Conclusion is that IF the umbrella is strong enough - this trick will actually work!

However, if you imagine a typical 6-spoke umbrella - then 1000N is 166N of upward force per spoke. It's hard to believe you could hang a 1.6kg weight off of each spoke of an umbrella without it bending.

The title text emphasizes that Cueball did indeed reach a high altitude - so we must conclude that his umbrella is some specially made high-strength device.

Since the fountain tops out at about 10m - and presumably it would be somewhat reduced with Cueball's weight on it - his feet might only be about 6 to 8 meters above the ground when he stops moving upwards. A fall from that height is survivable - especially if the drag of the umbrella slows him down somewhat.

Transcript[edit]

[A full color image of a fountain with three massive water jets. A Cueball seen from afar walks up to these while holding an umbrella.]
[Cueball splashes through the pond which makes sounds:]
Splish
Splash
[Cueball gets to one of the jets.]
[Cueball opens up the umbrella with a:]
Click
[Cueball opens the umbrella and swings it into water jet stream (which is outside the image). The umbrella makes a sound when opened:]
Fwoop
[Cueball is pulled up by the water jet stream (which is outside the image). Only his feet and the water dripping of them into the pond can be seen. The sound he makes follows him up with longer and longer distance between the letters (written in lower-case).]
Cueball: Wheeeeeeee!


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Discussion

A pulley and a rock do this pretty well too, although it's a wee bit dangerous. What with the falling rock and all. Davidy22[talk] 07:07, 2 February 2013 (UTC)

The military method of rescueing personnel consists of an plane snagging a balloon attached to the person. If the line is long enough, the rescue is safe, for any speed of aircraft! Quite a thrill! --DrMath 05:41, 7 September 2013 (UTC)

Can someone post a link to the principle at work here? 108.162.216.13 02:31, 16 August 2015 (UTC)