xkcd: What If?

[bystander]

Rocket Golf

• Assuming that you have a spaceship in orbit around the Earth, could you propel your
  ship to speeds exceeding escape velocity by hitting golf balls in the other direction?
  If so, how many golf balls would be required to reach the Moon? — Dan (Kanta, Ontario)

[neufer]

Rocket Golf

• According to research from the US Air Force Academy and BTG Research, a potato cannon fueled by acetylene can launch a potato at 140 m/s (310 mph). If it were capable of launching golf balls at that speed, our ship would have a diameter of only 150 miles!

A small golf ball moon with a diameter of 150 miles has its own escape velocity of ~180 mph.

Hence the "potato" cannon would have to have an actual muzzle velocity of (310 + 180 =) 490 mph in order to work as stated.

(The "potato" cannon would also have to eject golf balls at a fast enough rate
that the air drag on the 150 mile diameter golf ball moon could be ignored!)

However, by the time the space ship attained an altitude of around a thousand miles above the Earth's surface (such that the Coriolis forces of the orbiting system dominated both the air drag and the Earth's tidal forces on the ejected balls) the golf balls would simply "hook" into (~150 mile in diameter) circular trajectories that takes them from the trailing side of the small golf ball moon to crash back again on the leading side of the small golf ball moon. Even with a hard outer casing such that the golf balls elastically bounced off into space again the final result would be an orbiting ring of golf balls with roughly the same angular velocity as the golf ball moon, itself. Either way the space craft is stuck in its slightly elevated orbit.

[geckzilla]

Far-Traveling Objects

• In terms of human-made objects, has Voyager 1 travelled the farthest distance?
  It's certainly the farthest from Earth we know about. But what about the edge of
  ultracentrifuges, or generator turbines that have been running for years, for example? — Matt Russell

[geckzilla]

Enforced by Radar

• I've occasionally seen "radar enforced" on speed limit signs, and I can't help but ask:
  How intense would radio waves have to be to stop a car from going over the speed limit,
  and what would happen if this were attempted? — joausc

[geckzilla]

Also, Randall is publishing a What-If book. It's impossible to miss the link at the top of the page, but maybe you didn't actually click to read the what-if...

What if I wrote a book?

[geckzilla]

Soda Sequestration

• How much CO2 is contained in the world's stock of bottled fizzy drinks? How much
  soda would be needed to bring atmospheric CO2 back to preindustrial levels? — Brandon Seah

[bystander]

Tungsten Countertop

• How far would a tungsten countertop descend if I dropped it into the Sun? — Michael Leuchtenburg

[BMAONE23]

Tungsten Countertop

• How far would a tungsten countertop descend if I dropped it into the Sun? — Michael Leuchtenburg

Yeah, someone needs to let these French People know that even If you don't have a wine glass, It is still OK to drink the wine directly from the box

[bystander]

Great Tree, Great Axe

• If all the seas were one sea,
  What a great sea that would be!
  If all the trees were one tree,
  What a great tree that would be!
  If all the men were one man,
  What a great man that would be!
  If all the axes were one axe,
  What a great axe that would be!
  And if the great man took the great axe,
  And cut down the great tree,
  And let if fall into the great sea,
  What a great splish-splash that would be!
  
  ... How great would all of these things be?
  
  — John Eifert (quoting a Mother Goose rhyme)

[neufer]

---

<<Calaveras Big Trees State Park is a state park of California, USA, preserving two groves of giant sequoia trees. The North Grove included the 'Discovery Tree' noted by Augustus T. Dowd in 1852 and felled in 1853, leaving a giant stump which is the only remainder of the tree. It measured 24 feet (7.3 m) in diameter at its base and was determined by ring count to be 1,244 years old when felled.

John Muir wrote an article about 'Discovery Tree' titled:

"The Vandals Then Danced Upon the Stump!"

However, the largest tree was believed to be the Mother of the Forest (667 BCE – 1854 CE) was an ancient and huge Sequoiadendron tree. This tree stretched 321 feet (98 m) into the air with a girth of 90 feet (27 m) at ground level, and was the largest of 92 giant sequoias growing in the valley in 1852 when a man named George Gale discovered the massive tree. Gale named the massive tree the "Mother of the Forest" before he sent men to strip the tree of its bark. Once the bark was removed, the tree did not survive for long. In 1908, a fire that swept through the area burned away much of what was left of the tree. The massive tree had thick bark, 2 feet (0.61 m) thick in some spots, which Gale had stripped. Gale toured with the bark, showing it off to crowds.>>

---

Great Tree, Great Axe

<<In Avatar, the Hometree of the Omaticaya clan was obliterated by the Resources Development Administration's security forces on August 20, 2154 using high explosive missiles, after driving the Na'vi out with tear gas, forcing the remaining Na'vi to relocate.

Colonel Quaritch referred to it as "one big damn tree" before destroying it.>>

[geckzilla]

[bystander]

Faucet Power

• I just moved into a new apartment. It includes hot water but I have to pay the electric bill. So
  being a person on a budget ... what's the best way to use my free faucet to generate electricity? — David Axel Kurtz

[Chris Peterson]

Faucet Power

• I just moved into a new apartment. It includes hot water but I have to pay the electric bill. So
  being a person on a budget ... what's the best way to use my free faucet to generate electricity? — David Axel Kurtz

I'm skeptical that this is the best answer. Since it isn't just water that's included in the rent, but hot water, I think a better approach to getting electricity would be some type of heat pump (like like a thermocouple between the hot and cold water streams). I'll need to do a few calculations, but I'm guessing that would deliver more energy than using the gravitational potential of the water.

[neufer]

<<You can just hook the faucet up to your device, and let the water pressure drive the generator directly. For a [typical] bathtub faucet, this works out to almost 200 watts [= 17.3 Megajoules/day]...[but] eventually someone from the city would probably show up to ask why you're using 40 tons of water every day.>>

I'm skeptical that this is the best answer. Since it isn't just water that's included in the rent, but hot water, I think a better approach to getting electricity would be some type of heat pump (like like a thermocouple between the hot and cold water streams). I'll need to do a few calculations, but I'm guessing that would deliver more energy than using the gravitational potential of the water.

• 1) It is highly unlikely that the new apartment will actually provide 40 tons of hot water every day
  
  2) Even if it did one would be hard pressed to efficiently make use of even a tiny fraction (1-3%) of the 168 Megajoules/(day ºC) low grade thermal energy with a [hot/cold running water] thermopile device or Rankine cycle generator.

<<Ocean thermal energy conversion (OTEC) uses the temperature difference between cooler deep and warmer shallow or surface ocean waters to run a heat engine and produce useful work, usually in the form of electricity. A heat engine gives greater efficiency when run with a large temperature difference. In the oceans the temperature difference between surface and deep water is greatest in the tropics, although still a modest 20 to 25 °C. Closed-cycle engines use working fluids that are typically thought of as refrigerants such as ammonia or R-134a. These fluids have low boiling points, and are therefore suitable for powering the system’s generator to generate electricity. The most commonly used heat cycle for OTEC to date is the Rankine cycle using a low-pressure turbine.

The main technical challenge of OTEC is to generate significant amounts of power efficiently from small temperature differences. It is still considered an emerging technology. Early OTEC systems were 1 to 3 percent thermally efficient, well below the theoretical maximum 6 and 7 percent for this temperature difference. Modern designs allow performance approaching the theoretical maximum Carnot efficiency and the largest built in 1999 by the USA generated 250 kW.>>

[b][color=#FF0000]View of the Thermopylae pass from the area of the Phocian Wall. Thermopylae derives its name from its hot sulphur springs.[/color][/b]

---

<<A thermocouple consists of two dissimilar conductors that contact each other at one or more spots. It produces a voltage when the temperature of one of the spots differs from the reference temperature at other parts of the circuit. Thermocouples can also convert a temperature gradient into electricity. Type J (iron – constantan) thermocouples have a sensitivity of about 50 µV/°C.

Thermocouples can be connected in series to form a thermopile, where all the hot junctions are exposed to a higher temperature and all the cold junctions to a lower temperature. The output is the sum of the voltages across the individual junctions, giving larger voltage and power output. Thermopiles heated by kerosene lamps were used to run batteryless radio receivers in isolated areas. There are commercially produced lanterns that use the heat from a candle to run several light-emitting diodes, and thermoelectrically-powered fans to improve air circulation and heat distribution in wood stoves. In a radioisotope thermoelectric generator, the radioactive decay of transuranic elements as a heat source has been used to power spacecraft on missions too far from the Sun to use solar power.>>

[bystander]

One-Second Day

• What would happen if the Earth's rotation were sped up until a day only lasted one second? — Dylan

[bystander]

Windshield Raindrops

• At what speed would you have to drive for rain to shatter your windshield? — Daniel Butler

[bystander]

Billion-Story Building

• My daughter—age 4.5—maintains she wants a billion-story building.
  It turns out not only is that hard to help her appreciate this size,
  I am not at all able to explain all of the other difficulties you'd
  have to overcome. — Keira, via Steve Brodovicz, Media, PA

[orin stepanek]

One-Second Day

• What would happen if the Earth's rotation were sped up until a day only lasted one second? — Dylan

You think you would get dizzy? After reading this thread I think I am!

[orin stepanek]

Billion-Story Building

• My daughter—age 4.5—maintains she wants a billion-story building.
  It turns out not only is that hard to help her appreciate this size,
  I am not at all able to explain all of the other difficulties you'd
  have to overcome. — Keira, via Steve Brodovicz, Media, PA

How close would that building come to the Moon?

[geckzilla]

It goes way past it. He's got some pretty funny cartoons depicting the tower in the article.

[bystander]

Pyramid Energy

• What took more energy, the building of the Great Pyramid of Giza or the Apollo Mission? If we could convert the
  energy to build the Great Pyramid, would it be enough to send a rocket to the Moon and back? — Michael Marmol

[bystander]

$2 Undecillion Lawsuit

• What if Au Bon Pain lost this lawsuit and had to pay the plaintiff $2 undecillion? — Kevin Underhill

[BMAONE23]

The lawyer should just present Au Bon Pain with THEIR $667 Decillion (%33) bill
What a Pain

[bystander]

Burning Pollen

• What if you were to somehow ignite the pollen that floats around in the air in spring?
  Other than being a really bad idea, what effect would it have? — Jessica Thornburg

[bystander]

Blood Alcohol

• Could you get drunk from drinking a drunk person's blood? — Fiona Byrne