## Search found 40 matches

- Thu Apr 20, 2017 2:54 pm UTC
- Forum: News & Articles
- Topic: The Thread To Remind Me We're Living In The Future
- Replies:
**1548** - Views:
**212952**

### Re: The Thread To Remind Me We're Living In The Future

In all the above 'mass' refers to gravitational mass. There's another form of mass called inertial mass (how difficult it is to move something). That's the m in Newton's second law: F = m * a. For everything in the universe, as far as we can tell, gravitational and inertial mass seem to be identica...

- Mon Apr 17, 2017 5:16 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1825: "7 Eleven"
- Replies:
**85** - Views:
**12358**

### Re: 1825: "7 Eleven"

Moving past the post that pre-emptively dismissed any responses as "Excuses",...

The 7-11s I have actually checked have all opened at 7am, although that isn't very many of them.

The 7-11s I have actually checked have all opened at 7am, although that isn't very many of them.

- Sat Mar 25, 2017 2:16 pm UTC
- Forum: Science
- Topic: Toothbrush, glasses, and clock
- Replies:
**15** - Views:
**2425**

### Re: Toothbrush, glasses, and clock

This is completely idle speculation, but perhaps the image on the display is only showing a line at a time, and depends on persistence of vision to form an entire image? In that case, vibration of the eye might cause different parts of the image to be displaced differently. I'm not sure how would ch...

- Sat Mar 25, 2017 1:09 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1815: "Flag"
- Replies:
**49** - Views:
**5686**

### Re: 1815: "Flag"

Mikeski wrote:I don't see another flag with three stars, so it can't be a perfect rating.

Burundi has three stars.

- Thu Mar 23, 2017 11:46 pm UTC
- Forum: News & Articles
- Topic: The Darker Side of the News
- Replies:
**3123** - Views:
**357521**

### Re: The Darker Side of the News

Anyway, the average cargo hold of a civilian plane is pressurized, although maybe only to the equivalent of 2100 meters. An automobile engine should work, if they allow it to have fuel. Flammable liquids are usually prohibited in both checked and carry-on luggage. For that matter, knives are allowed...

- Tue Mar 07, 2017 12:24 pm UTC
- Forum: Mathematics
- Topic: vertex cut / graph / combinatorics problem
- Replies:
**5** - Views:
**1549**

### Re: vertex cut / graph / combinatorics problem

It follows from the generalized max-flow min-cut theorem that the minimum number of airports required to be destroyed equals the maximum number of airport-disjoint paths that visit every country in order. Call this number M. Call two paths p1 and p2 crossing if p1 visits a higher airport than p2 in ...

- Sun Mar 05, 2017 2:59 pm UTC
- Forum: Science
- Topic: How can we experminetly determine the acceleration due to gravity?
- Replies:
**20** - Views:
**3047**

### Re: How can we experminetly determine the acceleration due to gravity?

But for an object rolling without slipping, some of the gravitational potential energy is transformed into rotational energy. If I do the calculations right, the proportion depends on the ratio of the moment of inertia of the object and the product of its mass and the square of its rolling radius, w...

- Tue Feb 28, 2017 5:50 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1804: "Video Content"
- Replies:
**64** - Views:
**7671**

### Re: 1804: "Video Content"

Native wrote:SpaceX to fly two tourists around the moon. Kiss at 11.

It may help to explain that that image was of two robots exchanging information, as imagined by Hitoshi Ashinano in Yokohama Kaidashi Kikō.

- Sat Feb 25, 2017 3:36 pm UTC
- Forum: Mathematics
- Topic: Probability problem
- Replies:
**13** - Views:
**2635**

### Re: Probability problem

Perhaps Goahead52 can clarify the values of B, R, and N for the following situation, because I don't think it follows lorb 's bound. RRRR BBBB BBBB RRRR To me, it looks like the red cells all have one more blue neighbor than red neighbors and the blue cells all have two more blue neighbors than red ...

- Fri Feb 24, 2017 9:14 pm UTC
- Forum: Gaming
- Topic: real RTS decision-based idea
- Replies:
**8** - Views:
**1917**

### Re: real RTS decision-based idea

A real "real time strategy" game that involves construction of buildings should take months to play out, if construction workers around here are representative. Maybe the Sea-Bees can work faster. I've only seen one real-time video game where you can send out orders one day, then next day ...

- Thu Jan 26, 2017 2:24 am UTC
- Forum: Mathematics
- Topic: Combinatorics with constraint
- Replies:
**14** - Views:
**2763**

### Re: Combinatorics with constraint

A distinction is usually drawn between "Maximal", meaning cannot be extended to something larger, and "Maximum", meaning nothing larger exists. Finding a maximal square submatrix of all "1"s in a symmetric 0-1 matrix is pretty easy -- the greedy algorithm will work. Fin...

- Sat Jan 21, 2017 3:30 pm UTC
- Forum: Mathematics
- Topic: Combinatorics with constraint
- Replies:
**14** - Views:
**2763**

### Re: Combinatorics with constraint

I don't know anything for certain about exact values, but the biggest set of such 5-tuples I can find out of 10 numbers is given by the rows of the following matrix: b = 1 1 1 1 1 0 0 0 0 0 1 0 0 0 1 1 1 1 0 0 1 1 0 0 0 0 0 1 1 1 0 1 1 0 0 1 1 0 0 1 0 0 0 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 1 0 And I can ...

- Sun Jan 08, 2017 3:12 pm UTC
- Forum: Mathematics
- Topic: How to Calculate Pi without a Calculator?
- Replies:
**66** - Views:
**8518**

### Re: How to Calculate Pi without a Calculator?

In the 50?*?<=2000, and such, you can use division and 1 or 2 multiplications. It's just (a+x)*x<=b, with x a digit. I am pretty sure that x=floor(b/a)-1 (with -1 mapped to 0) will always work, but I think that x=floor(b/a) might work sometimes, so unless someone can prove otherwise, that's 2 digit...

- Mon Dec 26, 2016 2:44 pm UTC
- Forum: Mathematics
- Topic: chain rules matrix derivatives
- Replies:
**2** - Views:
**1352**

### Re: chain rules matrix derivatives

As the wikipedia page says, there are several competing notations. You will need to pick one, and stick with it. Also, you need to keep clear the distinction between row vectors and column vectors. If you use the notation where the derivative of Ax + b with respect to x is A , where x is a column ve...

- Fri Dec 23, 2016 4:56 pm UTC
- Forum: Mathematics
- Topic: Probability distribution on the power set of natural numbers?
- Replies:
**12** - Views:
**1889**

### Re: Probability distribution on the power set of natural numbers?

On a second note, selecting each element independently from N with equal probability p, with 0 < p < 1, gives finite and cofinite sets with probability 0. What do you mean "with equal probability p"? There can be no such p if you're considering all of N. I see the confusion now. I meant s...

- Fri Dec 23, 2016 3:39 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1776: "Reindeer"
- Replies:
**32** - Views:
**4797**

### Re: 1776: Reindeer

Is this new, associating Santa Claus with Odin? (Sleipnir is Odin's steed) Most theories I have heard that connect Santa with Norse mythology associate Santa with Thor, whose chariot is pulled by a pair of goats.

- Fri Dec 23, 2016 3:14 pm UTC
- Forum: Mathematics
- Topic: Probability distribution on the power set of natural numbers?
- Replies:
**12** - Views:
**1889**

### Re: Probability distribution on the power set of natural numbers?

Let f() be the natural mapping from the power set of N to [0,1] defined as: f(S) = sum_{i in S) 2^{-i}. We know f() doesn't provide a bijection, but I think g() below provides a bijection between the power set of N and [0,1). g(S) = f(S)/2 if S is finite; = 1-(f(S)/2) if S is cofinite; = f(S) otherw...

- Fri Dec 02, 2016 11:43 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1767: "US State Names"
- Replies:
**59** - Views:
**8945**

### Re: 1767: "US State Names"

thunk wrote:So what about Portwine Rocky?

It goes an a separate page, along with Gum and the Northern Marinara Islands.

- Sat Nov 26, 2016 2:42 pm UTC
- Forum: News & Articles
- Topic: Where Aren't They Now (Obituaries)
- Replies:
**156** - Views:
**21188**

### Re: Where Aren't They Now (Obituaries)

Angua wrote:Zoboomafoo

But the lemur died two years ago.

- Sat Nov 12, 2016 3:51 pm UTC
- Forum: Mathematics
- Topic: Three externally tangent circles sharing a single point of tangency
- Replies:
**19** - Views:
**4041**

### Re: Three externally tangent circles sharing a single point of tangency

Not exactly answering the last question, but as long as you can say things like: (1) If B is on a shortest path from A to C, and B is also on a shortest path from A to D, then either C is on the shortest path from A to D, or D is on the shortest path from A to C; and (2) If the distance from A to B ...

- Fri Oct 28, 2016 4:50 pm UTC
- Forum: Mathematics
- Topic: Finding a Quadrilateral for N Points
- Replies:
**20** - Views:
**3843**

### Re: Finding a Quadrilateral for N Points

For N=5, I think it works if we divide into cases based on the number S of sides of the convex hull of the points. For S=5 you should always be able to remove one side and extend two adjacent sides to meet, forming a quadrilateral. A detailed argument could involve the total exterior angles of a pen...

- Fri Jul 29, 2016 3:53 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1713: "50 ccs"
- Replies:
**46** - Views:
**5820**

### Re: 1713: "50 ccs"

I was figuring carbon copies.

- Thu Jul 28, 2016 4:27 pm UTC
- Forum: Movies and TV Shows
- Topic: Wander Woman
- Replies:
**71** - Views:
**7909**

### Re: Wonder Woman

Lawrence of Arabia and The African Queen are both WWI movies, although that may not be obvious to everybody. And there must be a slew of WWI aviation movies, going as far back as the 1927 Wings.

- Wed Jul 27, 2016 1:23 pm UTC
- Forum: Science
- Topic: Is the Hyperloop remotely possible?
- Replies:
**62** - Views:
**6380**

### Re: Is the Hyperloop remotely possible?

If you don't want passenger fatalities in event of cabin pressure loss, and you don't want your passengers to be wearing pressure suits, and you want the tube to be at 1 millibar, you will need to be able to repressurize the tube swiftly and safely. Or at least the segment containing your vehicle. I...

- Tue Jul 26, 2016 2:50 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1708: "Dehydration"
- Replies:
**109** - Views:
**10914**

### Re: 1708: "Dehydration"

Do I figure things correctly that nilane looks a lot like molecular hydrogen, that is, H

_{2}?- Fri Jul 22, 2016 11:54 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1710: "Walking Into Things"
- Replies:
**22** - Views:
**3742**

### Re: 1710: "Walking Into Things"

I do like the addition of "sky" to Randall's list: a useful reminder that one doesn't need devices (or collections of paper) to be distracted. I was certainly at risk a few days ago when we had a pretty impressive rai nb ow . Only a double rainbow, not any of the extreme forms mentioned i...

- Wed Jul 20, 2016 5:00 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1708: "Dehydration"
- Replies:
**109** - Views:
**10914**

### Re: 1708: "Dehydration"

HES wrote:What tree has a bowl?

A lava tree?

**Spoiler:**

- Mon Jul 18, 2016 4:17 am UTC
- Forum: Science
- Topic: How do satellites not crash into each other?
- Replies:
**94** - Views:
**11878**

### Re: How do satellites not crash into each other?

Right, but is that also the median in some sense? Let's see. The distance from the Earth on an elliptical orbit increases monotonically from perigee to apogee, and then decreases again. Half of the ellipse is on the Earth side of the minor axis, half on the other side, so the median points should b...

- Sat Jul 16, 2016 1:10 pm UTC
- Forum: Science
- Topic: How do satellites not crash into each other?
- Replies:
**94** - Views:
**11878**

### Re: How do satellites not crash into each other?

But all those orbits will pass through the same (comparatively small) volume occupied by geostationary satellites every day, right? An inclined, eccentric geosynchronous orbit has to pass through the same altitude as the geostationary satellites, but it doesn't have to do so anywhere near the equat...

- Wed Jul 13, 2016 8:13 pm UTC
- Forum: Mathematics
- Topic: Trisecting an angle
- Replies:
**63** - Views:
**8918**

### Re: Trisecting an angle

You mean that formula enumerates all constructible angles. A nonconstructible angle can be trisectable. For example 3*pi/7 is not constructible, but if we were presented with it, we could take its supplement 4*pi/7, and then bisect to get 2*pi/7, and then again to get pi/7.

- Wed Jul 13, 2016 7:37 pm UTC
- Forum: Mathematics
- Topic: Trisecting an angle
- Replies:
**63** - Views:
**8918**

### Re: Trisecting an angle

Maybe it is that if an angle x is constructable, but x/3 is not constructible, then x is not trisectable, and that this formula implies that there are many such constructable but not trisectable angles, for example whenever a is not a multiple of 3 and b

_{1}is one in the given formula.- Wed Jul 13, 2016 3:37 pm UTC
- Forum: Science
- Topic: How do satellites not crash into each other?
- Replies:
**94** - Views:
**11878**

### Re: How do satellites not crash into each other?

Geostationary orbits, written as GEO, should fit in the torus. Geosynchronous orbits, written as GSO, don't need to be equatorial, which is probably why the shell was proposed. But they don't need to be circular, either. Wikipedia mentions an Infrared Space Observatory at a geosynchronous orbit with...

- Mon Jun 20, 2016 12:48 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1696: "AI Research"
- Replies:
**12** - Views:
**3116**

### Re: 1696: "AI Research"

What's "updog"?

(Is it anything like updoc?)

(Is it anything like updoc?)

- Thu Apr 14, 2016 4:29 pm UTC
- Forum: The Help Desk
- Topic: Diagnosing a Dead Dell Inspiron Touchpad
- Replies:
**9** - Views:
**5403**

### Re: Diagnosing a Dead Dell Inspiron Touchpad

Have you looked in the BIOS? Sometimes hardware can be enabled/disabled there.

- Wed Mar 16, 2016 3:34 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 1655: "Doomsday Clock"
- Replies:
**63** - Views:
**9201**

### Re: 1655: "Doomsday Clock"

Before the advent of mechanical clocks, one option taken was to start the reckoning of hours at dawn, and divide the day into twelve hours, and the night also into twelve hours. An hour of day would equal an hour of night only at the equinoxes. It wasn't too hard to make sundials for these hours. Wi...

- Sat Mar 12, 2016 11:51 pm UTC
- Forum: Mathematics
- Topic: does every unit length curve lie inside the semicircle?
- Replies:
**12** - Views:
**2968**

### Re: does every unit length curve lie inside the semicircle?

If you use that argument, you can't add the lengths from the two sectors together. Consider the path e-f-g-h defined as follows. Let theta be some small positive angle. Let e = (sin(theta), cos(theta)), f = (-sin(theta),cos(theta)), g = (-sin(theta),0), h = (sin(theta),0). This has a point on each o...

- Sat Mar 12, 2016 7:43 pm UTC
- Forum: Mathematics
- Topic: does every unit length curve lie inside the semicircle?
- Replies:
**12** - Views:
**2968**

### Re: does every unit length curve lie inside the semicircle?

I think the reasoning for Cradarc 's Claim 3 is faulty. Yes, the curve has to connect P, Q, and R, but you haven't shown that it has to do it within the sector. I think the following approach works, extending measure 's idea. Let S be a curve of length x. Let a and b be the two endpoints of S. Let R...

- Tue Mar 01, 2016 8:46 pm UTC
- Forum: General
- Topic: ITT: We make xkcd slightly worse.
- Replies:
**8617** - Views:
**1389958**

### Re: ITT: We make xkcd slightly worse.

I think Sableagle is off by six orders of magnitude, converting 6 x 10

^{6}g to 4.6 x 10^{12}cm^{3}can't be right for something with a density only slightly greater than water.- Fri Feb 26, 2016 6:23 pm UTC
- Forum: Mathematics
- Topic: looking for a probability theorem
- Replies:
**3** - Views:
**1568**

### Re: looking for a probability theorem

Let x_1 and x_2 be independent standard normal random variables (mean 0, variance 1). Let y_1 be another normal random variable, with mean -0.1, variance 1, and y_2 = y_1. Then for all i,k, P(x_i > k) > P(y_i > k). But X = x_1+x_2 is a normal random variable with mean 0, variance 2, while Y = y_1+y_...

- Thu Feb 25, 2016 6:12 pm UTC
- Forum: Mathematics
- Topic: looking for a probability theorem
- Replies:
**3** - Views:
**1568**

### Re: looking for a probability theorem

I don't know of a general theorem, but you do need to state that the individual x_i are independent, and the individual y_i are independent. Otherwise there are counterexamples.