Search found 40 matches

by DavidSh
Thu Apr 20, 2017 2:54 pm UTC
Forum: News & Articles
Topic: The Thread To Remind Me We're Living In The Future
Replies: 1494
Views: 200246

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...
by DavidSh
Mon Apr 17, 2017 5:16 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1825: "7 Eleven"
Replies: 85
Views: 10590

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.
by DavidSh
Sat Mar 25, 2017 2:16 pm UTC
Forum: Science
Topic: Toothbrush, glasses, and clock
Replies: 15
Views: 2080

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...
by DavidSh
Sat Mar 25, 2017 1:09 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1815: "Flag"
Replies: 49
Views: 5076

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.
by DavidSh
Thu Mar 23, 2017 11:46 pm UTC
Forum: News & Articles
Topic: The Darker Side of the News
Replies: 2865
Views: 326573

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...
by DavidSh
Tue Mar 07, 2017 12:24 pm UTC
Forum: Mathematics
Topic: vertex cut / graph / combinatorics problem
Replies: 5
Views: 1354

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 ...
by DavidSh
Sun Mar 05, 2017 2:59 pm UTC
Forum: Science
Topic: How can we experminetly determine the acceleration due to gravity?
Replies: 20
Views: 2610

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...
by DavidSh
Tue Feb 28, 2017 5:50 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1804: "Video Content"
Replies: 64
Views: 6759

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.
by DavidSh
Sat Feb 25, 2017 3:36 pm UTC
Forum: Mathematics
Topic: Probability problem
Replies: 13
Views: 2302

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 ...
by DavidSh
Fri Feb 24, 2017 9:14 pm UTC
Forum: Gaming
Topic: real RTS decision-based idea
Replies: 8
Views: 1539

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 ...
by DavidSh
Thu Jan 26, 2017 2:24 am UTC
Forum: Mathematics
Topic: Combinatorics with constraint
Replies: 14
Views: 2494

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...
by DavidSh
Sat Jan 21, 2017 3:30 pm UTC
Forum: Mathematics
Topic: Combinatorics with constraint
Replies: 14
Views: 2494

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 ...
by DavidSh
Sun Jan 08, 2017 3:12 pm UTC
Forum: Mathematics
Topic: How to Calculate Pi without a Calculator?
Replies: 66
Views: 7818

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...
by DavidSh
Mon Dec 26, 2016 2:44 pm UTC
Forum: Mathematics
Topic: chain rules matrix derivatives
Replies: 2
Views: 1245

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...
by DavidSh
Fri Dec 23, 2016 4:56 pm UTC
Forum: Mathematics
Topic: Probability distribution on the power set of natural numbers?
Replies: 12
Views: 1705

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...
by DavidSh
Fri Dec 23, 2016 3:39 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1776: "Reindeer"
Replies: 32
Views: 4355

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.
by DavidSh
Fri Dec 23, 2016 3:14 pm UTC
Forum: Mathematics
Topic: Probability distribution on the power set of natural numbers?
Replies: 12
Views: 1705

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...
by DavidSh
Fri Dec 02, 2016 11:43 am UTC
Forum: Individual XKCD Comic Threads
Topic: 1767: "US State Names"
Replies: 59
Views: 8208

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.
by DavidSh
Sat Nov 26, 2016 2:42 pm UTC
Forum: News & Articles
Topic: Where Aren't They Now (Obituaries)
Replies: 142
Views: 17527

Re: Where Aren't They Now (Obituaries)

Angua wrote:Zoboomafoo

But the lemur died two years ago.
by DavidSh
Sat Nov 12, 2016 3:51 pm UTC
Forum: Mathematics
Topic: Three externally tangent circles sharing a single point of tangency
Replies: 19
Views: 3645

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 ...
by DavidSh
Fri Oct 28, 2016 4:50 pm UTC
Forum: Mathematics
Topic: Finding a Quadrilateral for N Points
Replies: 20
Views: 3629

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...
by DavidSh
Fri Jul 29, 2016 3:53 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1713: "50 ccs"
Replies: 46
Views: 5383

Re: 1713: "50 ccs"

I was figuring carbon copies.
by DavidSh
Thu Jul 28, 2016 4:27 pm UTC
Forum: Movies and TV Shows
Topic: Wander Woman
Replies: 71
Views: 6602

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.
by DavidSh
Wed Jul 27, 2016 1:23 pm UTC
Forum: Science
Topic: Is the Hyperloop remotely possible?
Replies: 62
Views: 5848

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...
by DavidSh
Tue Jul 26, 2016 2:50 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1708: "Dehydration"
Replies: 109
Views: 10062

Re: 1708: "Dehydration"

Do I figure things correctly that nilane looks a lot like molecular hydrogen, that is, H2?
by DavidSh
Fri Jul 22, 2016 11:54 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1710: "Walking Into Things"
Replies: 22
Views: 3471

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...
by DavidSh
Wed Jul 20, 2016 5:00 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1708: "Dehydration"
Replies: 109
Views: 10062

Re: 1708: "Dehydration"

HES wrote:What tree has a bowl?

A lava tree?

Spoiler:
lavatory, pronounced lavat'ry
by DavidSh
Mon Jul 18, 2016 4:17 am UTC
Forum: Science
Topic: How do satellites not crash into each other?
Replies: 94
Views: 11192

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...
by DavidSh
Sat Jul 16, 2016 1:10 pm UTC
Forum: Science
Topic: How do satellites not crash into each other?
Replies: 94
Views: 11192

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...
by DavidSh
Wed Jul 13, 2016 8:13 pm UTC
Forum: Mathematics
Topic: Trisecting an angle
Replies: 63
Views: 8583

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.
by DavidSh
Wed Jul 13, 2016 7:37 pm UTC
Forum: Mathematics
Topic: Trisecting an angle
Replies: 63
Views: 8583

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 b1 is one in the given formula.
by DavidSh
Wed Jul 13, 2016 3:37 pm UTC
Forum: Science
Topic: How do satellites not crash into each other?
Replies: 94
Views: 11192

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...
by DavidSh
Mon Jun 20, 2016 12:48 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1696: "AI Research"
Replies: 12
Views: 3013

Re: 1696: "AI Research"

What's "updog"?



(Is it anything like updoc?)
by DavidSh
Thu Apr 14, 2016 4:29 pm UTC
Forum: The Help Desk
Topic: Diagnosing a Dead Dell Inspiron Touchpad
Replies: 9
Views: 5041

Re: Diagnosing a Dead Dell Inspiron Touchpad

Have you looked in the BIOS? Sometimes hardware can be enabled/disabled there.
by DavidSh
Wed Mar 16, 2016 3:34 pm UTC
Forum: Individual XKCD Comic Threads
Topic: 1655: "Doomsday Clock"
Replies: 63
Views: 8403

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...
by DavidSh
Sat Mar 12, 2016 11:51 pm UTC
Forum: Mathematics
Topic: does every unit length curve lie inside the semicircle?
Replies: 12
Views: 2791

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...
by DavidSh
Sat Mar 12, 2016 7:43 pm UTC
Forum: Mathematics
Topic: does every unit length curve lie inside the semicircle?
Replies: 12
Views: 2791

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...
by DavidSh
Tue Mar 01, 2016 8:46 pm UTC
Forum: General
Topic: ITT: We make xkcd slightly worse.
Replies: 8592
Views: 1343197

Re: ITT: We make xkcd slightly worse.

I think Sableagle is off by six orders of magnitude, converting 6 x 106 g to 4.6 x 1012 cm3 can't be right for something with a density only slightly greater than water.
by DavidSh
Fri Feb 26, 2016 6:23 pm UTC
Forum: Mathematics
Topic: looking for a probability theorem
Replies: 3
Views: 1517

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_...
by DavidSh
Thu Feb 25, 2016 6:12 pm UTC
Forum: Mathematics
Topic: looking for a probability theorem
Replies: 3
Views: 1517

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.

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