this post was submitted on 14 May 2026
441 points (99.1% liked)

Lemmy Shitpost

39820 readers
3691 users here now

Welcome to Lemmy Shitpost. Here you can shitpost to your hearts content.

Anything and everything goes. Memes, Jokes, Vents and Banter. Though we still have to comply with lemmy.world instance rules. So behave!


Rules:

1. Be Respectful


Refrain from using harmful language pertaining to a protected characteristic: e.g. race, gender, sexuality, disability or religion.

Refrain from being argumentative when responding or commenting to posts/replies. Personal attacks are not welcome here.

...


2. No Illegal Content


Content that violates the law. Any post/comment found to be in breach of common law will be removed and given to the authorities if required.

That means:

-No promoting violence/threats against any individuals

-No CSA content or Revenge Porn

-No sharing private/personal information (Doxxing)

...


3. No Spam


Posting the same post, no matter the intent is against the rules.

-If you have posted content, please refrain from re-posting said content within this community.

-Do not spam posts with intent to harass, annoy, bully, advertise, scam or harm this community.

-No posting Scams/Advertisements/Phishing Links/IP Grabbers

-No Bots, Bots will be banned from the community.

...


4. No Porn/ExplicitContent


-Do not post explicit content. Lemmy.World is not the instance for NSFW content.

-Do not post Gore or Shock Content.

...


5. No Enciting Harassment,Brigading, Doxxing or Witch Hunts


-Do not Brigade other Communities

-No calls to action against other communities/users within Lemmy or outside of Lemmy.

-No Witch Hunts against users/communities.

-No content that harasses members within or outside of the community.

...


6. NSFW should be behind NSFW tags.


-Content that is NSFW should be behind NSFW tags.

-Content that might be distressing should be kept behind NSFW tags.

...

If you see content that is a breach of the rules, please flag and report the comment and a moderator will take action where they can.


Also check out:

Partnered Communities:

1.Memes

2.Lemmy Review

3.Mildly Infuriating

4.Lemmy Be Wholesome

5.No Stupid Questions

6.You Should Know

7.Comedy Heaven

8.Credible Defense

9.Ten Forward

10.LinuxMemes (Linux themed memes)


Reach out to

All communities included on the sidebar are to be made in compliance with the instance rules. Striker

founded 2 years ago
MODERATORS
 
you are viewing a single comment's thread
view the rest of the comments
[–] ArbitraryValue@sh.itjust.works 39 points 1 day ago* (last edited 1 day ago) (4 children)

You don't need calculus to do this. Neither one is accelerating, so "5 seconds after they started moving" is irrelevant. Just calculate the velocity of one in the reference frame of the other by subtracting the vectors: from the point of view of the boy, the girl's velocity vector has orthogonal components of -5 ft/sec north and 1 ft/sec east, so the magnitude is 26^0.5 ft/sec.

[–] Rivalarrival@lemmy.today 0 points 5 hours ago (1 children)

The boy's speed is given as 5 ft/sec, but the question is ambiguous as to whether his position remains due north of the starting point, or due north of the girl. Your approach assumes the former, but his 5ft/sec speed may include the girl's 1ft/sec eastward component.

[–] kogasa@programming.dev 1 points 3 hours ago (1 children)

The implication that the boy is strafing while maintaining a straight vertical line to the girl is hilarious

[–] Rivalarrival@lemmy.today 1 points 1 hour ago* (last edited 1 hour ago)

Technically, his starting location is defined contradictorily, being both the same position as hers, as well as "due north". We can only proceed if a point can be considered "due north" of itself.

Technically, his initial direction of travel isn't actually defined, nor is that direction specified as constant. Only his initial position and "5ft/sec" speed has actually been defined. The problem doesn't actually constrain him to a northerly heading, or even to a constant heading. He could choose to orbit her as she travels eastward.

In five seconds, he could be a maximum of 25 feet away from the starting point. That puts him up to 30 feet west of her, or 20 feet east of her. His possible position will be defined by an ellipse with foci at the origin and her position 5 feet to the east, with a major axis length of 50 feet. If his direction is fixed, he will be on the circumference of the ellipse. If not fixed, somewhere within the ellipse.

If his direction is not fixed, and he elects to minimize his distance from her, as his distance from her approaches zero, his revolutions around her in a given time will approach infinite, and we will have to consider relativistic effects. His body will be ripped apart into a pink mist, which he will experience for all eternity. Poetic, I suppose.

[–] Mirodir@discuss.tchncs.de 25 points 1 day ago (2 children)

The “5 seconds after they started moving” is relevant. If we assume this takes place on Earth (i.e. on the surface of a sphere with a set pair of north/south poles), the angle between the two vectors changes depending on their current position.

If it's not on the equator, it's also slightly up to interpretation if "Due East" means they'll turn to stay on the same latitude, always adjusting to stay moving east forever or if they'll do a great circle. In the former case, the north moving one will eventually get stuck at the north-pole too instead of continuing their circle around the globe. Most likely not within 5 seconds though, unless the place they started was within 25 feet of the north-pole.

To actually do the math we'll need to know (or somehow deduce) where "the place where everything about them began" is though.

[–] ChaoticNeutralCzech@feddit.org 10 points 20 hours ago* (last edited 11 hours ago) (1 children)

You actually only need to know the latitude for that... except the local terrain will play a larger role anyway, unless they started very close to a pole and follow rhumb lines (in this case ♂︎ a meridian and ♀︎ a circle of latitude) as opposed to great circles, so better just ask for full coordinates.

What? The teacher does not want to talk about it? Let's find out anyway, to the best of my abilities. For now, we'll be assuming Earth is a fully walkable ellipsoid.

We don't have many data points in the question so let's extrapolate their movement into the past. There is the hint that they met 8 years earlier at the same spot, during which he'd have gone 1 262 304 000 ft or 384 750.2592 km, completing 9.617 polar circumferences of the Earth (40 007.863 km each).

Huh, that's not a whole number. In some languages, "eight long years" might mean "a little over 8 years" so let's assume he finished exactly 10 polar circumnavigations, which took 8 years and about 116 days. Her walked distance over that time is 5x smaller, 2 polar circumnavigations' worth or 80 015.726 km. This is only exactly 2 great circles (ellipses, really) if they are polar, but we know that it's impossible to go due east from either pole. Therefore, we'll use the other option you pointed out, of her having gone at a constant bearing of 090, her path being a circle of latitude (aka a "parallel"). To end up in the same spot, she must have not-quite-circumnavigated-but-enough-for-Phileas-Fogg the Earth (aka crossed every meridian but not the equator) an integer number of times. After a simple conversion, we can construct a table of the options.

To calcuate latitude from circle-of-latitude circumference (colc), we'll be using geodetic ↔ ECEF conversion equations (except those with the perverse prime vertical radius of curvature 𝑁 of course) and their notation (simplified with 𝑦 = 0, 𝜆 = 0, ℎ = 0 to ignore longitude and elevation) with values of the WGS-84 ellipsoid. The relationship we're seeking is between colc/2𝛑 = 𝑝, circle-of-latitude radius, which is at zero longitude equal to ECEF 𝑋, and 𝜙 (latitude). See also Wikipedia on Earth radius by location but remember to skip anything with 𝑁, we're not doing that.

The geocentric radius (𝑅) is related to 𝜙 (latitude) like this but we only need the distance to axis of rotation 𝑝.

(𝑍/𝑝)(cot 𝜙) = (1 − 𝑒²) → (𝑏²/𝑎²)(𝑍/𝑝) = 1/(cot 𝜙) = tan 𝜙 → 𝜙 = atan((𝑏²/𝑎²)(𝑍/𝑝))
(using 𝑒² = 1 −  𝑏²/𝑎²)

Since sin² 𝛂 = 1 − cos² 𝛂 and we can normalize 𝑍 and 𝑝 to the unit circle with ellipsoid radii 𝑏 and 𝑎 respectively:
𝑍²/𝑏² = (𝑍/𝑏)² = 1 − (𝑝/𝑎)² = 1 − 𝑝²/𝑎², therefore 𝑍 = √(𝑏²(1 − 𝑝²/𝑎²)).

All in all, 𝑝 → 𝜙 conversion is:
𝜙 = atan((𝑏²/𝑎²)(√(𝑏²(1 − 𝑝²/𝑎²))/𝑝))

(Presumably, this could be simpified further but I can just put this into a calculator so idc)
Per WGS-84:
𝑎 = 6378.137 km
𝑏 = 6356.752 km

Here are the results. Finding appropriate meeting locations at some of the 25+ possible latitudes on either hemisphere is left as an exercise to the reader. Also note that "rainy days" don't occur in some places, which is why I didn't bother adding more rows after I got within 500 km of the pole.

nqcbefPFs colc/2𝛑 = 𝒑 [km] Latitude [°N/°S]
1 too big N/A
2 6 367.449 3.277975
3 4 244.966 47.934779
4 3 183.724 59.758044
5 2 546.979 66.211738
6 2 122.483 70.346611
7 1 819.271 73.238734
8 1 591.862 75.380740
9 1 414.988 77.033209
10 1 273.489 78.347789
11 1 157.718 79.419029
12 1 061.241 80.309059
13 979.607 81.060439
14 909.635 81.703329
15 848.993 82.259706
16 795.931 82.745975
17 749.111 83.174629
18 707.494 83.555355
19 670.257 83.895779
20 636.744 84.201988
21 606.423 84.478901
22 578.859 84.730536
23 553.691 84.960206
24 530.620 85.170671
25 509.395 85.364245
26 489.803 85.542885

Rows where the number of not-quite-circumnavigations is divisible by 2, 5, or 10 are especially interesting because then the couple would meet 3, 6 and 11 times over the 8.32-year relationship, respectively, rather than just twice.

(Fun fact: another set of latitude circles whose sizes are an inverse-integer sequence is a lesser-known solution to the famous cheeky bear color problem (the well-known solution is a 1-radian arc around the North Pole), although obtaining the ultimate answer then fails due to the lack of bears in Antarctica)

[–] Mirodir@discuss.tchncs.de 2 points 20 hours ago
[–] ByteJunk@lemmy.world 12 points 1 day ago* (last edited 1 day ago)

What a convoluted way of asking the teacher to spill the beans. I like it.

[–] rustydrd@sh.itjust.works 3 points 1 day ago

I guess the calculus portion of this is to write the separation as a function of time, s = √26*t, and then realize that the "speed" of separation is the same regardless of time, because the first derivative is a constant.

[–] andros_rex@lemmy.world 1 points 21 hours ago

I mean, there’s quite a few calculus problems that you can do without calculus. Pretty much 80% of the optimization problems/max’s and minimums in a typical Calc 1 class can be done if you remember that -b/2a is the vertex of a quadratic.