/r/theydidthemath

And they said math has no real world applications

Proper title tags are

**required**for all posts or it will be**removed!**The tags accepted are at the end of the sidebar down there.State clearly what is being or what you want calculated in the title. [Request] posts must have a calculable answer; otherwise, they will be removed.

Give credit where credit is due—include the user who made the calculations in any post you submit: ex. "[RDTM] /u/FragTheWhale calculates...".

For easy and quick math results (ex. How many feet are in a mile?) use Wolfram|Alpha™. Posts containing "simple math" will be removed, as well as requests whose answers are easily searchable online, and any other post at the moderators' discretion. For more abstract math, try /r/math or /r/learnmath.

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Do not submit top-level comments in [Request] posts that are not an attempt at an answer or a request for clarification. Jokes or off-topic top-level comments will be removed.

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**LaTeX and subscripts/superscripts**

`[; e^{\pi i} + 1 = 0 ;]`

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**TAGS!**

We require title tags. Here are the tags we accept and what they are for. Please keep them at the start of your post.

**[Request]**- Asking the /r/theydidthemath community to do the math for you!**[Self]**- You did the math and want to share it!**[RDTM]**- "Reddit Did The Math", linking to a comment or post on Reddit where someone else does the math!**[Off-Site]**- Use when linking to any site, including Imgur, that is**not**reddit.com.**[Meta]**- Non-math posts regarding the subreddit itself. Don't use this unless you really have to.

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/r/theydidthemath

2

1 Comment

2023/06/03

23:30 UTC

23:30 UTC

0

20 Comments

2023/06/03

20:49 UTC

20:49 UTC

0

3 Comments

2023/06/03

15:52 UTC

15:52 UTC

0

The problem:

I have a very expensive, high-end wind generator on my sailing vessel. Unfortunately, it has broken blades. Replacing the blades is prohibitively expensive ($2000). Many sailing vessels utilise hydrogenerators that are dragged behind the vessel under way. I am proposing to remove the hub that holds the blades, couple on a shaft and propellor, and drag the prop behind the boat to create a DIY hydrogenerator. The generator itself would remain in a dry location on deck, driven by a ~2m shaft.

I have a table of the RPM required at the wind generator's shaft to produce a given wattage. I also have a number of spare props lying around. I would like a formula to calculate how fast a given prop would turn the shaft, given its dimensions and particulars (pitch, blades, diameter, rake if necessary).

- I do not need the RPM figure to be very precise, +/- 20% would be fine

- assume there is no friction from the shaft and hub

- ideally I want to achieve something like 900 rpm at 5 knots (5.7mph)

- Example problem using a propellor I have lying around: the diameter is 9.5 inches, the pitch 10 inches, number of blades 3. How fast would this propellor spin when dragged at 5kts through salt water?

Here is the RPM to power table if it helps. It would be ideal if I could achieve 900rpm or higher while travelling at 5 to 7 kts.

thank you for your help!

2 Comments

2023/06/03

15:38 UTC

15:38 UTC

4

Even with it being /r/science I thought this was worthy of recognition for the effort put in.

1 Comment

2023/06/03

15:34 UTC

15:34 UTC

0

Should be about 1L glass, does someone have a corn volume occupancy number?

2 Comments

2023/06/03

13:51 UTC

13:51 UTC

5

This question is working under the following parameters

- Such a building was physically possible
- Each floor needs to be tall enough to live in (for an adult).

5 Comments

2023/06/03

10:09 UTC

10:09 UTC

3

Given that pregnant women will have 4, but that’s offset by some people with no legs, or 1 leg.

3 Comments

2023/06/03

09:46 UTC

09:46 UTC

0

4 Comments

2023/06/03

04:10 UTC

04:10 UTC

0

1 Comment

2023/06/03

03:43 UTC

03:43 UTC

3

1 Comment

2023/06/03

02:57 UTC

02:57 UTC

7

10 Comments

2023/06/03

02:25 UTC

02:25 UTC

1

Alright, reddit. I'm sick of night and day. I'm sick of morning people and night owls. I want to stop this planet from spinning -- can it be done? From an engineering perspective, what will it take, and how long do I have to keep dealing with these crazies?

4 Comments

2023/06/02

21:22 UTC

21:22 UTC

0

1 Comment

2023/06/02

18:24 UTC

18:24 UTC

789

Using the relation between meters, liters and grams in the metric system and this redditor's proposal, how many liters is a deciminute ?

55 Comments

2023/06/02

17:44 UTC

17:44 UTC

1

TLDR - The prism shown on the cover of Dark Side of the Moon is not physically possible due to the angles of light coming out. Form over function in this case.

1 Comment

2023/06/02

16:07 UTC

16:07 UTC

3

tried to ask ChatGPT, but it kept spewing out different answers

for example, when asked about **77** evenly spaced towers:

If you have 77 towers that are evenly spaced across the Earth's land, we can calculate the distance between each tower.

Assuming the land area of the world is approximately 148.94 million square kilometers, we can divide this by the number of spaces between the towers, which is one less than the number of towers (76 spaces).

Land area between each tower = Land area of the world / Number of spaces = 148.94 million square kilometers / 76 ≈ 1.96 million square kilometers

Now, let's convert the square kilometers to meters by multiplying by 1,000,000.

Land area between each tower = 1.96 million square kilometers * 1,000,000 ≈ 1,960,000,000 square meters

To find the distance between each tower, we take the square root of the land area between each tower.

Distance between each tower = √(1,960,000,000 square meters) ≈ 44,246.81 meters

Therefore, if you have 77 towers evenly spaced across the Earth's land, the distance between each tower would be approximately 44,246.81 meters (or about 44.25 kilometers, 27.49 miles).

If you have 77 towers evenly spaced across the land area of the Earth, we can calculate the average distance between each tower. Assuming that the land area is approximately 148.94 million square kilometers, we can proceed with the calculation.

To determine the average distance between each tower, we divide the circumference of the Earth (at the equator) by the number of towers.

Circumference of the Earth = 40,075 kilometers

Average distance = Circumference / Number of towers = 40,075 kilometers / 77 ≈ 519.48 kilometers

Therefore, if 77 towers were evenly spaced across the land area of the world, they would be approximately 519.48 kilometers (or about 322.61 miles) apart from each other on average.

So... Which one is it? Maybe neither?

7 Comments

2023/06/02

09:27 UTC

09:27 UTC

3

- Shuffle a deck of 52 cards.
- Turn over the first card by saying "Ace"
- If the card is an Ace, you lose.
- Turn the second card by sayiing "Two"
- If the card is a 2, you lose.
- Continue like that counting like that: Ace,2,3,4,5,6,7,8,9,10,J,Q,K,A,2,3,...

What are the chances of passing the whole deck without dying?

I did some Monte Carlo sims and I get around 1.6%.

How do you solve that analytically. It just seems too much conditionals and a product of a product with a general term that I'm too lazy to try and resolve...

Edit: (12/13)^52 = 1.557%

5 Comments

2023/06/02

00:23 UTC

00:23 UTC

768

109 Comments

2023/06/01

22:12 UTC

22:12 UTC

5

My brother is on the hunt for a new car. The one he likes, he was told, has a 9 year waitlist. It is electric, and I know those are in demand, but it just seems nuts to me!

My question- how many cars behind is the factory?!! (Obvs the people on the waitlist may drop off etc) but if it is truly 9 years, how many cars will be made before his name comes up?

Before retiring, my dad worked on the assembly line at Ford, and they put out 1 car every 60 seconds, 24 hours a day. I’m not sure that’s the same for electric- but I’m happy with that as a starting point!!

TIA! We are so curious!

5 Comments

2023/06/01

16:52 UTC

16:52 UTC

1

I'm not a smart person and cannot figure this out. I have a wall that is 10 feet wide. I have 4 pictures that are 17.5 inches wide a piece. They all hang from a single point in the center of the picture. I want to hang them spaced evenly from each other. I also want a little bit of a buffer(?) on the left and right side near the edges of the wall making them look centered instead of exactly evenly spaced across the whole wall. I hope this makes sense.

8 Comments

2023/06/01

16:51 UTC

16:51 UTC

776

Let's assume that's an early modem where 2400 baud actually means 2,4 kbps. If it's actually 9,6 kbps we'll just divide by 4. Let's factor in possible bottle necks given by poor land-line signal, LAN interface between my C64 and the storage medium, etc. (although I believe the narrowest bandwidth is definitely the modem itself). How many days and hours would it take to download Skyrim from Steam?

68 Comments

2023/06/01

10:47 UTC

10:47 UTC

2

Hoping someone can help settle a disagreement on how to calculate the chances of the scenario below and an explanation of why and how:

There is a box with 25 switches

Each switch can only be up or down

You have to get the right combination for only 4 of them to 'win' (in this case triggered a lock to open)

You do not know which 4 switches are the relevant ones

What are the chances that you randomly (not methodically) mess around with the switches and get the right combination?

Also:

- does it matter if the action was random versus methodical (e.g. going through one by one)
- Does it matter if only one person randomly messed around with it or multiple people with the only the last person getting lucky

Does the calculation change if there is also a master switch--a master on/off switch?

3 Comments

2023/06/01

07:39 UTC

07:39 UTC

2

Edit: in English with a laser

10 Comments

2023/06/01

03:36 UTC

03:36 UTC

6

It's 4AM but I don't need sleep, I need answers.

5 Comments

2023/06/01

01:59 UTC

01:59 UTC

8

16 Comments

2023/06/01

01:22 UTC

01:22 UTC