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Old 05-16-2018, 11:52 AM   #1
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Basic Physics
I went on a Wikipedia binge, and wound up reading about Pulsars. According to Wikipedia:

Quote:
PSR J1748-2446ad is the fastest-spinning pulsar known, at 716 Hz,[2] or 716 times per second. This pulsar was discovered by Jason W. T. Hessels of McGill University on November 10, 2004 and confirmed on January 8, 2005.

It has been calculated that the neutron star contains slightly less than two times the mass of the Sun, within the typical range of neutron stars. Its radius is constrained to be less than 16 km. At its equator it is spinning at approximately 24% of the speed of light, or over 70,000 km per second.
https://en.wikipedia.org/wiki/PSR_J1748-2446ad

I shared this on Facebook, because it's fucking nuts to think about, but one of my friends said that the math seemed wrong, and that the proper answer was 1/8 the speed of light; not 1/4.

So I did the math myself and I got... the same answer he got. Are we missing something obvious? I did some more Googling and everyone has 25% listed, so I'm thinking either we are doing something dumb, or everyone else is copy-pasting from the same, mistaken source.

Our math:

Diameter = 16,000 meters
RPS = 716
Speed of Light = 299792458 m/s

(16000)*(pi) = 50265m

(50265)*(716) = 35990000m/s

299792458/35990000 = ~1/8 = ~ 12% the speed of light, not 24%.

What are we missing?
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Old 05-16-2018, 12:05 PM   #2
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Isn't Wikipedia possible to edit for anybody? Why not just change it to the answer you think it is?
Edit: Nevermind, I just changed it for you.
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Old 05-16-2018, 12:36 PM   #3
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Originally Posted by Impureclient View Post
Isn't Wikipedia possible to edit for anybody? Why not just change it to the answer you think it is?
Edit: Nevermind, I just changed it for you.
Hahaha. I figured you were kidding about changing it but you really did.

https://en.wikipedia.org/w/index.php...ldid=819250061
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Old 05-16-2018, 12:51 PM   #4
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Lol, I was gonna, but wanted to make sure I wasn't being a stupid first!
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Old 05-16-2018, 01:00 PM   #5
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Quote:
Originally Posted by Stang70Fastback View Post

299792458/35990000 = ~1/8 = ~ 12% the speed of light, not 24%.

What are we missing?
You didn't include the perceived speed increases of using a lightweight pulley.
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Old 05-16-2018, 01:10 PM   #6
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You didn't include the perceived speed increases of using a lightweight pulley.
Oh man, I went to bat for him in front of the whole science community and he forgot about the pulleys in the equation?!
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Old 05-16-2018, 01:16 PM   #7
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Oh man, I went to bat for him in front of the whole science community and he forgot about the pulleys in the equation?!
We can call in the expert. Probably the same guy that enters the wrong stuff in Wiki to start with though. Does Wiki qualify as a "Scientific Journal"?
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Old 05-16-2018, 01:37 PM   #8
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Quote:
Originally Posted by Stang70Fastback View Post
What are we missing?
Sorry guys, but you are wrong. You mixed up radius and diameter.

The circumference is pi*D = 2*pi*R. You used the D in your equation but the article quoted the radius as 16,000 m, not the diameter.

I'm surprised @Tcoat didn't catch this.
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Old 05-16-2018, 01:45 PM   #9
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Quote:
Originally Posted by jvincent View Post
Sorry guys, but you are wrong. You mixed up radius and diameter.

The circumference is pi*D = 2*pi*R. You used the D in your equation but the article quoted the radius as 16,000 m, not the diameter.

I'm surprised @Tcoat didn't catch this.
I KNEW it was something stupid! Thanks... it all makes sense now!
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Old 05-16-2018, 01:57 PM   #10
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I should make this my avatar.
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Old 05-16-2018, 02:10 PM   #11
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Science is fun!
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Old 05-16-2018, 02:16 PM   #12
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Okay boys... change it back. Hahaha
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Old 05-16-2018, 02:22 PM   #13
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Okay boys... change it back. Hahaha
LOL
And THAT is why you don't trust Wikipedia!
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Old 05-16-2018, 03:12 PM   #14
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Quote:
Originally Posted by Tcoat View Post
LOL
And THAT is why you don't trust Wikipedia!


So true. In theory someone would correct it. It would be a fun experiment to just leave it and see how long it takes for someone else to fix. Hahaha
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