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August 6th, 2010, 02:59 PM   #1
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Rotate A Circle About A Circle

I have a five inch circle, with the point (for simplicity) (3,4). Now, I want to offset that circle by (1,1). Making my new point (4,5). That seems simple enough. If I rotate the offset circle around the original circle, I would like to be able to calculate the new point. In my example the point was rotated up ~53 degrees from the X axis. If I rotate it to 60 degrees, where would it be? This has me puzzled. I've come up with a few things, but they don't pan out, when I test it in CAD.


Thanks for any help,

GP
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August 7th, 2010, 03:30 AM   #2
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Re: Rotate A Circle About A Circle

What is your center of rotation?
It seems to me as a translation by the vector (1,1).
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August 7th, 2010, 04:06 AM   #3
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Re: Rotate A Circle About A Circle

I have a picture to help explain it, but it won't let me upload it. I'm also posted in another forum, with the picture. Your answer is right, and getting there may be simple, but I've been having problems with it. http://www.freemathhelp.com/forum/vi...p?f=10&t=40678

Thanks,

GP
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August 7th, 2010, 05:17 AM   #4
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Re: Rotate A Circle About A Circle

Your first circle: x+y=25
After translation with vector (1,1): (x-1)+(y-1)=25
Test it with (3,4).
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August 7th, 2010, 09:08 AM   #5
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Re: Rotate A Circle About A Circle

Thanks...I figured it out. The problem wasn't calculating the new point. I was calculating the point when rotated. Obviously, I could go into cad then shift the point, but I needed it to be dynamic. The piece I was missing, is that the new point creates a new angle to the origin. That seems insanely obvious now.

Thanks again,

GP
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August 8th, 2010, 05:19 AM   #6
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Re: Rotate A Circle About A Circle

OK, good luck.
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