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 July 14th, 2014, 11:29 PM #1 Senior Member     Joined: Oct 2013 From: Far far away Posts: 422 Thanks: 18 A moving experience This is a stupid question but I have to ask. Is there any object in this universe which is at rest? 1) It is absolutely certain that the earth is in motion relative to the sun. 2) Consider an object x 3) x can be either at rest or in motion relative to the earth 4) if x is in motion relative to the earth, then x is in motion 5) if x is at rest relative to the earth, then x is in motion relative to the sun 6) Therefore, x is always in motion relative to earth or the sun 7) Therefore, everything in the universe is in motion If my argument is sound one could move the whole universe just by picking your nose! I'm not quite sure about statement (5). I tried thinking of a scenario where x could be at rest relative to both the sun and the earth but I couldn't. Is it possible for an object x to be at rest relative to two bodies that are themselves in relative motion???? Thanks Last edited by shunya; July 14th, 2014 at 11:39 PM.
 July 15th, 2014, 12:38 AM #2 Senior Member     Joined: Apr 2014 From: Europa Posts: 571 Thanks: 176 The Sun is a special river, the Earth is a simple boat, and the man ... sleeping in the boat. Thanks from shunya and Benit13
July 23rd, 2014, 10:13 AM   #3
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Quote:
 Originally Posted by aurel5 The Sun is a special river, the Earth is a simple boat, and the man ... sleeping in the boat.
Hi aurel5. You should write these down and save it for posterity. If the answer to my question is somewhere in there I didn't get it

 July 23rd, 2014, 10:21 AM #4 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms I don't even think that "at rest" even make sense, except relative to a (possibly implicit) base object. Thanks from perfect_world
July 23rd, 2014, 10:51 AM   #5
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Math Focus: Wibbly wobbly timey-wimey stuff.
Quote:
 Originally Posted by shunya \ If my argument is sound one could move the whole universe just by picking your nose!
Now that's what I call a proper "butterfly effect!"

-Dan

July 23rd, 2014, 10:55 AM   #6
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Math Focus: Wibbly wobbly timey-wimey stuff.
Quote:
 Originally Posted by shunya \ I'm not quite sure about statement (5). I tried thinking of a scenario where x could be at rest relative to both the sun and the earth but I couldn't. Is it possible for an object x to be at rest relative to two bodies that are themselves in relative motion???? Thanks
Sure. Consider an object at the center of mass of the Earth and Sun (or use the geometric center, whatever.) The CM will change position as the Earth moves about the Sun. To a good approximation the CM will move in a circle at more or less constant speed.

You can do this for any number of objects, but your CM may have a very complicated motion in any given reference frame.

-Dan

 July 23rd, 2014, 04:19 PM #7 Senior Member   Joined: Apr 2014 From: Glasgow Posts: 2,127 Thanks: 716 Math Focus: Physics, mathematical modelling, numerical and computational solutions Here's one for you: imagine a rotating unbreakable frisbee that's finite in size but REALLY big. The frisbee spins round at a constant angular velocity about its centre, but the frisbee is so big that the edges of the disc start to approach the speed of light. Now... what does the frisbee look like?
July 24th, 2014, 01:38 AM   #8
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Math Focus: Wibbly wobbly timey-wimey stuff.
Quote:
 Originally Posted by Benit13 Here's one for you: imagine a rotating unbreakable frisbee that's finite in size but REALLY big. The frisbee spins round at a constant angular velocity about its centre, but the frisbee is so big that the edges of the disc start to approach the speed of light. Now... what does the frisbee look like?
Ooooooo! I love this effect!

The frisbee keeps the same shape. But... Draw a line on the non-rotating frisbee, from the edge to the center. When you rotate the frisbee this line turns into a spiral!

-Dan

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