July 15th, 2014, 12:29 AM  #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 15th, 2014 at 12:39 AM. 
July 15th, 2014, 01:38 AM  #2 
Senior Member Joined: Apr 2014 From: Europa Posts: 575 Thanks: 176  The Sun is a special river, the Earth is a simple boat, and the man ... sleeping in the boat. 
July 23rd, 2014, 11:13 AM  #3 
Senior Member Joined: Oct 2013 From: Far far away Posts: 422 Thanks: 18  
July 23rd, 2014, 11:21 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.

July 23rd, 2014, 11:51 AM  #5 
Math Team Joined: May 2013 From: The Astral plane Posts: 2,040 Thanks: 815 Math Focus: Wibbly wobbly timeywimey stuff.  
July 23rd, 2014, 11:55 AM  #6  
Math Team Joined: May 2013 From: The Astral plane Posts: 2,040 Thanks: 815 Math Focus: Wibbly wobbly timeywimey stuff.  Quote:
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, 05:19 PM  #7 
Senior Member Joined: Apr 2014 From: Glasgow Posts: 2,134 Thanks: 720 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, 02:38 AM  #8  
Math Team Joined: May 2013 From: The Astral plane Posts: 2,040 Thanks: 815 Math Focus: Wibbly wobbly timeywimey stuff.  Quote:
The frisbee keeps the same shape. But... Draw a line on the nonrotating frisbee, from the edge to the center. When you rotate the frisbee this line turns into a spiral! Dan  

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