MadSci Network: Physics

Re: Followup to 1210359712.Ph,Re freefalling frames as inertial

Date: Mon May 19 08:46:58 2008
Posted By: Jim Guinn, Staff, Science, Georgia Perimeter College
Area of science: Physics
ID: 1211134713.Ph

Dear Ray,

Thank you for responding to my post and letting me try to be clearer.

You are absolutely right.  In general, a frame on the Earth is not going 
to be in free fall, and will therefore not be an inertial frame.  I just 
meant that since the Earth, and any frames on the Earth, are in orbit 
around the Sun, the SUNís gravitational force, and subsequent acceleration 
of the Earth because of it, is not what causes a frame on the Earth to be 
non-inertial.  As you say, it is the Earthís gravitational force that 
causes an Earth-bound frame to be non-inertial.  A frame, like the 
International Space Station, that is in free fall about the Sun AND the 
Earth, IS inertial.  In your initial question, you asked that since the 
Earth was in orbit around the Sun, and therefore accelerating, shouldnít 
that preclude the application of Special Relativity.  I was trying to draw 
a distinction between the gravitational force due to the Sun and that due 
to the Earth.  The effects of the force from the Sun are largely removed 
by the motion of the Earth and therefore the Sunís force is not what 
causes an Earth-bound frame to be non-inertial.  The gravitational force 
due to the Earth is what causes an Earth-bound frame to be non-inertial.  
For a frame to be inertial, the effects of that force also have to be 
removed by an additional acceleration, like that of the motion of the ISS.

I hope that clears up any confusion that I caused with my first post.  
Please let us know if you would like some more information.


Jim Guinn
Georgia Perimeter College

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