Mass and Gravity

Question:  Is it possible to measure mass in a weightless environment?  Also, 
why does mass exert gravitational forces?  Does it go down to the atomic level 
where different particles have different charges, therefore attracting each 
other?
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Yes, mass can be measured in a weightless environment.  
Basically, "inertial" mass is defined by Newton's law:  F = m a.  So, if you 
can measure a force and acceleration on an object, you can measure its mass.  
The standard method on earth uses force-balance, with one of the forces being 
the gravitational one, which is itself proportional to the "gravitational" 
mass, which has been experimentally shown to be exactly the same as "inertial" 
mass up to quite high precision.  In weightlessness you could not use the 
gravitational force to measure mass and so you would have to measure it by the 
inertial approach, using an unbalanced force.  For example, measuring the 
frequency of oscillation of a spring with the mass to be measured attached 
would give the mass pretty accurately.  Gravity is not associated with the 
charges on things - otherwise it would not depend just on mass, but on what 
kind of microscopic relation there was between charge and mass.  It is a very 
weak force, at least on a human scale, but it is indeed completely independent 
from the electrical forces.  It was a good suggestion though - practically all
the interactions of matter we run into in our lives are through 
electromagnetism - gravity is the one exception.  There are two other forces 
at very short distances in the nucleus.  One of the goals of physics is to try 
and unify all four forces to treat them as one, but there is no working theory 
of this yet!

A. Smith
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I add that gravitational mass equals inertial mass by definition.  
That is the way the gravitation constant (big "G") is defined, in the equation 
F=GmM/r^2 in the equation for the force between masses m and M separated by a 
distance r (r^2 means "r-squared).  What is important is that G is the same 
for ALL pairs of masses, no matter what they are made of.  This fact, which is 
what has been verified to high precision by experiment, is sometimes referred 
to as "the principle of equivalence".

jlu
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