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Name: Unknown
Status: N/A
Age: N/A
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Date: Around 1993

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?

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

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 Correction Submitted November, 2013

At the bottom of the page it states I add that gravitational mass equals inertial mass by definition. This statement is wrong. Gravitational mass and inertial mass aren't equal by definition but by observation. i.e. it is an empirical fact. Thank you.

Best wishes, Peter M. Brown

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