Inertial Mass vs. Gravitational Mass

AI Thread Summary
The discussion centers on the intriguing question of why inertial mass and gravitational mass are equivalent, a topic that has puzzled scientists for years. It is noted that while classical mechanics defines mass based on the amount of matter, the proportionality of inertial and gravitational effects raises deeper questions, especially in the context of general relativity. Some participants argue that a violation of the equivalence principle would disrupt fundamental laws like Newton's third law and conservation of momentum. The conversation reflects a mix of curiosity and confusion about the underlying reasons for this equivalence. Ultimately, the relationship between different types of mass remains a significant topic in physics.
runningninja
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An interesting idea that my physics teacher posed to us yesterday, and apparently one that scientists have been puzzling over for quite a while: why is the mass as a measure of inertia equal to the mass in terms of gravity in our universe? My teacher said that this doesn't need to be the case, but it just so happens to be. Why?
 
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runningninja said:
An interesting idea that my physics teacher posed to us yesterday, and apparently one that scientists have been puzzling over for quite a while: why is the mass as a measure of inertia equal to the mass in terms of gravity in our universe? My teacher said that this doesn't need to be the case, but it just so happens to be. Why?
To me that always sounded (and still does) somewhat as misunderstanding. In classical mechanics it was found that both inertia and gravitation depend on "amount of matter" as measured with a balance (read Newton); by next finding equations for these effects, by definition there was only one kind of "mass". I can't find a logical reason for wonder about that simple fact.

The real question, I think, is why the inertial effect of mass is proportional to its gravitational effects (there are two); but if I correctly understand it, this is not the case in GR for fast moving objects (and if I'm mistaken, I'll be happy to hear a detailed correction!).
 
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runningninja said:
An interesting idea that my physics teacher posed to us yesterday, and apparently one that scientists have been puzzling over for quite a while: why is the mass as a measure of inertia equal to the mass in terms of gravity in our universe? My teacher said that this doesn't need to be the case, but it just so happens to be. Why?

I've heard statements similar to the one from your teacher. However, according to everything I've read, a violation of the equivalence principle, as it relates to active gravitational mass, would result in a violation of Newton's third law of motion and the conservation of momentum.

This wikipedia section shows the classical reasoning as to why the three different types of mass must be proportionally equivalent.
http://en.wikipedia.org/wiki/Equivalence_principle#Active.2C_passive.2C_and_inertial_masses
 

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