When I read discussions about physics, especially when it comes to relativity, things tend to become quite twisted, just because I have no clear definition of the most basic concepts. Mass From: http://wiki.answers.com/Q/What_is_the_definition_of_mass" There is inertial mass, gravitational mass, and "potential energy" (E = mc^2). My interpretation is that "mass" is different depending on what force you consider (gravity, electromagnetism, or the strong and weak forces). So my question is, can a particle's inertial mass differ from its gravitational mass? And what do we mean when we say that a photon have zero mass? It seems to have gravitational mass at least, doesn't it? And it does, obviously, have potential energy...!? Another interesting question about potential energy... When charging a battery, we move electrons from one metal plate to another. No particles are added, but we have added potential energy. Does this mean that the mass of the battery has increased? Does it weigh more if you place it on a scale? (This is basically the same question as before. Is gravitational mass different from potential energy?) Energy From: http://www.businessdictionary.com/definition/joule.html" "As a unit or work, it is defined as the work done by a force of one newton acting on an object to move it through a distance of one meter in the direction the force is applied." This definition is obviously not correct (if my understanding of the english language is sufficient). Applying a force to an object already in motion, in the same direction as the motion, will result in different work depending on the initial speed of the object (since it takes different amount of time to travel that meter). The definition should be something like "the work done by overcoming a force of one newton acting on an object...". However, how do we define one newton? My point is... In order to define what "force" is, you need a definition of "mass". But in order to define "mass", you need "force". And round it goes. As in math, you need some basic axioms, but they can't be based on each other using circular arguments. So are there any sensible definitions of mass and energy, that does not refer to each other? Velocity In my opinion, velocity is defined as "the difference in distance between two objects over time". And we all know the fact that "nothing can travel with a speed faster than the speed of light". But if the space between two objects is expanding, it seems as if it is totally ok that these objects fly apart at speeds greater than the speed of light, since they are not travelling faster than the speed of light relative to the, eh... Spacetime? Now, what did Einstein say about reference frames? You can't have the spacetime as your reference frame, or can you? Because if you can, we can suddenly speak about things as "absolute velocity". Does the term "aether" ring a bell? Please help me get these things straight.