Mass-Energy Equivalence: The concept that any mass has an associated energy, and that, conversly, any energy has an associated mass. In Einstein’s Special Theory of Relativity, this relationship is expressed in the famous mass-energy equivalence formula, E = mc2, where E = total energy, m = mass and c = thespeed of light in a vacuum. Given that c is a very large number, it becomes apparent that mass is in fact a very concentrated form of energy.
Matter: Anything that has both mass and volume (i.e. takes up space). Matter is what atoms and moleculesare made of, and it exists in four states or phases: solid, liquid, gas and plasma (although other phases, such as Bose-Einstein condensates, also exist).
A collection of atoms glued together by electromagnetic forces. A more formal definition might be: a sufficiently stable electrically neutral group of at least two atoms, in a definite arrangement, held together by very strong chemical bonds. A molecule may consist of atoms of the same chemical element(e.g. oxygen: O2) or of different elements (e.g. water: H2O). Organic molecules are those which include carbon, and the others are called inorganic.
Momentum: A measure of how much effort is required to stop a body, defined as the body’s mass multiplied by its velocity. Thus, a large heavy body (e.g. a train) going relatively slowly may have more momentum than a smaller body going very fast (e.g. a racing car). The Law of Conservation of Momentum rules that the total momentum of an isolated system (one in which no net external force acts on the system) does not change.