# Matter, Gravity, Mass

Give examples of four states of matter. Discuss how gravity affects weight and density. Why is the mass density more important than weight density?

The four states of the matter are solid, liquid, gas and plasma. Examples of the solid state is the frozen water (ice), vast majority of metals in normal conditions (below the melting point), a large number of chemical compounds in normal conditions (like the table salt NaCl). Examples of liquids are water in normal conditions, molten state of chemical elements (like Br in normal conditions), etc. Gas state is for example the mixture of chemical elements we call air (a mixture of nitrogen, oxygen and traces of other gas chemical elements or compounds – like CO2 or Helium He). Other example of gas is the water at a temperature higher than its critical temperature vapor . This means that by increasing its pressure the substance in gas state can not be condensed without reducing its pressure. Examples of plasma is any ionized gas. Plasma can be found in the flames of a burning gas, a plasma bulb lightning or into the earth ionosphere at very high altitudes. Also all the stars are made out of plasma.

By DEFINITION the gravity is the acceleration with which a body is attracted by the Earth or any other very large mass (planets). The weight of a body is the force with which that body is attracted by Earth (or another planet). The relationship between the gravity (g) and the weight of a body (W) is as the second physics principle states W =m*g, hence they are interrelated (proportional) . A smaller gravity implies a smaller weight and a bigger gravity implies a bigger weight. On the other hand the density of a body is DEFINED as the mass over the volume of that body, hence the gravity in its original definition (as acceleration) does not affect the density. (However if by gravity one understands the force of attraction one can say that the density becomes higher and higher as one moves from the outside of the planet to its core.)

If by density is by definition the mass of a body over its volume. If by mass density one understands the definition given before and by weight density the weight of a body over its volume, the mass density is more important because it is independent of gravity (or gravitational field of another body), and hence it is an intrinsic property of that body. (it does not depend on the presence of other bodies).