Except for cases where nuclear reactions are involved mass in conserved; meaning that it cannot be destroyed nor created. Of course a particular object might loose mass, but whatever is lost can be found elsewhere given enough care. For Stirling engines that means that the mass of gas inside the engine remains constant over time as long as there are no leaks. The mass of gas found in one section of the engine may i vary over time but if it increases in one section another section(s) has to loose mass an equal amount of mass. In terms of a mathematical equation we say that the sum of all masses in the individual sections of an engine is equal to the total mass, the latter being constant. (1)
m
m There is an alternative form in which the law of conservation of mass
can be written. It is cast in terms of changes of mass.
Let (2)
dm Mass is measured in
Opposite to that is the
weight of an object which we obtain by putting an object on a scale. But a
scale, of whatever variety, measures only the force the object exerts onto
it. In metric system scales show weight (the force) in terms of
Of course the force an object exerts onto a scale is equal to the force gravity exerts on your object and as gravity changes from location to location so does the weight of an object. Here on earth gravity is about 0.5% higher at the poles than at the equator ( not including effects of centripetal acceleration) and even more due to elevation. If you could go to the moon you would see a reduction of weight by a factor of 6 and on mars by a factor 2.66. Confusion between weight and mass arises for historical reasons and
imprecision in the usage of language in daily life. In daily usage
- including on the internet - we rarely distinguish between the
force-pound (lb ## Density and specific volumeInstead of only measuring the mass of an object one can consult the density of the kind of material the mass consists of. Or if you want to determine the density (greek symbol ρ (rho) is most often used) of an unknown substance you can measure the total mass,m [kg], and volumei, V [m³], and devide :
ρ = m/V [kg/m³] The inverse of the density is call specific volume v [m³/kg] which in many instances is easier to use : v = V/m = 1/ρ [m³/kg] |

Last revised: 12/09/14