Given the occurrence of outside forces, in practice the Law of Conservation of
Linear Momentum does not rigorously occur. The boundary of the system can,
however, be modified to include these forces. For example, if we consider as a
system a vertical falling body, there is no conservation of momentum, since the
force of gravity exerted by the Earth, considered as external, acts on the body
causing change in its linear momentum. The system can then be altered to include
the Earth so that the total momentum of the new system (Earth + body) remains
unchanged, with gravity being considered as the inner force. In this case, we
assume a movement of the Earth towards the falling body, whose velocity, as
mentioned above, is practically null, given the enormous mass of the Earth.
A2.4 Topics on Fluid Mechanics
A2.4.1 Fundamental Principles
Fluids are substances, liquid or gaseous, which do not maintain a fixed shape, and
can flow with greater or lesser ease, because their particles do not occupy fixed
positions. Contrary to solids, liquids do not maintain a fixed shape assuming the
shape of their container. A liquid such as a solid is not readily compressible and its
volume can only be significantly modified by a very significant force. The liquid
particles move adjacent to each other, with some internal friction due to their
viscosity. For its part a gas does not have a fixed shape or volume, expanding to fill
in full and simultaneously, the volume of its container and not from the bottom, as
in the case of liquids. Under normal conditions, the gas molecules are very far apart,
in the order of about one hundred diameters, moving randomly and quickly within
their container space.
Density and relative density are two known physical properties fundamental to
the characterization of fluids. The density of a substance, q is defined as the mass,
m, per unit volume, V, given by the ratio m/V. The density is expressed in kgm
−3
(units IS). For example, density values (in gcm
−3 ) of liquids such as water at 4 °C or
ethyl alcohol are 1 and 0.79, respectively. The values of density of gases such as
carbon dioxide or air are 1.98 and 1.29 (in kgm
−3 ) and the values of solids density
(in gcm
−3 ) as wood, cork, or steel are of the order of 0.3–0.9, 0.24 and 7.8,
respectively. In turn, the relative density of a substance relative to a standard
substance is a dimensionless quantity defined as the ratio between the masses of
these substances, occupying the same volume. The standard substance normally
considered for solids and liquids is water at 4 °C. The standard substance usually
considered for gases is air, at the same pressure and temperature as the gas whose
relative density is to be determined.
A fluid is in hydrostatic equilibrium when, in macroscopic terms, at any point in
its container space there is no accumulation or decrease of particles in the fluid. This
situation of macroscopic equilibrium does not occur at the level of atoms and
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Annex A2: Basic Topics on Laws of Motion and Evaporation
Linear Momentum does not rigorously occur. The boundary of the system can,
however, be modified to include these forces. For example, if we consider as a
system a vertical falling body, there is no conservation of momentum, since the
force of gravity exerted by the Earth, considered as external, acts on the body
causing change in its linear momentum. The system can then be altered to include
the Earth so that the total momentum of the new system (Earth + body) remains
unchanged, with gravity being considered as the inner force. In this case, we
assume a movement of the Earth towards the falling body, whose velocity, as
mentioned above, is practically null, given the enormous mass of the Earth.
A2.4 Topics on Fluid Mechanics
A2.4.1 Fundamental Principles
Fluids are substances, liquid or gaseous, which do not maintain a fixed shape, and
can flow with greater or lesser ease, because their particles do not occupy fixed
positions. Contrary to solids, liquids do not maintain a fixed shape assuming the
shape of their container. A liquid such as a solid is not readily compressible and its
volume can only be significantly modified by a very significant force. The liquid
particles move adjacent to each other, with some internal friction due to their
viscosity. For its part a gas does not have a fixed shape or volume, expanding to fill
in full and simultaneously, the volume of its container and not from the bottom, as
in the case of liquids. Under normal conditions, the gas molecules are very far apart,
in the order of about one hundred diameters, moving randomly and quickly within
their container space.
Density and relative density are two known physical properties fundamental to
the characterization of fluids. The density of a substance, q is defined as the mass,
m, per unit volume, V, given by the ratio m/V. The density is expressed in kgm
−3
(units IS). For example, density values (in gcm
−3 ) of liquids such as water at 4 °C or
ethyl alcohol are 1 and 0.79, respectively. The values of density of gases such as
carbon dioxide or air are 1.98 and 1.29 (in kgm
−3 ) and the values of solids density
(in gcm
−3 ) as wood, cork, or steel are of the order of 0.3–0.9, 0.24 and 7.8,
respectively. In turn, the relative density of a substance relative to a standard
substance is a dimensionless quantity defined as the ratio between the masses of
these substances, occupying the same volume. The standard substance normally
considered for solids and liquids is water at 4 °C. The standard substance usually
considered for gases is air, at the same pressure and temperature as the gas whose
relative density is to be determined.
A fluid is in hydrostatic equilibrium when, in macroscopic terms, at any point in
its container space there is no accumulation or decrease of particles in the fluid. This
situation of macroscopic equilibrium does not occur at the level of atoms and
342
Annex A2: Basic Topics on Laws of Motion and Evaporation