# 9.2.2: Basic Units Blocks

In Thermo–Fluid science there are several basic physical quantities which summarized in Table 9.1. In the table contains two additional physical/basic units that appear in magnetohydrodynamics (not commonly use in fluid mechanics). Many (almost all) of the engineering dimensions used in fluid mechanics can be defined in terms of the four basic physical dimensions MM, LL ,tt and θθ. The actual basic units used can be S.I. such as kilograms, meters, seconds and Kelvins/Celsius or English system or any other system. In using basic new basic physical units, MM, LL, tt, and θθ or the old system relieves the discussion from using particular system measurements. The density, for example, units are Mass/Length3Mass/Length3 and in the new system the density will be expressed as M/L3M/L3 while in S.I. kg/m3kg/m3 and English system it slug/ft3slug/ft3. A common unit used in Fluid Mechanics is the Force, which expressed in SI as Newton [NN]. The Newton defined as a force which causes a certain acceleration of a specific mass. Thus, in the new system the force it will be defined as MLt−2MLt−2. There are many parameters that contains force which is the source reason why the old (or alternative) system use the force instead the mass. There many physical units which are dimensionless by their original definition. are the angle, strains, ratio of specific heats, kk, friction coefficient, ffand ratio of lengths. The angle represented by a ratio of two sides of a triangle and therefore has no units nor dimensions. Strain is a ratio of the change of length by the length thus has no units. Quantities used in engineering can be reduced to six basic dimensions which are presented in Table 9.1. The last two are not commonly used in fluid mechanics and temperature is only used sometimes. Many common quantities are presented in the following Table 9.3.

Table 9.3 Physical units for two common systems. Note the second (time) in large size units appear as "ss'' while in small units as "secsec."

Standard System |
Old System |
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Name |
Letter |
Units |
Name |
Letter |
Units |

Area | L2L2 | [m2m2] | Area | L2L2 | [m2m2] |

Volume | L3L3 | [m3m3] | Volume | L3L3 | [m3m3] |

Angular Velocity | 1t1t | [m3m3] | Angular Velocity | L3L3 | [m3m3] |

Acceleration | Lt2Lt2 | [msec2][msec2] | Acceleration | Lt2Lt2 | [msec2][msec2] |

Angular Acceleration | 1t21t2 | [1sec2][1sec2] | Angular Acceleration | 1t21t2 | [1sec2][1sec2] |

Force |
MLt2MLt2 |
[kgmsec2][kgmsec2] |
Force |
Ft2LFt2L |
[Ns2m][Ns2m] |

Density |
ML3ML3 |
[kgL2][kgL2] |
Density | Ft2L4Ft2L4 | [Ns2m4][Ns2m4] |

Momentum |
MLtMLt |
[kgmsec][kgmsec] | Momentum | FtFt |
[Nsec][Nsec] |

Angular Momentum | ML2tML2t | [kgm2sec][kgm2sec] | Angular Momentum | LFtLFt | [mNsec][mNsec] |

Torque |
ML2t2ML2t2 |
[kgm2s2][kgm2s2] |
Torque |
LFLF |
[mN][mN] |

Absolute Velocity |
MM1t2MM1t2 |
[kgms][kgms] |
Absolute Velocity |
tFL2tFL2 |
[Nssec][Nssec] |

Kinematic Viscocity |
L2tL2t |
[m2s][m2s] |
Kinematic Viscocity |
L2tL2t |
[m2s][m2s] |

Volume Flow Rate |
L3tL3t |
[m3sec][m3sec] |
Volume Flow Rate |
L3tL3t |
[m3sec][m3sec] |

Mass Flow Rate |
MtMt |
[kgsec][kgsec] |
Mass Flow Rate |
FtLFtL |
[Nsm][Nsm] |

Pressure |
MLt2MLt2 |
[kgms2][kgms2] |
Pressure |
FL2FL2 |
[Nm2][Nm2] |

Surface Tension |
Mt2Mt2 |
[kgs2][kgs2] |
Surface Tension |
FLFL |
[Nm][Nm] |

Work or Energy |
ML2t2ML2t2 |
[kgm2s2][kgm2s2] |
Work or Energy |
FLFL |
[Nm][Nm] |

Power |
ML2t3ML2t3 |
[kgm2s3][kgm2s3] |
Power |
FLtFLt |
[Nmsec][Nmsec] |

Thermal Conductivity |
MLt3θMLt3θ |
[kgms3K][kgms3K] |
Thermal Conductivity |
FLtFLt |
[NsK][NsK] |

Specific Heat |
L2t2θL2t2θ |
[m2s2K][m2s2K] |
Specific Heat |
L2Tt2L2Tt2 |
[m2s2K][m2s2K] |

Entropy |
ML2t2θML2t2θ |
[kgm2s2K][kgm2s2K] |
Entropy |
LFTLFT |
[mNK][mNK] |

Specific Entropy |
L2t2θL2t2θ |
[m2s2K][m2s2K] |
Specific Entropy |
L2t2TL2t2T |
[m2s2K][m2s2K] |

Molar Specific Entropy |
ML2t2MθML2t2Mθ |
[m2s2Kmol][m2s2Kmol] |
Molar Specific Entropy |
FL2TMFL2TM |
[NmKmol][NmKmol] |

Enthalpy |
ML2t2ML2t2 |
[kgm2s2][kgm2s2] |
Enthalpy |
LFLF |
[mN][mN] |

Specific Enthalpy |
L2t2L2t2 |
[m2s2][m2s2] |
Specific Enthalpy |
L2t2L2t2 |
[m2sec2][m2sec2] |

Thermo Dynamic Force |
MLt2MMLt2M |
[kgms2mol][kgms2mol] |
Thermo Dynamic Force |
NMNM |
[Nmol][Nmol] |

Catalytic Activity |
MtMt |
[molsec][molsec] |
Catalytic Activity |
MtMt |
[molsec][molsec] |

Heat Transfer Rate |
ML2t3ML2t3 |
[kgm2sec3][kgm2sec3] |
Heat Transfer Rate |
LFtLFt |
[mNsec][mNsec] |