ENGINEERING MECHANICS – Lecture Note

PHYSICAL QUANTITIES, UNITS AND UNIT SYSTEMS

PHYSICAL QUANTITIES

  • A property which can be measured or quantified
  • It can be represented as a number and a corresponding unit

UNITS

  • A unit is usually mentioned along with the measured physical quantity expressed in numbers. This distinguishes different physical quantities.

       > Fundamental units are those which cannot be expressed in terms of other quantities and have physical                   independence of their own

       > 7 fundamental units are:

        > 2 supplementary quantities are:

          > Derived units: All other units which can be expressed in terms of fundamental units.

             Example:

                Area                               m2

                Volume                          m3

                Density                          kg/m3

                Velocity                          m/s

                Acceleration                   m/s2

                Force                              Newton = 1 kg m/s2

                Stress, pressure              F/A = N/m2= Pascal

                 Moment                         Fxr = Nm

                 Work, energy                  F x s = Nm=Joule

         > A vector is often represented by an arrow pointing in the direction of quantity

  • Eg., Force vector of magnitude 5N is expressed in X direction as                                                                  Fx = 5i N (where i is a vector of magnitude 1 in the x direction.)

Dimensional Formula

> Method of expressing a physical quantity in terms of fundamental quantities with proper manner

 

UNIT SYSTEMS

1. CGS system of units [Centimeter-Gram-Second]

  • Length ———-> cm
  • Mass ————> gram
  • Time ————>second
  • Force————>dyne
  • Speed———–>kyne
  • Acceleration —->gal
  • Energy———–>erg
  • Power————>erg/s

2. FPS system of units [Foot-Pound-Second]

  • Length ———–>foot
  • Mass ————->pound
  • Time ————->second
  • Force ————>poundal
  • Energy ———–>poundal-foot
  • Power ————>horse power

3. MKS system of units [Meter-kilogram-second]

  • Length ————>meter
  • Mass ————–>kilogram
  • Time ————–>second
  • Force ————->newton kgf
  • Energy ————>joule
  • Power ————->watt
  • Pressure ———->Pascal

4. SI system of units

> French – system international d/ unites (International system of units)

> Adopted in 1960 by the General conference on weight and measures

  • Length ——————->meter
  • Mass ———————>kilogram
  • Time ———————>second
  • Force ——————–>Newton
  • Energy ——————->Joule
  • Power ——————–>Watt
  • Pressure —————–>Pascal
  • Capacitance ————->Farad
  • Electric charge ———–>Coulomb
  • Electric conductance —–>Siemens
  • Magnetic conductance —>Henry
  • Electric potential ———->Volt
  • Electric resistance ———>Ohm
  • Illuminance —————->Lux
  • Luminous flux ————–>Lumen
  • Magnetic flux ————–>Weber
  • Magnetic flux density ——>Tesla
  • Plane angle —————->Radian
  • Solid angle —————–>Steradian

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