ABSTRACT
The correct unit to express the weight W, of an object is the newton since the weight is the gravitational force that causes a downward acceleration of the object.
Newton, N, is defined as the force that causes a 1 kg mass to accelerate 1 m/s2
Since, a newton, is too small a unit for engineering usage, multiples of newtons expressed as kilonewton, kN, and meganewton, MN, are used. Some of the useful relationships are
1 kilonewton, kN =103 newton =1000 N meganewton, MN =106 newton =103 kN =1000 kN
Stress and Pressure The unit of stress and pressure in SI units is the pascal (Pa) which is equal to 1 newton per square meter (N/m2). Since a pascal is too small a unit, multiples of pascals are used as prefixes to express the unit of stress and pressure. In engineering practice kilopascals or megapascals are normally used. For example,
1 kilopascal =1 kPa =1 kN/m2 =1000 N/m2
1 megapascal =1 MPa =1 MN/m2 =1000 kN/m2
Density Density is defined as mass per unit volume. In the SI system of units, mass is expressed in kg/m3. In many cases, it may be more convenient to express density in megagrams per cubic meter or in gm per cubic centimeter. The relationships may be expressed as
It may be noted here that the density of water, Pw' is exactly 1.00 g/cm3 at 4 DC, and the variation is relatively small over the range of temperatures in ordinary engineering practice. It is sufficiently accurate to write
P w
= 1.00 g/cm3 = 103 kg/m3 = 1 Mg/m3
Unit weight Unit weight is still the common measurement in geotechnical engineering practice. The relationship between unit weight, y, and density p, may be expressed as y= pg. For example, if the density of water, Pw = 1000 kg/m3, then
kg m kg m r =p g=1000-x9.81-=9810-·-
Since, 1 N = 1 kg m, r =9810 ~3 =9.81 kN/m3 s2 W m
Table A.3 Conversion factors
SI to FPS FPS to SI To convert
From To Multiply by From To Multiply by
m ft 3.281 ft m 0.3048 Length m in 39.37 in m 0.0254
cm in 0.3937 in cm 2.54 mm in 0.03937 in mm 25.4
m2 ft2 10.764 fe m2 929.03 x10-4 Area m
2 in2 1550 in2 m2 6.452 x10-4 cm2 in2 0.155 in2 cm2 6.452 mm2 in2 0.155 x 10-2 in2 mm2 645.16
m3 ft3 35.32 ft3 m3 28.317 x 10-3 Volume m3 in3 61,023.4 in3 m3 16.387 x 10-6
cm3 in3 0.06102 in3 cm3 16.387
N lb 0.2248 lb N 4.448
Force kN lb 224.8 lb kN 4.448 x 10-3
kN kip 0.2248 kip kN 4.448 kN US ton 0.1124 US ton kN 8.896
N/m2 lb/fe 20.885 x10-3 lb/fe N/m2 47.88 kN/m2 lb/fe 20.885 lb/fe kN/m2 0.04788
Stress kN/m2 US ton/ft2 0.01044 US ton/ft2 kN/m2 95.76 kN/m2 kip/ft2 20.885 x 10-3 kip/ft2 kN/m2 47.88 kN/m2 Ib/in2 0.145 Iblin2 kN/m2 6.895
Unit weight kN/m 3 lb/fe 6.361 Ib/ft3 kN/m3 0.1572
kN/m3 Ib/in3 0.003682 Ib/in3 kN/m3 271.43
Moment N-m lb-ft 0.7375 lb-ft N-m 1.3558N-m lb-in 8.851 lb-in N-m 0.11298
Moment of inertia mm 4 in4 2.402 x 10-6 in4 mm4 0.4162 x 106
m4 in4 2.402 x 106 in4 m4 0.4162 x 10-6
Section modulus mm 3 in3 6.102 x 10-5 in3 mIn3 0.16387 x lOS
m3 in3 6.102 x 104 in3 m3 0.16387 x 1<J4
m/min ft/min 3.281 ft/min m/min 0.3048 Hydraulic cm/min ft/min 0.03281 ft/min cm/min 30.48 conductivity m/sec ft/sec 3.281 ft/sec m/sec 0.3048
cm/sec in/sec 0.3937 in/sec cm/sec 2.54
Coefficient cm 2/sec in2/sec 0.155 in2/sec cm2/sec 6.452
of consolidation m 2/year in2/sec 4.915 x 10-5 in2/sec m2/year 20.346 x 103
cm2/sec ft2/sec 1.0764 x 10-3 fe/sec cm2/sec 929.03
