Newtons

The unit of force, the newton, is derived from the unit of
mass through the relationship that force is equal to mass
times the gravitational pull of 9.81 metres per second per second
(9.81 m/s2
), in the direction of the force,
e.g. 1 kilogram f = 9.81 newtons.
For approximate purposes 100 kgf = 1 kN.
Alternatively one newton is that force which, if applied to a
mass of one kilogram, gives that mass an acceleration of one
metre per second per second (1 m/s2
) in the direction of the
force, so 1 N = 1 kg  1 m/s2
.
When calculating the weight of materials for structures, the
kilograms must be multiplied by 9.81 to get the equivalent figure
in newtons (or 9.81 ÷ 1000 for kN).
As a general rule, the following expressions are used:
superimposed loads kN/m2
mass loads kg/m2 or kg/m3
stress N/mm2
1 kN.m = 106
.Nmm (often written 1 kNm = 106 Nmm)
1 N/mm2 = 103 kN/m2