Page 234 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 205

Weight density

Whereas weight is the force exerted on a mass by gravitational attraction, weight
density
is the weight per unit volume of the substance. The force per unit
volume due to gravity is equal to the weight of the substance per unit volume.
The properties and
can be related via Newton's second law of motion
(F ˆ ma) to give
ˆ g where is the density of the solid and g is the
ÿ2
acceleration due to gravity, 9.81 ms .
Individual particles of different weight density settle at different rates in a
fluid according to the resistance to movement imposed on the particles by the
fluid properties of density and viscosity. Consider two spherical particles of gold
totally immersed in water of density ( ˆ 1). One particle is of high-grade gold,
density 19.0; the other is a lower grade particle (electrum) of density 16.75. The
effective specific weight of the first particle is 19:0 ÿ 1† 9:81 ˆ 176:58 that
of the second particle 16:75 ÿ 1† 9:81 ˆ 154:51. The rate of settling of the
lower grade particle, having a lower density and hence a higher surface area to
volume ratio than the high-grade particle, will be slower because of higher drag
forces.

4.2 Fluvial hydrology
The flow of water in stream channels is governed by the interaction of two
opposing forces: gravity and friction. Gravitational forces act to pull the water
downslope and exert pressure on the confining channel walls. Resistance to flow
is provided by a combination of:
· viscous shearing between the fixed channel boundaries and the moving water
· turbulence and eddying within the fluid
· expenditure of fluid energy when impact and viscous forces build up
sufficiently to overcome the inertia of particles at rest.
These forces, which involve velocity and acceleration possess both magnitude
and direction and hence are vector quantities. Pressure, temperature, length, area
and volume, as scalar quantities, have magnitude alone and can achieve
directional status and show a gradient only when mapped spatially.

4.2.1 Dimensions and units

Measurements of physical quantities are expressed both in terms of a numerical
magnitude and as a unit of measurement. The concept of length is fundamental
and is applied to measurements of depth, width, length, height and diameter.
Three other fundamental dimensions are mass (force), time and temperature.
Units of measurement are grouped dimensionally into three main categories
geometric, kinematic and dynamic: