Page 239 - Handbook of Gold Exploration and Evaluation

210 Handbook of gold exploration and evaluation

· fields of force exerted by magnetic and electrostatically charged devices in
processing plant
· forces of electrostatic attraction, repulsion and ionisation in cohesive soils
· magnetic forces due to the Earth's magnetic field
· centrifugal forces arising from rotation of the Earth-Moon planetary system
· gravitational forces exerted by the Sun, Moon and Earth.

Prime consideration is given to gravitational body forces in this chapter and
throughout the book generally.

Surface forces
Surface forces act by direct contact between the surface of a fluid element or
submerged body and its surroundings. A force that acts perpendicular to the
surface of action is a normal force with pressure intensity. Shear stress is
pressure intensity acting tangentially to the surface of action.

Dynamic pressure
Solids and fluids can exert dynamic pressures and impact forces only when they
result from changes in momentum. By definition, momentum or impact I is the
product of mass times velocity I ˆ MV† and the force F changing the
momentum is its time derivative:
F ˆ Mdv=dt 4.10
As applied to sluicing operations where a high velocity jet of water is directed
against a bank of alluvium, dv=dt ˆ 0 at impact and the momentum of the jet is
totally destroyed in the direction of flow. For a vertical bank, since force equals
the rate of change of momentum, the pressure exerted by the wall on the jet
2
equals pAV in the horizontal plane, where A is the cross-sectional area of the jet
at the point of impact. As applied to centrifugal pumping equipment, when a free
body of fluid in steady motion undergoes a change in angular motion, the
resultant of the external forces acting on the body is a torque equal to the time
rate of change of the angular motion.

Shear stress
A force that acts tangentially to a surface (shear stress) expends part of its
kinetic energy in overcoming viscous forces. Shear forces are developed both
from intermolecular friction between adjacent fluid elements and from fluid
drag along surfaces in contact with and in relative motion with the fluid. Shear
stress and rates of deformation remain constant for any given pressure and
temperature, regardless of the duration of the action.