Forces and dynamics - Physical Processes in Earth and Environmental Sciences - page 43

Fig. 3.10 This normal fault scarp formed as the ground to the right dropped quickly downward, with a smaller motion upward of the ground
to the left (person standing). The 1.5 m displacement affected a c .10 km 3 crustal block, giving a force at the base of the block of some
2.75

10 14 N, equivalent to work done in a second or so of some 4.13 10 14 J. 1983 Borah Peake earthquake, Idaho.

forces result. This follows from Newton's Third Law:
An action is always opposed by an equal reaction; or, the
mutual actions of two bodies are always equal and act in
opposite directions.

A

∆ x b

This means that natural forces come in opposing pairs. If
substance of mass m 1 at position x 1 exerts force F on mass
m 2

∆ h

at position x 2 , the latter exerts an exactly equal force

F on the former. Examples would be an atmospheric
boundary layer shearing over the ground surface, exerting
force F ABL , and opposed by equal and opposite force

u

B

As it descends
vertical distance, ∆ h,
the fluid loses potential
energy,

F GR ; a fluid flowing down an inclined channel
(Fig. 3.11); a lithospheric plate sliding over the asthenos-
phere exerting a force, call it F L , which is opposed by an
equal and opposite force from the asthenosphere of

-F

hmg , which is
converted into
kinetic energy, 0.5 mu 2
plus energy losses to
friction

∆

F A
(Fig. 3.12). In all such cases, the total rate of change of
momentum with respect to time is zero, and momentum
is said to be conserved.

F = mg sin b

∆ x

A

b

∆ h

tan b = ∆ h / ∆ x

3.3.5 Other forces and the nature of equilibrium

b

mg

B

In subsequent sections, we will come across natural forces
apart from those due to gravity including viscous, buoy-
ant, pressure, radial, and rotational forces. Each, together
with the gravitational and inertial forces described above
(including contact forces due to friction), may contribute
to the total force acting on any substance. For the moment
we simply say that Newton's Second Law states that the

Fig. 3.11 A mass, m , of water, debris, or lava flowing at mean velocity,
u , under influence of gravity down a channel sloping at angle, ,

exerts a sharing force, mg sin , along the base.