文档介绍：Lecture 19 Stresses (1)
Force
Force is a kind of mechanical action between different objects, it tends to change the shape, volume or movement state of the object with a force upon it.
Force = mass × acceleration (kg m s-2) [Newton][N]
Force is a vector quantity, and thus possesses both magnitude and direction; it can be represented by an arrow whose length specifies the magnitude and whose orientation specifies the orientation of the force.
F
Unit: Newton
1 Newton = 1 kilogram meter per second squared
vector
scalar
(only magnitude)
Resolution and resultant of forces
A Force F resolved into ponents F1 and F2.
B Two forces F1 and F2 represented by resultant F
F1
F2
F
F1
F2
F
A
B
Surface Forces and Body Forces
Surface forces: the forces acting on the contact surface between adjacent parts of rock system, between adjacent blocks or adjacent lithosphere plates. The contact surface may be or may be not a visible material boundary. It can be a imaginary surface inside the object considered.
Body forces: the forces can work at a distance and depend on the amount of material affected, so, we can call body forces distant forces. Gravitational force is an example of body forces. The gravitational force on a rock body of mass m is
F = mg
where g is the acceleration of gravity. g varies with depth in the earth and with position on the earth’s surface, but for the purpose of structural geology, it is a constant .
Body forces
Uniform forces
Nonuniform forces
External forces
Imaginary plane
Uniform
Internal
Forces
a
b
a
F
F
F
N=F
=N/A=F/A
Internal forces and stresses
Stress on a plane: internal forces acting on unit area of the
given plane within the considering body.
x
External
forces
Internal forces
area
stress
Stress acting at a point m on a plane n is a vector, it can be resolved into ponents and , is normal to the plane, called normal stress, is tangential to the plane, called shear stress.
Normal stress and