Force and Stress

Question 1

Body force vs surface force

Answer 1

Body forces result from action of a field at every point within a body. It can work at a distance and depends somehow on the amount of the material
affected. Examples: gravity, magnetic.
 Surface forces act only on surfaces, operate
across contact area between adjacent parts of a
body, and these forces originate when one body
pushes or pulls another. VERY important to
geologists and to deforming rocks.
 Body forces and surface forces are closely related
in the earth because body forces give rise to
spatial variations or gradients in the surface
forces

Question 2

compressive and tensile forces

Answer 2

compressive push against each other
-positive (in structural geology)

tensile forces pull away from each other
-negative (in structural geology)

The former push the particles on either side of a plane closer and the latter pull them farther apart.

Question 3

what is stress on a plane (on a surface) and its vector components

Answer 3

force per unit area
σ = F/A

Stress on a plane is a vector quantity

2 vector components:
 Normal stress (90 degree to the plane)
 Shear stress (parallel to the plane)