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Forces, Tractions and Stresses |
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We have seen, on the former chapter, that the orientation of newly formed (as opposed as reactivated) faults and fractures can be used to infer the orientation of the principal stress axes that characterise the state of stress that produced faulting. In this chapter, we are going to define the concepts of force, traction and stress. Forces, tractions, and stresses are the causes for deformation.
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The concept of Force: A force is a physical entity which tends to either change the state of rest or the uniform motion in a straight line of a body. A force is described by a vector, its SI unit is the Newton (N). Its magnitude is equal to the rate of change of momentum of the body. The force F producing the acceleration a (meter.s-2) of a body of mass m (kg) is therefore given by: F = m.a this is Newton's second law of motion. Hence, a force of 1 Newton accelerates a 1 kg object by 1 m per second per second.
There is two types of forces: Surfaces forces act on the surface of a body. For instance the tectonic forces (e.g. ridge push and slab pull) acting on a lithospheric plate are surface forces, they are often called external forces. In contrast body forces act inside the body, they are often called internal forces. In a lithospheric plate, body forces arise due to lateral heterogeneity in density.
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The concept of Traction: In mechanic a traction is the pressure vector that represent the limit, as area tends to zero, of the ratio of force over area. This force can have any orientation with respect to the surface. A traction is represented by a vector whose orientation is that of the force and its magnitude the ratio of the force (N) over the area (m2) of the surface. The SI unit of a traction is therefore that of pressure: N.m-2. |
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Tractions can be decomposed into a shear component (parallel to the surface) and normal component (normal to the surface). |
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