Excerpt from Strength of Materials
I. - Whenever a force or a number of forces act on a rigid body the general effect is to put the body in what is called a state of stress. By this is meant that there is a tendency for one part of the body to move relatively to another as a result of the force or forces acting on the body. For example, a colliery winding rope is said to be in a state of stress because the force acting along the rope, due to the weight of the cage, tends to separate the lower portion of the rope from the upper.
As another example we may take the case of an engine crank shaft. Here again, the shaft is in a state of stress because the thrust along the connecting rod, acting at the crank pin, turns the shaft and tends to separate one section of the shaft from another by twisting the one relatively to the other.
Stresses are of different kinds. When the forces act in one straight line in opposite directions, away from each other, as represented by Fig. I, the stress is known as a tensile stress. Thus the colliery winding rope above referred to is subjected to a tensile stress due to the forces acting on it, viz, the weight of the cage and the force or reaction resisting this weight; these forces act in the same straight line but in opposite directions, away from each other. The student is of course aware that wherever there is an action there is an equal and opposite reaction. This is Newton's Third Law of Motion.
If the forces act in one straight line but in opposite directions, towards each other, as represented by Fig. 2, the stress produced is known as a compressive stress.
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