22 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 
MECHANICAL PROPEKTIES. 
Mechanical properties are the properties of wood which enable it 
to resist deformations, loads, shocks, or forces. Thus the ability 
to resist shearing forces is a mechanical property of timber. (See 
"Strength.") 
MODULUS OF ELASTICITY. 
(Seep. 14.) 
Modulus of elasticity is the ratio of stress per unit area to cor- 
responding strain per unit length, the distortion or strain being 
within the elastic limit. 
Numerically, the modulus of elasticity of a material is the force 
in pounds required to stretch a sample of that material with a cross- 
sectional area of 1 square inch to double its length, on the assump- 
tion that the fibers would not be stressed beyond their elastic limit. 
India rubber has a very low modulus of elasticity, while that of 
steel is very high. It is, then, the measure of the stiffness or rigidity 
of a substance. 
MODULUS OF RUPTURE. 
(See p. 13.) 
PHYSICAL PROPERTIES. 
Physical properties, as the term is used in this bulletin, are those 
properties of wood which have to do with its structure, such as 
density, cell arrangements, fiber length, etc. In its broad sense the 
term physical properties includes all those properties listed as me- 
chanical properties as well as those pertaining to its structure. 
RADIAL. 
Radial means extending outward from a center or an axis. Thus 
a radial surface in a tree is one extending from the pith of the tree 
outward, such as the wide faces of a quarter-sawed board. 
RINGS. 
(Seep. 8.) 
Rings are those circular markings around the center of a tree 
section which are produced by the contrast in density, hardness, 
color, etc., between springwood and summerwood. One ring, known 
as an annual ring, consists of a layer of springwood and a layer 
of summerwood. 
SHEAR. 
(Seep. 17.) 
Shear is the name of the stress which tends to keep two adjoining 
planes or surfaces of a body from sliding, one on the other, under 
the influence of two equal and parallel forces acting in opposite 
directions. A force which produces shear (or shearing stress) in a 
material is called a shearing force. 
