The outlines of physics: an elementary text-book . plate from sliding. (c) Press the plates together for an instant, thus reducing theinterveniBg layer of water to a thin fiber. The plane may now beraised to 90°, as in Fig. 72, without causing the upper plate to slip. (The coefficient of starting friction has risen from an infinitesimal valueto more than unity.) If the experiment be repeated with oil the result will be the same. 94 THE OUTLINES OF PHYSICS 86. Rolling Friction. —? The term friction has been ap-plied, although inaptly, also to the resistance which arolling body experiences. The
The outlines of physics: an elementary text-book . plate from sliding. (c) Press the plates together for an instant, thus reducing theinterveniBg layer of water to a thin fiber. The plane may now beraised to 90°, as in Fig. 72, without causing the upper plate to slip. (The coefficient of starting friction has risen from an infinitesimal valueto more than unity.) If the experiment be repeated with oil the result will be the same. 94 THE OUTLINES OF PHYSICS 86. Rolling Friction. —? The term friction has been ap-plied, although inaptly, also to the resistance which arolling body experiences. The French physicist. Coulomb,who investigated the subject with great care, found theresistance to rolling to be directly proportional to the pres-sure and inversely proportional to the radius of the wheel. 87. Friction of a Shaft in its Bearings. — This is an inter-esting and important case of sliding friction. It may bebriefly stated that the shaft rolls in its bearing until theline of contact is upon a portion of the surface of the latter,. Fig. 73. c (Fig. 73), the tangent to which makes the critical anglewith the horizon. Then it begins to slide. The materialsof the shaft and bearing are selected with a view to thereduction of the angle a to a minimum, and lubricantsare applied for the same purpose. ULASTICITT 95 CHAPTER XIELASTICITY 88. Stress and Strain. — All material bodies owe theirstructure, volume, and form to the interaction of the molec-ular forces between the particles of which they are com-posed. These forces are in equilibrium. When forcesfrom without are brought to bear, this equilibrium is dis-turbed, and there are movements of all the particles withreference to each other. These movements continue untilequilibrium is re-established. The body is then said to beunder stress, and the distortion which results is calledstrain. The changes which are thus brought about are of twokinds, viz.: (1) Changes of form. (2) Changes of volume. When the body is again rele
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