Mechanics of deformable solids

Within a certain kind of environment, an activity may be checked so that the only meaning which accrues is of its direct and tangible isolated outcome. One may cook, or hammer, or walk, and the resulting consequences may not take the mind any farther than the consequences of cooking, hammering, and walking in the literal — or physical — sense. But nevertheless the consequences of the act remain far-reaching. To walk involves a displacement and reaction of the resisting earth, whose thrill is felt wherever there is matter. It involves the structure of the limbs and the nervous system; the principles of mechanics. To cook is to utilize heat and moisture to change the chemical relations of food materials; it has a bearing upon the assimilation of food and the growth of the body. The utmost that the most learned men of science know in physics, chemistry, physiology is not enough to make all these consequences and connections perceptible. The task of education, once more, is to see to it that such activities are performed in such ways and under such conditions as render these conditions as perceptible as possible.

People get a lot of confusion, because they keep trying to think of quantum mechanics as classical mechanics.

Another conspicuous failure of classical mechanics was with one aspect of the problem of radiation. ...Imagine a crowd of steel balls rolling about on a steel floor. ...There must... be a steady leakage of energy from... causes, such as air resistance and the friction of the floor, so the balls will eventually lose energy, and, after no great length of time, will be found standing at rest on the floor. The energy of their motion seems to have been lost... most of it has been transformed into heat. The classical mechanics predicts that this must happen; it shows that all energy of motion, except possibly a minute fraction of the whole, must be transformed into heat whenever such a transformation is physically possible. It is because of this that perpetual-motion machines are a practical impossibility.