CHAPTER IX
MOMENTUM AND ENERGY
For spirits and men by different standards mete
The less and greater in the flow of time.
By sun and moon, primeval ordinances—
By stars which rise and set harmoniously—
By the recurring seasons, and the swing
This way and that of the suspended rod
Precise and punctual, men divide the hours,
Equal, continuous, for their common use.
Not so with us in the immaterial world;
But intervals in their succession
Are measured by the living thought alone
And grow or wane with its intensity.
And time is not a common property;
But what is long is short, and swift is slow
And near is distant, as received and grasped
By this mind and by that.
Newman, Dream of Gerontius.
One of the most important consequences of the relativity theory is the unification of inertia and gravitation.
The beginner in mechanics does not accept Newton's first law of motion without a feeling of hesitation. He readily agrees that a body at rest will remain at rest unless something causes it to move; but he is not satisfied that a body in motion will remain in uniform motion so long as it is not interfered with. It is quite natural to think that motion is an impulse which will exhaust itself, and that the body will finally come to a stop. The teacher easily disposes of the arguments urged in support of this view, pointing out the friction which has to be overcome when a train or a bicycle is kept moving uniformly. He shows that if the friction is diminished, as when a stone is projected across ice, the motion lasts for a longer time, so that if all interference by friction were removed uniform motion might continue indefinitely. But he glosses over the point that if there were no interference with the motion—if the ice were abolished altogether—the motion would be by no means uniform, but like that of a falling body. The teacher probably insists that the continuance of uniform motion does not require anything that
