MOTION MOUNTAIN

Chapter 10
CLASSICAL PHYSICS IN A NUTSHELL
Page 8
Classical electrodynamics, with mechanics, thermodynamics and relativity,
ompletes our walk through classical physics. In the structure of physics,
lassical physics encompasses four of the eight points that make up all of physics,
the science of motion. As a whole, classical physics describes the motion of everyday
bodies, the motion of heat, the motion of extremely fast objects, the motion of empty
space, and the motion of light and electric charge. By completing classical physics, we
have covered one half of our adventure. Let us summarize what we have found out about
motion so far – and what we did not.
What can move?
In nature, four entities can move:objects,radiation,space-time andhorizons. In all cases,
their motion happens in such a way as to minimize change. Change is also called (physical)action.Inshort,all
motionminimizesaction.
In all cases of motion, we distinguish the permanent or intrinsic properties from the
varying state. We learned to distinguish and to characterize the possible intrinsic propertiesandthepossiblestatesofeachmoving
entity.
Aboutobjects, we found that in everyday life, all sufficiently small objects or particles
are described completely by their mass and their electric charge. There is no magnetic
charge. Mass and electric charge are thus the only localized intrinsic properties of classical,
everyday objects. Both mass and electric charge are defined by the accelerations
they produce around them. Both quantities are conserved; thus they can be added (with
certain precautions). Mass, in contrast to charge, is always positive. Mass describes the
interaction of objects in collisions and in gravitation, charge the interaction with electromagneticfields.
All varying aspectsofobjects,i.e., theirstate,canbedescribedusingmomentumand
position, as well as angular momentum and orientation. These four quantities can vary
continuously in amount and direction. Therefore the set of all possible states forms a
space, the so-calledphasespace.Thestateofextended,shape-changingobjectsisgiven by
the states of all its constituent particles.These particles make up all objects by interacting
electromagnetically.
The Lagrangian determines the action, or total change, of any kind of motion. Action,
or change, is independent of the observer; the state is not.The states found by different
observers are related: the relations are called the ‘laws’ or properties of motion. Fordifferenttimestheyarecalledevolutionequations,fordifferentplacesandorientationsthey
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net