Newton's laws of motion. Newton's laws of motion are a unifying theory explaining both terrestrial and cosmic motion. A body's state of motion, which is defined by its velocity as specified in some reference frame, can be changed only by a force with the change being an acceleration in the direction of the force. The natural state of motion is to continue at a constant ·peed in the same direction. The resistance or inertia a body shows to changing its state of motion is mea·ured by its mass. Newton envisioned space and time as absolute properties of the universe against which absolute motion could be measured. The total quantity of motion or momenta of all bodies in the universe constant with their interactions with each other redistributing it.
Gravity. Gravity is an intrinsic property of matter in ich each body in the universe attracts every other y with a force proportional to the product of their asses and inversely to their separation. Gravity as the force holding bodies in mutual orbit about each other prescribes that the paths allowed will be conic sections: ellipses, circles, parabolas, hyperbolas. From his laws of motion and gravitational law, Newton showed that the constant of proportionality in kepler's third law depends on the masses of the or'ng bodies, which provides astronomers with a means of determining masses for celestial bodies.
Energy. Energy is a measure of the ability of a phys~ system to perform useful work when changing in describable way. Energy can be either or both the -etic energy of motion or the potential energy in a field of force. Energy can neither be created nor deed, only transformed from one form to another; that is, energy is conserved. Waves, which are propagating disturbances, transport energy from one place to another.
Atomic motion. Motion on the atomic scale is governed by electromagnetic forces rather than gravity, and the Newtonian laws of motion have a restricted applicability there. The ato.ms and molecules com .. posing matter are in constant motion, colliding and exchanging kinetic energy in a state known as random thermal motion. Energy added to a gas is distributed among all its particles by random thermal motion with temperature being a measure of the average kinetic energy of the gas particles. Hence energy will flow from high temperatu re (high energy) regions to ones of low temperature (low energy).
Relativity. All motion is relative in that there is no absolute reference frame; one's perception of space and time is dependent on one's frame of reference. In Einstein's special relativity, physical laws are the same in all reference frames moving uniformly relative to each other, and all observers measure the velocity of light as a constant regardless of their reference frame or relative motion. What differs between different frames of reference moving uniformly is the measurement of space and time, meaning that masses, lengths, and time are not measured as being the same in different reference frames. Contrary to Newton's vision, Einstein conceived of different realms of space and time in the universe being defined by the universe itself on a local basis. General relativity is an alternative to Newtonian gravity in which gravitation and acceleration are equivalent interpretations of the same action. Curved space-time tells matter how to move and in turn matter tells local space-time how to curve.
