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Study the following words and word combinations
account for, v −
agent, n − ,
along with, prep −
attract, v − ,
coalesce, v − ',
consistent, adj −
definitive, adj − ,
density, n − , ; density of matter
disperse, v − ,
draw upon, v − ,
drop, v −
elaboration, n − , ,
essential, adj − ,
familiar, adj − ,
fluid, n −
intact, adj −
intertwine, v − ,
link, n − , ,
observe, v
phenomenon, n
refusal, v
remain, v
responsible, adj (for) −
reveal, v − ,
strict, adj − , ,
tide, n
Read the following text
Text 1
THE ROLE OF GRAVITY
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. In everyday life, gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, and coalesced matter to remain intact, thus accounting for the existence of the Earth, the Sun, and most of the macroscopic objects in the universe. Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth; for the formation of tides; for natural convection, by which fluid flow occurs under the influence of a density gradient and gravity; for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena observed on Earth.
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Gravitation is one of the four fundamental interactions of nature, along with electromagnetism, and the nuclear strong force and weak force. It is, first of all, the only truly universal force. It acts on every material thing from electron to galaxy, and, as we learned, it even acts on particles like photons and neutrinos or energy in any form. Second, gravity has played a uniquely important role in the development and growth of mechanics. Newton, in his definitive formulation of mechanics, drew upon studies of motion near the earth, influenced by local gravity and of planetary motion far from earth, influenced by the Suns gravity.
Since Newtons time, motion governed by gravitational force has provided the strictest tests of mechanics, has served as stimulus for much of the mathematical elaboration of the theory of mechanics, has led to the discovery of distant new planets and artificial satellites, and has revealed new details of the shape and structure of the earth. Through the study of the orbit of Mercury came to the first hint of an imperfection of Newtonian mechanics. Mercurys refusal to follow precisely the laws of classical mechanics stands now as one of the experimental supports of the new mechanics of Einsteins general relativity.
Finally, gravity has a special place in mechanics because of its essential link with motion. Most forces on earth can be measured with a spring balance or other device that involves no motion. The same is obviously not true of the forces acting on the planets. These can be determined only if we study the motion of the planets. The motion determines the force, but at the same time the force determines the motion. Planetary mechanics must be self-consistent theory in which the law of motion (Newtons second law) and the law of gravitational force are intertwined.
