测绘工程转业英语 Unit 16.docx

Unit 16Gravity MeasurementAs known from daily experience, the most conspicuous force present on the surface of theEarth is gravity. Gravity is the force that pulls things towards the center of the Earth.Gravity affects almost everything in our lives. From clocks to hydroelectric dam s, from thetides of the oceans to plant life, gravity plays an important role. Gravity governs our heightand shape and keeps us from falling off the surface of the Earth.Concepts of GravityGravitation is the force that pulls or attracts all bodies in the universe towards one another.The famous la w of universal gravitation, first formulated mathematically by Newton statesthe fact that any two physical bodies attract each other. This gravitation force, which isproportional to the product of the t w o m asses (Mandm) and inversely proportional to thesquare of their distance, can be expressed as follows:in which G is known as Newton’s gravitational constant . Since the Earth is spinning, anyobject within or on the Earth’s surface pursues a circular path as the Earth rotates on its axis. If the body is on the Earth’s surface, it follows the circular path because it is held ontothe Earth’s surface by the gravitational force. However, the body as it pursues a circularpath exerts an outward force called the centrifugalforce. The sum of the gravitation and thecentrifugal force is called the gravity force. The gravity force is stronger on the poles thanon the equator, since the centrifugal force vanishes on the pole whereas the value of thecentrifugal force on the equator is obtained biggest. Even though centrifugal force is thestrongest on the equator, it is about 0.35% of the gravitation force. Therefore the gravityis mainly governed by the gravitation force. The gravity causes an object to have weightand, if the object is free to move, to fall with increasing speed (i.e., the body accelerates)toward the center of the Earth. The acceleration experienced by that object as it moves toward the Ear