The Guinness Encyclopedia

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Extensible Markup Language | 1995-08-15 | 6KB | 34 lines

<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE card PUBLIC "-//Apple, Inc.//DTD card V 2.0//EN" "" > <card> <id>14265</id> <filler1>0</filler1> <cantDelete> <false /> </cantDelete> <showPict> <true /> </showPict> <dontSearch> <false /> </dontSearch> <owner>5472</owner> <link rel="stylesheet" type="text/css" href="stylesheet_3106.css" /> <content> <layer>background</layer> <id>25</id> <text>otion and Force (2 of 5)MotionWhen a body is in motion it can be thought of as moving in space and time. If the body moves from one position to another, the straight line joining its starting point to its finishing point is its displacement. This has both magnitude and direction, and is therefore said to be a vector quantity. The motion is linear.The rate at which a body moves, in a straight line or rectilinearly, is its velocity. Again, this has magnitude and direction and is a vector quantity. In contrast, the speed, which has magnitude, but is not considered to be in any particular direction, is a scalar quantity. The average velocity of the body during this rectilinear motion is defined as the change in displacement divided by the total time taken. Its dimensions are therefore length divided by time, and are given in meters per second (m s-1). The instantaneous velocity (the velocity of any instant) at any point is the rate of change of velocity at that point.If the body moves with a changing velocity, then the rate of change of the velocity is the acceleration. This is defined as the change in velocity in a given time interval. Its dimensions are velocity divided by time, and are given in metros per second per second (m s-2). When a body moves with uniform acceleration (uniformly accelerated motion), the displacement, velocity and acceleration are related. These relationships are described in the kinematic equations, sometimes called the laws of uniformly accelerated motion. Kinematics is the study of bodies in motion, ignoring masses and forces.To use the equations to solve a problem in kinematics it is necessary to identify the information given in the problem, then to identify which of the four equations can be manipulated to give the answer required.Galileo Galilei (1564-1642) was an Italian physicist and astronomer who investigated the motion of objects falling freely in air. He believed that all objects falling freely towards the Earth have the same downward acceleration. This is called the acceleration due to gravity or the gravitational acceleration. Near the surface of the Earth it is 9.80 m s-2, but there are small variations in its value depending upon latitude and elevation. In the idealized situation, air resistance is neglected, although in a practical experiment it would have to be considered. In a demonstration on the Moon in August 1971, an American astronaut showed that, under conditions where air resistance was negligible, a feather and a hammer, released at the same time from the same height, would fall side by side. They landed on the lunar surface together.When real motion is considered, both the magnitude and the direction of the velocity have to be investigated. A golf ball, hit upwards, will return to the ground. During flight its velocity will change in both magnitude and direction. In this case, instead of average velocity, the instantaneous velocities have to be evaluated. The velocity can, at any instant, be considered to be acting in two directions, vertical and horizontal. Then the velocity at that instant can be separated into a vertical and a horizontal component. Each component can be considered as being uniformly accelerated rectilinear motion, so the kinematic equations can be applied in each direction. Then the instantaneous velocity and position at any point of the flight can be calculated.</text> </content> <content> <layer>background</layer> <id>26</id> <text>. Instantaneous velocity v can be expressed as a horizontal component and a vertical component. The velocity at point P is v. It has a horizontal component v cos q and a vertical component v sin q.</text> </content> <content> <layer>background</layer> <id>23</id> <text>ΓÇó THE HISTORY OF ASTRONOMYΓÇó FORCES AFFECTING SOLIDS AND FLUIDSΓÇó THERMODYNAMICSΓÇó QUANTUM THEORY AND RELATIVITYΓÇó THE HISTORY OF SCIENCEΓÇó FUNCTIONS, GRAPHS AND CHANGE</text> </content> <content> <layer>background</layer> <id>36</id> <text>162224266070</text> </content> <name>p020-2</name> <script></script> </card>