Text - "Relativity: The Special and General Theory" Albert Einstein
It is not clear what is to be understood here by "position" and "space." I stand at the window of a railway carriage which is travelling uniformly, and drop a stone on the embankment, without throwing it. Then, disregarding the influence of the air resistance, I see the stone descend in a straight line. A pedestrian who observes the misdeed from the footpath notices that the stone falls to earth in a parabolic curve. I now ask: Do the "positions" traversed by the stone lie "in reality" on a straight line or on a parabola? Moreover, what is meant here by motion "in space" ? From the considerations of the previous section the answer is self-evident. In the first place we entirely shun the vague word "space," of which, we must honestly acknowledge, we cannot form the slightest conception, and we replace it by "motion relative to a practically rigid body of reference." The positions relative to the body of reference (railway carriage or embankment) have already been defined in detail in the preceding section. If instead of " body of reference " we insert " system of co-ordinates," which is a useful idea for mathematical description, we are in a position to say : The stone traverses a straight line relative to a system of coordinates rigidly attached to the carriage, but relative to a system of co-ordinates rigidly attached to the ground (embankment) it describes a parabola. With the aid of this example it is clearly seen that there is no such thing as an independently existing trajectory, but only a trajectory relative to a particular body of reference. In order to have a complete description of the motion, we must specify how the body alters its position with time ; i.e. for every point on the trajectory it must be stated at what time the body is situated there. These data must be supplemented by such a definition of time that, in virtue of this definition, these time-values can be regarded essentially as magnitudes (results of measurements) capable of observation. If we take our stand on the ground of classical mechanics, we can satisfy this requirement for our illustration in the following manner. We imagine two clocks of identical construction ; the man at the railway-carriage window is holding one of them, and the man on the footpath the other. Each of the observers determines the position on his own reference-body occupied by the stone at each tick of the clock he is holding in his hand. In this connection we have not taken account of the inaccuracy involved by the finiteness of the velocity of propagation of light.
next lessonTask Single lesson Lessons group 1000 most popular words Back to test
the best way to learn touch typing is to use AgileFingers! *sample text Start typing Start typing. Good luck! Touch the keyboard and start typing Press any key on your own keyboard, and then start typing the text Welcome to AgileFingers! This is your last attempt for this task! Set your goal Choose a lesson Choose a text Choose or upload a text Choose a task Congratulations! Your score meets your current goal. Too slow! Your current goal is Too inaccurate! Minimum accuracy is Goal achieved!!! , but first sign in and your scores will be saved! Options are stored in your browser, not on your user's account. Every constructive feedback is a great gift! Ah, what a fine day for Science!! Invalid e-mail You are not signed in practice Error saving data Use your own, not the virtual keyboard for typing. Random words Text words per minute characters per minute WPM CPM chart hide Lesson Task Single lesson Lessons group Text s m h Touch typing lessons - learn to type faster - AgileFingers Type much faster without looking at your keyboard. This top free online touch typing course will help you to increase your speed and accuracy." Decide how fast you want to type - AgileFingers When learning to type faster it is important to set a goal. Try to achieve it. If you achieve the goal, set a higher one. Touch typing lessons - AgileFingers You can type faster. Each of your finger type in a different speed. Each finger is responsible for clicking a particular area of your keyboard. Practice fast typing on full texts - AgileFingers Pick a full text for practicing fast typing. Touch typing test - AgileFingers Test if you type as fast as you think you do. If you don't, don't worry. Just practice more with AgileFingeres, eventually you will achieve your goal. Test task Touch typing test - task: Your opinion about AgileFingers Share your thoughts about AgileFingers. We can surely make this course better. Let's work together on that Lesson Fast typing exercise: My own text - AgileFingers Sample text - AgileFingers Typing exercise Fast touch typing exercise Learn touch typing by playing games - AgileFingers Learning touch typing is not the most pleasurable thing to do, but AgileFingers makes it more interesting. Learn to type faster by playing a game! Sheep Rescue - touch typing game - AgileFingers Learn touch typing by playing "Sheep Rescue" - game that teaches how to type faster and more accurately. Press the correct key and get points for your bravery. Star Words - touch typing game - AgileFingers Master touch typing by playing "Star Words" - game that will make you type much faster. This game is a part of AgileFingers course. User profile - AgileFingers User profile for AgileFingers Log in / register - AgileFingers Authentication for AgileFingers Settings - AgileFingers Settings for AgileFingers - touch typing online tutor Teach your students how to type faster - AgileFingers Zone for teachers who wish to improve typing skills of their students. You can start a lesson in classroom or assign your students. Observe how they type faster and faster. Student zone for those who wish to learn touch typing - AgileFingers Take touch typing classes. Your teacher will guide you in touch typing matters. index middle ring little all fingers row Lesson: fingers inner Random words Random words & numbers Most common words numbers homework classroom work One more go Try again edit English