Topic 1-1: The intuition of distance
The intuition of distance is a natural human instinct. When confronted with two nearby objects, say A and B, a human observer can always tell whether A is farther away than B, B is farther away than A, or A and B are roughly at the same distance from the observer. Physics cannot and does not explain what distance is, but distance is one of the essential elements of the foundation of physics.
This intuition of distance, though very clear to the observer himself, is rather awkward if the observer tries to explain it to another person. How do you explain how much farther away A is than B? Thus, standards have emerged to define the measure of distances. For example many experts from various countries have agreed to call a specifically produced rod "one meter long" and agreed to store this master "meter" rod in Paris. People then made numerous copies of this master meter rod to bring back to their own countries to mass produce meter rods for measurements. They also agreed to call 1/100-th meter "a centimeter", 1/10-th centimeter "a millimeter" and 1000 meters a kilometer. With the measurement system defined, the observer has a concrete way of saying that A is certain meters farther away than B, and thus everyone else will understand what he is talking about. In this manner the concept of "length" evolves from the intuition of distance. Of course, the meter system is not the only measuring system of distance. In the United States of America the inch-foot-yard-mile system is still widely used, but most of the countries of the rest of the world, including the birthplace of the foot system, England, have switched to the millimeter-centimeter-meter-kilometer, or metric, system. The conversion between those two systems can be confusing and causes a great deal of frustration for American students who have just started to learn physics, but it is not much of a concern for students in other countries who do not need to bother with the aged inch-foot-system.
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