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lecture_2 - "Principle of the invariance of coincidences...

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1 “Principle of the invariance of coincidences”: when one observer says two events coincide in space and time, so will all other observers. Moving Clocks run slowly Moving Sticks shrink
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2 The Length of a Moving Stick: Part 1  The length of a meter stick that moves with uniform velocity along a line perpendicular to its length is the same as the length of a meter stick at rest. 100 90 80 70 60 50 40 30 20 10 100 90 80 70 60 50 40 30 20 10 The stationary meter stick A measures the length of stick B, which moves with speed v in the direction of the arrows. A B Direction of motion of stick B There is a single instant of time when B crosses A, they are on top of each other. At this moment we note the points on A that are in contact with the 0-cm and 100-cm points of B. If they are the 0-cm and 100-cm of A, the sticks are obviously of the same length.
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3 The Length of a Moving Stick: Part 1  Now suppose, on the contrary, that moving stick B shrank so that, for example, the 100-cm and 0-cm coincided with 95-cm and 0-cm mark of A. 100 90 80 70 60 50 40 30 20 10 100 90 80 70 60 50 40 30 20 10 Direction of motion of stick B B A We can invoke the principle of relativity to show that this is impossible. From B point of view, A is moving, and he must also find that the 100-cm and 0- cm coincided with 95-cm and 0-cm mark of A. So he concludes that a stick moving perpendicular to its length (for him, stick A) gets longer than the stationary sick (for him, stick B). Rule 1 : A meter stick moving with uniform  velocity in a direction perpendicular to itself has  the same length as a meter stick at rest.
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4 QuickTime  and a Animation decompressor are needed to see this picture.
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