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Measuring Relative Velocity
How can we measure the relative velocity of two bodies when the distance between them is very large? Whatever method we choose, it will involve the use of light or some other electro-magnetic radiation. Consider two observers, A and B, moving with relative velocity v as shown below. Notice that the diagram is drawn from Aís point of view.
A sends pulses of light to B at intervals of T seconds (as measured on A's clock). If A and B are moving away from each other, as shown in the diagram, B receives the pulses at longer intervals because each pulse has further to go than the preceding pulse. To help in measuring the relative velocity, we will define the constant, k as follows
B has a mirror which reflects the pulses back to A. We will consider the following sequence of events