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suppose there is two point A and B they are connected by straight railway line on which there is moving locootive on figure a there is shown time graphic with relative distance S_A from point A till locooenter image description heretive

from figure B we should choose which one is graphic of the in the same process that express graph of time-distance relation from the B point to locootive measure S_B distance enter image description here

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up vote 3 down vote accepted

Think about this: If points A and B are, say, 3 km apart, then the distance from A to the train plus the distance from the train to B should always be 3 km. For example, if the train is 1 km from A, then it is 2 km from B. If it is 2 km from A, then it is 1 km from B. Which of the choices for graphs of S_B has the property that, for all times t, the value S_A(t) + S_B(t) is the same constant throughout?

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