According to clocks on the ground (system s), streetlights a and b (situated 4 km apart) were both turned on at precisely 8: 00p. m. which one went on first according to an observer on a train (system s') which moves from a to b at 3/5 times the speed of light? how much later (in seconds) did the other light go on?

acceleration = (change in speed) / (time for the change)

change in speed = (20-120)= -100 m/s.

1.5 min = 90 sec

acceleration = -100/90 = -1.11 m/s^2.

we don't care about the 150kg or the 2,500 meters. they don't make any difference.

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