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A train 400 m long is moving on a straight track with a speed of 80.1 km/h. The engineer applies the brakes at a crossing, and later the last car passes the crossing with a speed of 16.9 km/h. Assuming constant acceleration, determine how long the train blocked the crossing. Disregard the width of the crossing

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The average speed of the train, from the time deceleration begins until the last car passes, is
(80.1 + 16.9)/2 = 48.5 m/s.
The train moves 400 m during that time, so it took 0.4 km/48.5 km = 8.247*10^-3 h to leave the crossing. That is 29.7 seconds

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