physics
posted by sally .
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

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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