For 1 minute the train moved at a speed of 72 km / h, then for 5 minutes at a speed of 15 m / s.

For 1 minute the train moved at a speed of 72 km / h, then for 5 minutes at a speed of 15 m / s. Determine the average train speed over the entire observation period.

Average travel speed:

Vav. = S / t, where S is the total distance traveled (S = S1 + S2), t is the total travel time (t = t1 + t2, where t1 = 1 min = 60 s, t2 = 5 min = 5 * 60 s = 300 s).

Track section S1:

S1 = V1 * t1, where V1 = 72 km / h (72 km / h = 72 * 1000/3600 = 20 m / s).

Track section S2:

S2 = V2 * t2, where V2 = 15 m / s.

Vav. = (V1 * t1 + V2 * t2) / (t1 + t2) = (20 * 60 + 15 * 300) / (60 + 300) = 5700/360 = 15.83 m / s (57 km / h).

Answer: The average speed is 15.83 m / s or 57 km / h.