How long does it take for a fast train 280 meters long, which travels at a speed of 72 km / hour

How long does it take for a fast train 280 meters long, which travels at a speed of 72 km / hour, to pass by a freight train 700 meters long, which travels towards it at a speed of 54 km / hour.

L1 = 280 m.

V1 = 72 km / h = 20 m / s.

L2 = 700 m.

V2 = 54 km / h = 15 m / s.

t -?

Let’s consider a freight train as a reference body. Since the trains are moving towards each other, the speed of the train relative to the freight one is V12 = V1 + V2.

The passage of trains begins when they meet the first cars and ends when the last cars are on the same level.

Therefore, when passing the train passes the distance L = L1 + L2.

t = L / V12 = (L1 + L2) / (V1 + V2).

t = (280 m + 700 m) / (20 m / s + 15 m / s) = 28 s.

Answer: a fast passage will pass by the goods vehicle during the time t = 28 s.