A train 130 meters long travelling at 36 km/hr crosses the bridge in 30 sec.…

Concept

When a train crosses a bridge (or any platform), the distance it must travel equals the train's own length PLUS the length of the bridge, because the train is fully clear only after its rear end leaves the far end of the bridge.

The governing relation is: Distance = Speed × Time, and Distance covered = Length of train + Length of bridge.

Application

1. Convert the speed from km/hr to m/s: multiply by 5/18. So 36 × 5/18 = 10 m/s.

2. Find the total distance covered in the given time: Distance = Speed × Time = 10 × 30 = 300 m.

3. This total distance equals train length + bridge length: 300 = 130 + (bridge length).

4. Solve for the bridge length: 300 − 130 = 170 m.

Cross-check

Add the parts back: train (130 m) + bridge (170 m) = 300 m, and at 10 m/s the train covers 300 m in 300/10 = 30 s, which matches the given time. Hence the bridge is 170 m long.