A train crossing a platform must cover its OWN length plus the platform's length — one of the most commonly missed details in speed problems.
Study Zone is free for our students — you just need to log in with your WhatsApp number first.
Log in with WhatsApp →The first two chapters of every book are free once you're logged in. To open this one, request an unlock and we'll get back to you on WhatsApp.
Speed, distance and time all connect through one formula — the only real skill is converting units correctly before using it.
📌 Speed = Distance ÷ Time
km/h to m/s: multiply by 5/18. 72 km/h = 72 × 5/18 = 20 m/s.
m/s to km/h: multiply by 18/5. 20 m/s = 20 × 18/5 = 72 km/h.
Always convert to the SAME units before adding, subtracting, or dividing.
✨ Worked example
Given
240 km covered in 4 hours
Speed = 240 ÷ 4 = 60 km/h
A train crosses a pole
speed 72 km/h, time 10 s
Length = 20 m/s × 10 s = 200 m
A train crossing something must cover a total distance that includes its OWN length — this is the detail every trap question tests.
📊 What distance to use
| Crossing | Total distance to cover |
|---|---|
| A pole or a standing person | Just the train's own length |
| A platform or bridge | Train's length + platform's length |
| Another train (opposite directions) | Sum of both lengths, at the SUM of both speeds |
| Another train (same direction) | Sum of both lengths, at the DIFFERENCE of both speeds |
Worked example: a 150 m train crosses a 250 m platform at 54 km/h (=15 m/s). Total distance = 150+250=400 m. Time = 400÷15 ≈ 26.67 seconds.
A boat's speed changes with the current — downstream it gets a boost, upstream it fights against the same current.
🏆 Boats and streams formulas
Downstream speed = boat speed + stream speed.
Upstream speed = boat speed − stream speed.
To find both: boat speed = (downstream + upstream) ÷ 2; stream speed = (downstream − upstream) ÷ 2.
Worked example: downstream speed 20 km/h, upstream speed 12 km/h. Boat speed = (20+12)÷2 = 16 km/h. Stream speed = (20−12)÷2 = 4 km/h.
12 questions on speed, trains, and boats and streams.
