Speed Unit Conversion ③ Problems Involving the Length of a Train
In problems about a train crossing a bridge or passing through a tunnel, divide the distance equal to “the train’s length plus the bridge’s length” by the speed to find the time required.
Basics of Train-Passing Problems
- Distance to completely cross a bridge = train’s length + bridge’s length
- Distance to completely pass through a tunnel = train’s length + tunnel’s length
- Distance for a train to pass a utility pole = train’s length only
Example 1: A train 200 m long is traveling at 72 km/hour. How many seconds does it take to completely cross an 800 m bridge?
Solution:
① Change the speed to meters per second → 72 ÷ 3.6 = 20 m/sec
② Calculate the distance traveled → 200 + 800 = 1000 m
③ Time = distance ÷ speed → 1000 ÷ 20 = 50 sec
Solution:
① Change the speed to meters per second → 72 ÷ 3.6 = 20 m/sec
② Calculate the distance traveled → 200 + 800 = 1000 m
③ Time = distance ÷ speed → 1000 ÷ 20 = 50 sec
Example 2: A train 150 m long takes 6 seconds to pass a utility pole. What is the train’s speed in kilometers per hour?
Solution:
① Distance to pass the utility pole = train’s length 150 m
② Find the speed in meters per second → 150 ÷ 6 = 25 m/sec
③ Change to kilometers per hour → 25 × 3.6 = 90 km/hour
Solution:
① Distance to pass the utility pole = train’s length 150 m
② Find the speed in meters per second → 150 ÷ 6 = 25 m/sec
③ Change to kilometers per hour → 25 × 3.6 = 90 km/hour
The distance to use changes depending on “what the train is passing.” Draw a diagram, check where the front and rear of the train are, and then set up the equation.