Time, Speed and Distance
The speed-distance-time relationship, converting between km/h and m/s, relative speed for objects moving in the same and opposite directions, trains crossing objects and platforms, and average speed over a there-and-back trip.
What you will be able to do
- Use speed equals distance over time and its rearrangements, with consistent units.
- Convert between km/h and m/s using the 5/18 factor.
- Apply relative speed for same-direction and opposite-direction motion.
- Solve train problems where the length of the train matters.
What is inside this lesson
- 1The one relationship
- 2Converting km/h and m/s
- 3Relative speed
- 4Worked example: a train crossing a platform
- 5Worked example: two trains passing
- 6Worked example: there and back
- 7Common mistakes
- 8Quick revision
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The one relationship
Everything in this topic comes from speed equals distance divided by time, rearranged as distance equals speed times time, or time equals distance divided by speed.
The only real discipline is units. Speed in kilometres per hour needs distance in kilometres and time in hours. Speed in metres per second needs metres and seconds. Mixed units are the main source of wrong answers.
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