Ratio, proportion & rates of change · Compound units & rates
Chapter 1 · 3
The idea
Speed, distance and time
Why speed is a rate (distance per one unit of time), how the three formulas are one relationship, why time must be in decimal hours, and why average speed is total distance over total time — never the mean of the speeds.
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Ratio, proportion & rates of change · Compound units & rates
Speed, distance and time
Why speed is a rate (distance per one unit of time), how the three formulas are one relationship, why time must be in decimal hours, and why average speed is total distance over total time — never the mean of the speeds.
Why it works
"Per" means divide
Speed is a rate: how much distance is covered in one unit of time. A speed of 60 km/h literally reads "60 kilometres per (each) hour". That "per" is a division, and it gives the defining formulaThe other two forms aren't separate facts to memorise — they are the same relationship rearranged. If each hour covers 60 km, then hours cover km (), and covering 150 km takes hours (). Sense-check every rearrangement: distance should come out biggest, and a faster speed should make the time smaller.
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The rest of the explanation, plus 2 worked examples you step through move by move.
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