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PRACTICE / Distance and average speed

Why isn’t the average speed simply five?

A runner travels 240 m at 4 m/s, then returns along the same route at 6 m/s. Ignore time spent turning.

What is the average speed for the whole trip?

Organize the givens & assumptions
  • Each leg: 240 m
  • Outward speed: 4 m/s
  • Return speed: 6 m/s

Each leg has constant speed; total distance includes both legs.

  1. A0 m s−10\ \mathrm{m\,s^{-1}}
  2. B4.8 m s−14.8\ \mathrm{m\,s^{-1}}
  3. C5 m s−15\ \mathrm{m\,s^{-1}}
  4. D10 m s−110\ \mathrm{m\,s^{-1}}

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