The Fastest Average
Maya runs one lap of a 400 m track in 50 s. Her average speed is 8.0 m/s, and her average velocity is zero.
How can both be true of the same run?
Page 1
Science · Physics
Chapter 1: Measurement and Motion in One Dimension
Average and instantaneous are different numbers.
Maya runs one lap of a 400 m track in 50 s. Her average speed is 8.0 m/s, and her average velocity is zero.
How can both be true of the same run?
Make your guess to turn the page.
Still to come: 10 pages · Lab · Words to Know · What We Learned · 4 questions · Explain It in Writing · Open Questions
The idea this lesson is working on — Students take "average velocity" to mean the average of two velocities, and it often gives the right number in textbook problems, which have constant acceleration. The book tests the shortcut against Bolt, whose acceleration was not constant, and it fails by more than 4 m/s. Ask: what does average velocity divide by what? The idea to reach is Δx / Δt, always.
Average speed is not the average of the speeds — Half a journey's distance at 30 km/h and half at 60 km/h averages 40 km/h, not 45, because more time is spent at the slower speed. Bolt's splits are the published figures for the 2009 Berlin final; the ±0.01 s uncertainty is the resolution of the published times, not a measured error.