10 Worked examples
Example 1 — average velocity of a two-part trip. A bus travels 120 km east in 1.5 h and then 60 km east in 0.5 h. Total displacement = 180 km; total time = 2.0 h. v̄ = 180/2 = 90 km/h east. (This is neither 80 km/h nor 120 km/h: do not average the two part-velocities, 80 and 120, unless the times are equal.)
✍️ Worked examples
Example 2 — return trip. A student walks 600 m east in 5 min and then 600 m back west in 5 min.
- Displacement = 0 → average velocity = 0.
- Distance = 1200 m, so average speed = 1200/600 s = 2 m/s. This shows that speed and velocity are different quantities.
Example 3 — how long? A car moves with an average velocity of 15 m/s. How long does it take to cover 4.5 km? t = 4500/15 = 300 s = 5 min.