Distance and displacement
Distance is how far something travels along its path. Displacement is how far it ends up from where it started, in a stated direction. Walk 3 m east and then 3 m back west: the distance is 6 m, the displacement is 0 m.
A quantity with a direction, like displacement, is a vector. One with size only, like distance, is a scalar.
Speed and velocity
Average speed is distance divided by time:
speed = distance ÷ time
Its unit is metres per second, m/s. Velocity is displacement divided by time, so it is a vector: it has a direction as well as a size.
Worked example
A runner covers 400 m in 50 s. Average speed = 400 m ÷ 50 s = 8 m/s.
Changing units
One kilometre is 1000 m and one hour is 3600 s, so 1 km/h = 1000 ÷ 3600 m/s. To turn km/h into m/s, divide by 3.6; to go back, multiply by 3.6.
Example: 90 km/h ÷ 3.6 = 25 m/s.
Guided practice
A bus travels 12 km in 20 minutes. First write both in base units: 12 000 m and 1200 s. Speed = 12 000 ÷ 1200 = 10 m/s, which is 10 × 3.6 = 36 km/h.