What speed measures
- Speed is the distance an object travels each second
- Speed is a scalar quantity: it has a magnitude (a numerical size) but no direction
- The SI unit is metres per second (m/s); other units include km/h and mph for everyday contexts
- An object's speed is rarely truly constant from second to second; in most real journeys the instantaneous speed drifts as the motion progresses
Average speed
- Average speed smooths the whole journey into one figure that ignores the moment-by-moment variation:
average speed = (total distance moved) / (total time taken)
- The same equation rearranges three ways:
- distance = average speed × time
- time = distance / average speed
Example — a high-speed ferry maintains an average speed of 18 m/s for 45 min. Calculate the distance it covers.
- Convert the time to SI units: 45 min = 45 × 60 = 2700 s
- distance = average speed × time = 18 × 2700 = 48 600 m (48.6 km)