Speed & Velocity
Speed measures how quickly an object covers distance — it is a derived quantity combining length and time. In everyday life, speed appears in road travel (miles per hour or kilometres per hour), aviation (knots), science (metres per second), and sport. Velocity adds a direction component to speed, but for conversion purposes only magnitude — speed — is relevant. The most common conversion need is between mph and km/h for travellers moving between the United States and metric countries.
Units
- Metres per second (m/s): The SI unit of speed — one metre of distance covered per second of time. Scale: A world-class sprinter runs at about 10–12 m/s; walking pace is roughly 1.4 m/s. Example: The speed of sound in air at 20°C is approximately 343 m/s.
- Kilometres per hour (km/h): 1 000 metres per 3 600 seconds — equal to 5/18 m/s exactly. Scale: Average city driving speed is 30–50 km/h; highway speeds are 100–130 km/h in most countries. Example: Highway speed limits in Europe are typically 120–130 km/h.
- Miles per hour (mph): 1 609.344 metres per 3 600 seconds — equal to approximately 0.447 04 m/s. Scale: US highway speed limits are typically 65–75 mph; residential streets are often 25–30 mph. Example: A professional baseball pitch travels at 85–100 mph.
- Knots (kn): One nautical mile per hour — exactly 1.852 km/h. Used in maritime and aviation navigation. Scale: Commercial aircraft cruise at 470–500 knots; sailing yachts race at 10–25 knots. Example: A commercial airliner's approach speed is typically 130–160 knots.
- Feet per second (ft/s): Exactly 0.3048 m/s. Used in some US engineering and ballistics contexts. Scale: The speed of sound in air is approximately 1 125 ft/s. Example: A sprinter running 100 m in 10 seconds covers about 32.8 ft/s.
Key Facts
- 1 mph = 1.609 344 km/h exactly. A quick approximation: multiply mph by 1.6 to get km/h; divide km/h by 1.6 to get mph.
- The speed of light in a vacuum is exactly 299 792 458 m/s — this exact number defines the metre.
- Knots are used in aviation and shipping because navigational distances are traditionally measured in nautical miles (one arc-minute of latitude).
- The cheetah, the fastest land animal, can sustain sprints of about 112 km/h (70 mph) for short distances.
- Sound travels about 4.3 times faster in water than in air, which is why underwater sonar is effective over long distances.
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