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Speed is a scalar quantity — it has magnitude only (e.g. 60 km/h). Velocity is a vector quantity — it has both magnitude and direction (e.g. 60 km/h due north). In everyday use, the terms are often used interchangeably, but in physics they are distinct. Average speed = total distance / total time; average velocity = displacement / time.
The speed of sound in dry air at 20°C is approximately 343 m/s (1,235 km/h, 767 mph, Mach 1). It increases with temperature (about 0.6 m/s per °C) and is faster in denser media — about 1,480 m/s in water and 5,100 m/s in steel. This is why you see lightning before you hear thunder.
The speed of light in a vacuum (c) is exactly 299,792,458 m/s (approximately 300,000 km/s or 186,000 miles/s). This is a universal physical constant — the maximum speed at which information or matter can travel. Light takes about 8 minutes to travel from the Sun to Earth, and about 1.3 seconds from the Moon to Earth.
Mach number is the ratio of an object\ speed to the local speed of sound. Mach 1 = speed of sound (≈343 m/s in air at sea level). Mach 2 = twice the speed of sound. Subsonic = below Mach 1, transonic = Mach 0.8–1.2, supersonic = Mach 1.2–5, hypersonic = Mach 5+. The Concorde flew at Mach 2.04.
Average speed ≠ average of the individual speeds. Correct method: total distance ÷ total time. For example, driving 100 km at 50 km/h (2 hrs) then 100 km at 100 km/h (1 hr): average speed = 200km / 3hrs = 66.7 km/h — not (50+100)/2 = 75 km/h. The time-weighted average must be used.
Terminal velocity is the maximum speed reached by a falling object when the drag force equals the gravitational force, producing zero net force and zero acceleration. For a skydiver in a spread-eagle position, terminal velocity is approximately 195 km/h (120 mph). In a head-down dive position, it can exceed 320 km/h (200 mph).
GPS receivers calculate speed by measuring how quickly position changes between consecutive location fixes (Doppler shift method gives even greater accuracy). Most automotive GPS systems update 1–10 times per second. The speed displayed is ground speed — speed relative to the Earth\ surface — not airspeed (used in aviation).
The Parker Solar Probe holds the record for the fastest human-made object, reaching approximately 692,000 km/h (430,000 mph, Mach 560) as it passed close to the Sun in 2023. The Voyager 1 spacecraft is the most distant at about 61,000 km/h, having left the solar system. Bullet trains reach ≈600 km/h; the SR-71 Blackbird held the manned aircraft record at 3,540 km/h.
