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Speed
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Speed is the rate of motion, or equivalently the rate of change of distance.
Speed is a scalar quantity with dimensions length/time; the equivalent vector quantity to speed is velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the magnitude component of velocity.
In mathematical notation, if an object traveling at constant speed moves a distance in time , its speed, denoted by , is simply given by
.
In many situations, objects do not move at a constant speed.

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Encyclopedia
Speed is the rate of motion, or equivalently the rate of change of distance.
Speed is a scalar quantity with dimensions length/time; the equivalent vector quantity to speed is velocity. Speed is measured in the same physical units of measurement as velocity, but does not contain the element of direction that velocity has. Speed is thus the magnitude component of velocity.
In mathematical notation, if an object traveling at constant speed moves a distance in time , its speed, denoted by , is simply given by
.
In many situations, objects do not move at a constant speed. For example, if a car goes 60 miles in 2 hours, its average speed during that time is 30 miles per hour, but its instantaneous speed may have varied. For an object which is accelerating or decelerating, the instantaneous speed is given by
,
where is the distance it travels in a very short period of time . If the object travels a total distance in time , its average speed over that time is given by
.
Units
Units of speed include:
c = 299,792,458 ms-1
Important conversions between units of speed include:
1 m/s = 3.6 km/h
1 mph = 1.609 km/h
1 knot = 1.852 km/h = 0.514 ms-1
Examples of different speeds
- Speed of a common snail = 0.001 ms-1; 0.004 km/h; 0.002 mph (1 millimeter per second).
- A brisk walk = 1.7 ms-1; 6 km/h; 4 mph (5.5 feet per second).
- Olympic sprinters (average speed over 100 metres) = 10 ms-1; 36 km/h; 22 mph.
- Speed limit on a French autoroute = 36 ms-1; 130 km/h; 80 mph.
- Taipei 101 observatory elevator = 1010 m/min ; 16.7 ms-1 ; 60.6 km/h; 37.6 mph
- Cruising speed of a Boeing 747-8 = 290 ms-1; 1050 km/h; 650 mph; (Mach 0.85)
- The speed of sound in dry air at sea-level pressure and 20 °C (293 kelvin) is 343 ms-1 ˜ 1235 km/h ˜ 768 mph ( = Mach 1 by definition).
- Official flight airspeed record = 980 ms-1; 3,530 km/h; 2,194 mph.
- Space shuttle on re-entry = 7,800 ms-1; 28,000 km/h; 17,500 mph.
- Average orbital speed of planet Earth = 29,783 ms-1; 107,218 km/h; 66,623 mph.
- Speed of light in vacuum (symbol c) 299,792,458 ms-1 (exactly, by definition).
See also
External links
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