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. Therefore, speed is always positive.
In mathematical notation, if an object traveling at constant speed
moves a distance
x in time
t, its speed
v is given by:
- v = \frac {x}{t}.
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
- v = \left|\frac {dx}{dt}\right|,
where dx is the distance it travels in a very short period of time
dt. If the object travels a total distance x in time t, its average
speed over that time is given by
- \bar{v} = \left|\frac {x}{t}\right|.
Units
Units of speed include:
- :c = 299,792,458 m/s
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 m/s
Examples of different speeds
- Speed of a common snail = 0.001 m/s; 0.004
km/h; 0.002 mph (1 millimeter per second).
- A brisk walk = 1.7 m/s; 6 km/h; 4 mph (5.5
feet per second).
- Olympic sprinters (average speed over 100 metres) = 10
m/s; 36 km/h; 22 mph.
- Speed
limit on a French autoroute = 36 m/s; 130 km/h; 80 mph.
- Taipei
101 observatory elevator = 1010 m/min ; 16.7 m/s ; 60.6
km/h; 37.6 mph
- Cruising speed of a Boeing 747-8 =
290 m/s; 1050 km/h; 650 mph; (Mach
0.85)
- The speed of sound in dry air at
sea-level pressure and 20 °C (293 kelvin) =
343 ms^{-1} ≈ 1235 km/h ≈ 768 mph ( = Mach 1
by definition).
- Muzzle velocity of an AK47 assault rifle = 710
m/s; 2,600 km/h; 1,600 mph.
- Muzzle velocity of a paintball marker = 90 m/s; 320 km/h; 200
mph.
- Official flight airspeed
record = 980 m/s; 3,530 km/h; 2,194 mph.
- Escape velocity on Earth: 11.2
km/s; 40,000 km/h; 25,000 mph
- Space shuttle on re-entry = 7,800
m/s; 28,000 km/h; 17,500 mph.
- Average orbital speed of planet Earth =
29,783 m/s; 107,218 km/h; 66,623 mph.
- Speed of light in vacuum (symbol c) 299,792,458 m/s
(exactly, by definition).
See also
References
External links