speed (physics) - Average speed, Cultural significance

The rate of change of distance with time; symbol v, units m/s; a scalar quantity. Speed is the magnitude of velocity, but unlike velocity specifies no direction.

Speed is the rate of motion, or equivalently the rate of change of position, many times expressed as distance d moved per unit of time t.

In mathematical notation, it is simply:

Units of speed include:

meters per second, (symbol m/s), the SI derived unit kilometers per hour, (symbol km/h) miles per hour, (symbol mph) knots (nautical miles per hour, symbol kt) Mach, where Mach 1 is the speed of sound; Mach 1 ≈ 343 m/s ≈ 1235 km/h ≈ 768 mph (see the speed of sound for more detail) speed of light in vacuum (symbol c) is one of the natural units c = 299,792,458 m/s [other important conversions] 1 m/s = 3.6 km/h 1 mph = 1.609 km/h 1 knot = 1.852 km/h = 0.514 m/s

Vehicles often have a speedometer to measure the speed.

Average speed

Speed as a physical property represents primarily instantaneous speed. For example, if you go 60 miles in 2 hours, your average speed during that time is 60/2 = 30 miles per hour, but your instantaneous speed may have varied.

In mathematical notation:

Instantaneous speed defined as a function of time on interval [t0,t1] gives average speed:

while instant speed defined as a function of distance (or length) on interval [l0,l1] gives average speed:

It is often intuitively expected, but incorrect, that going half a distance with speed va and second half with speed vb, produces total average speed .

Average speed can be derived also from speed distribution function (either in time or on distance):

Cultural significance

Speed or swiftness of motion plays a significant role in human culture and also animal culture (see racing).