speed (photography) - Average speed, Cultural significance

A rating of the sensitivity of a photographic material on a recognized numerical scale. Previously, ASA (in which doubling the sensitivity doubles the rating number), and DIN (a German industrial standard, where doubling sensitivity adds 3° to the rating) were used, now combined in an International Standards scale in a form such as ISO 200/24°. The arithmetic Exposure Index (EI) rating refers to speed values used in practice for non-standard processing or treatment of a material.

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).