Instruct: Given a question, retrieve relevant documents that answer the question.
Query: What unit is used to measure velocity?
The following example concerns definitions of quantities and units. The (average) velocity ("v") of an object is defined as the quantitative physical property of the object that is directly proportional to the distance ("d") traveled by the object and inversely proportional to the time ("t") of travel, i.e., "v" = "kd"/"t", where "k" is a constant that depends on the units used. Suppose that the meter (m) and the second (s) are base units; then the kilometer (km) and the hour (h) are noncoherent derived units. The meter per second (mps) is defined as the velocity of an object that travels one meter in one second, and the kilometer per hour (kmph) is defined as the velocity of an object that travels one kilometer in one hour. Substituting from the definitions of the units into the defining equation of velocity we obtain, 1 mps = "k" m/s and 1 kmph = "k" km/h = 1/3.6 "k" m/s = 1/3.6 mps. Now choose "k" = 1; then the meter per second is a coherent derived unit, and the kilometer per hour is a noncoherent derived unit. Suppose that we choose to use the kilometer per hour as the unit of velocity in the system. Then the system becomes noncoherent, and the numerical value equation for velocity becomes {"v"} = 3.6 {"d"}/{"t"}. Coherence may be restored, without changing the units, by choosing "k" = 3.6; then the kilometer per hour is a coherent derived unit, with 1 kmph = 1 m/s, and the meter per second is a noncoherent derived unit, with 1 mps = 3.6 m/s.
A Pitot-tube is used to measure fluid flow velocity. The tube is pointed into the flow and the difference between the stagnation pressure at the tip of the probe and the static pressure at its side is measured, yielding the dynamic pressure from which the fluid velocity is calculated using Bernoulli's equation. A volumetric rate of flow may be determined by measuring the velocity at different points in the flow and generating the velocity profile.
Because mass and weight are separate quantities, they have different units of measure. In the International System of Units (SI), the kilogram is the basic unit of mass, and the newton is the basic unit of force. The non-SI kilogram-force is also a unit of force typically used in the measure of weight. Similarly, the avoirdupois pound, used in both the Imperial system and U.S. customary units, is a unit of mass, and its related unit of force is the pound-force.
The cross-sectional area of the flow is calculated from a depth measurement and the average velocity of the flow is measured directly (Doppler and propeller methods are common). Velocity times the cross-sectional area yields a flow rate which can be integrated into volumetric flow. There are two types of area velocity flow meter: (1) wetted; and (2) non-contact. Wetted area velocity sensors have to be typically mounted on the bottom of a channel or river and use Doppler to measure the velocity of the entrained particles. With depth and a programmed cross-section this can then provide discharge flow measurement. Non-contact devices that use laser or radar are mounted above the channel and measure the velocity from above and then use ultrasound to measure the depth of the water from above. Radar devices can only measure surface velocities, whereas laser-based devices can measure velocities sub-surface.
Many other hybrid units are used such as mmHg/cm or grams-force/cm (sometimes as kg/cm without properly identifying the force units). Using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is prohibited in SI; the unit of force in SI is the newton (N).
A force meter is an instrument to measure the magnitude of a force. This is often just a spring contained within a cylinder that has a scale to mark the extension of the spring produced by the force. In the International System of Units, the unit of force is the newton (symbol N). A force meter that measures force in newtons may be referred to as a newtonmeter.
Other units are based on customary units, but use power-of-ten factors and metric prefixes. For example, distance to target for a U.S. submarine is expressed in kiloyards rather than some combination of miles, yards, and feet. Telephone transmission line length and loop distances are measured in kilofeet. In some fields of civil engineering (especially structural engineering), and architecture, large loads and forces (such as the weight of a building or the amount of load applied to a column) are measured in kips (kilopounds or 1000 pounds-force) instead of short tons-force (2000 pounds-force), which are used in virtually all other non-metric industries in the United States, as well as in common usage among the public, when dealing with large values of force.
The names of most derived units of measure in the imperial and US customary systems are concatenations of the constituent parts of the unit of measure, for example the unit of pressure is the "pounds [force] per square inch". Apart from the poundal, most of the named units of measure are non-coherent, but were adopted due to traditional working practice.
The expansion of the universe causes distant galaxies to recede from us faster than the speed of light, if proper distance and cosmological time are used to calculate the speeds of these galaxies. However, in general relativity, velocity is a local notion, so velocity calculated using comoving coordinates does not have any simple relation to velocity calculated locally. (See comoving distance for a discussion of different notions of 'velocity' in cosmology.) Rules that apply to relative velocities in special relativity, such as the rule that relative velocities cannot increase past the speed of light, do not apply to relative velocities in comoving coordinates, which are often described in terms of the "expansion of space" between galaxies. This expansion rate is thought to have been at its peak during the inflationary epoch thought to have occurred in a tiny fraction of the second after the Big Bang (models suggest the period would have been from around 10 seconds after the Big Bang to around 10 seconds), when the universe may have rapidly expanded by a factor of around 10 to 10.
This section is based on the work of Jong-Ping Hsu and Leonardo Hsu. They decided to use the word "Taiji" which is a Chinese word meaning the ultimate principles that existed before the creation of the world. In SI units, time is measured in seconds, but taiji time is measured in units of metres — the same units used to measure space. Their arguments about choosing what units to measure time in, lead them to say that they can develop a theory of relativity which is experimentally indistinguishable from special relativity, but without using the second postulate in their derivation. Their claims have been disputed.
The transformations that they derive involve the factor formula_2 where β is the velocity measured in metres per metre (a dimensionless quantity). This looks the same as (but should NOT be conceptually confused with) the velocity as a fraction of light v/c that appears in some expressions for the Lorentz transformations. Expressing time in metres has previously been done by other authors: Taylor and Wheeler in "Spacetime Physics" and Moore in "Six Ideas that Shaped Physics".
Measurement of light or photometry is generally concerned with the amount of useful light falling on a surface and the amount of light emerging from a lamp or other source, along with the colors that can be rendered by this light. The human eye responds differently to light from different parts of the visible spectrum, therefore photometric measurements must take the luminosity function into account when measuring the amount of useful light. The basic SI unit of measurement is the candela (cd), which describes the luminous intensity, all other photometric units are derived from the candela. Luminance for instance is a measure of the density of luminous intensity in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m). The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m. The amount of useful light emitted from a source or the luminous flux is measured in lumen (lm).
Many different units of measure are used for photometric measurements. People sometimes ask why there need to be so many different units, or ask for conversions between units that can't be converted (lumens and candelas, for example). We are familiar with the idea that the adjective "heavy" can refer to weight or density, which are fundamentally different things. Similarly, the adjective "bright" can refer to a light source which delivers a high luminous flux (measured in lumens), or to a light source which concentrates the luminous flux it has into a very narrow beam (candelas), or to a light source that is seen against a dark background. Because of the ways in which light propagates through three-dimensional space — spreading out, becoming concentrated, reflecting off shiny or matte surfaces — and because light consists of many different wavelengths, the number of fundamentally different kinds of light measurement that can be made is large, and so are the numbers of quantities and units that represent them.
The mass flow meter does not measure the volume per unit time (e.g., cubic meters per second) passing through the device; it measures the mass per unit time (e.g., kilograms per second) flowing through the device. Volumetric flow rate is the mass flow rate divided by the fluid density. If the density is constant, then the relationship is simple. If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles.
Actual density can be determined due to dependency of sound velocity on the controlled liquid concentration.
Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.
While comparing between various classes of animals, a different unit is used, body length per second for organisms. The fastest organism on earth, relative to body length, is the South Californian mite "Paratarsotomus macropalpis", which has a speed of 322 body lengths per second. The equivalent speed for a human running as fast as this mite would be . This is far in excess of the previous record holder, the Australian tiger beetle, "Cicindela eburneola", the fastest insect in the world relative to body size, which has been recorded at or 171 body lengths per second. The cheetah, the fastest land mammal, scores at only 16 body lengths per second while Anna's hummingbird has the highest known length-specific velocity attained by any vertebrate
Associated with propagation of a disturbance are several different velocities. For definiteness, consider an amplitude modulated electromagnetic carrier wave. The phase velocity is the speed of the underlying carrier wave. The group velocity is the speed of the modulation or envelope. Initially it was thought that the group velocity coincided with the speed at which "information" traveled. However, it turns out that this speed can exceed the speed of light in some circumstances, causing confusion by an apparent conflict with the theory of relativity. That observation led to consideration of what constitutes a "signal".
Test units of force measurements are most commonly newtons or pounds. The peak force is the most common result in force testing applications. It is used to determine if a part is good or not. Some examples of force measurement: door latch, quality of spring, wire testing, strength ... but more complicated tests can be performed like peeling, friction, texture.
In a medium, light usually does not propagate at a speed equal to "c"; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity "v". An actual physical signal with a finite extent (a pulse of light) travels at a different speed. The largest part of the pulse travels at the group velocity "v", and its earliest part travels at the front velocity "v".
The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a "refractive index". The refractive index of a material is defined as the ratio of "c" to the phase velocity "v" in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material-dependent constant. The refractive index of air is approximately 1.0003. Denser media, such as water, glass, and diamond, have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-"c" speeds in material substances. As an extreme example of light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light.
The smallest meaningful increment of time is the Planck time, the time light takes to traverse the Planck distance, many decimal orders of magnitude smaller than a second. The largest "realized" amount of time, given known scientific data, is the age of the universe, about 13.8 billion years - the time since the Big Bang as measured in the cosmic microwave background rest frame. Those amounts of time together span 60 decimal orders of magnitude. Metric prefixes are defined spanning 10 to 10, 48 decimal orders of magnitude which may be used in conjunction with the metric base unit of second. Metric units of time larger than the second are most commonly seen only in a few scientific contexts such as observational astronomy and materials science although this depends on author; for everyday usage and most other scientific contexts the common units of minutes (60 s), hours (3600 s or 3.6 ks), days (86 400 s), weeks, months, and years (of which there are a number of variations) are commonly used. Weeks, months and years are significantly variable units whose length crucially depends on the choice of calendar and is often not regular even with a calendar, e.g. leap years versus regular years in the Gregorian calendar. This makes them problematic for use against a linear and regular time scale such as that defined by the SI since it is not clear as to which version of these units we are to be using. Because of this, in the table below we will not use weeks and months and the year we will use is the Julian year of astronomy, or 365.25 days of 86 400 s exactly, also called an annum and denoted with the symbol "a", whose definition is based on the average length of a year of the Julian calendar which had one leap year every and always every 4 years against common years of 365 days each. This unit is used, following the convention of geological science, to form larger units of time by the application of SI prefixes to it at least up to "giga-annum", or Ga, equal to 1 000 000 000 a (short scale: one "billion" years, long scale: one "milliard" years).
A Number of units were used to measure mass. Some of units which were used in 1920s too in addition to metric system, and which belonged to old Spanish, American, and local, are provided below:
A flick is a unit of time equivalent to exactly 1/705,600,000 of a second. The figure was chosen so that frequencies of 24, 25, 30, 48, 50, 60, 90, 100 and 120 Hz, as well as 1/1000 divisions of all those, can be represented with integers. The unit was launched in January 2018 by Facebook. A flick is approximately 1.42 × 10 s, which makes it larger than a nanosecond but much smaller than a microsecond.
Velocity is a metric for work done, which is often used in agile software development. Measuring velocity is sometimes called "velocity tracking". The velocity metric is used for planning sprints and measuring team performance. There is no scientific evidence that measuring velocity improves planning effectiveness or team performance. Furthermore, the metric can be misleading.
To be able to measure the sound velocity, and more specifically the change in sound velocity, in a material subjected to some stress state, one can measure the velocity of an acoustic signal propagating through the material in question. There are several methods to do this but all of them use one of two physical relations of the sound velocity. The first relation is related to the time it takes a signal to propagate from one point to another (typically the distance between two acoustic transducers or two times the distance from one transducer to a reflective surface). This is often referred to as "Time-of-flight" (TOF) measurements, and use the relation
where formula_153 is the distance the signal travels and formula_51 is the time it takes to travel this distance. The second relation is related to the inverse of the time, the frequency, of the signal. The relation here is
where formula_73 is the frequency of the signal and formula_1 is the wave length. The measurements using the frequency as measurand use the pheonomenon of acoustic resonance where formula_5 number of wave lengths match the length over which the signal resonate. Both these methods are dependent on the distance over which it measure, either directly as in the Time-of-flight, or indirectly through the matching number of wavelengths over the physical extent of the specimen which resonate.
Automated direct measurement of streamflow discharge is difficult at present. In place of the direct measurement of streamflow discharge, one or more surrogate measurements can be used to produce discharge values. In the majority of cases, a stage (the elevation of the water surface) measurement is used as the surrogate. Low gradient (or shallow-sloped) streams are highly influenced by variable downstream channel conditions. For these streams, a second stream gauge would be installed, and the slope of the water surface would be calculated between the gauges. This value would be used along with the stage measurement to more accurately determine the streamflow discharge. Within the last ten years, the technological advance of velocity sensors has allowed the use of water velocity as a reliable surrogate for streamflow discharge at sites with a stable cross-sectional area. These sensors are permanently mounted in the stream and measure velocity at a particular location in the stream and related to flow in a manner similar to the use of traditional water level.
The U.S. military uses metric measurements extensively to ensure interoperability with allied forces, particularly NATO Standardization Agreements (STANAG). Ground forces have measured distances in "klicks", slang for kilometers, since 1918. Most military firearms are measured in metric units, beginning with the M-14 which was introduced in 1957, although there are a few legacy exceptions, such as .50-caliber guns. Aircraft ordnance is normally measured in pounds. Heavy weapon caliber is measured in millimeters. Military vehicles are generally built to metric standards. An exception is the U.S. Navy, whose guns are measured in inches and whose undersea fleet measures distances in terms of "kiloyards" (equivalent to 914.4 m), depth as "feet", and velocity, in some cases, as "feet per second". The Navy and Air Force continue to measure distance in nautical miles and speed in knots; these units are now accepted for use with SI by the BIPM. Furthermore, in military aviation NATO countries use feet for flight heights, as they do in the civilian aviation.