What unit is used to measure velocity?
Coherence (units of measurement)
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.
Velocity
Velocity is defined as the rate of change of position with respect to time, which may also be referred to as the "instantaneous velocity" to emphasize the distinction from the average velocity. In some applications the "average velocity" of an object might be needed, that is to say, the constant velocity that would provide the same resultant displacement as a variable velocity in the same time interval, , over some time period . Average velocity can be calculated as:
Kilometres per hour
Internationally, km/h is the most commonly used unit of speed on traffic signs and speedometers.
Power (physics)
The dimension of power is energy divided by time. The SI unit of power is the watt (W), which is equal to one joule per second. Other units of power include ergs per second (erg/s), horsepower (hp), metric horsepower (Pferdestärke (PS) or cheval vapeur (CV)), and foot-pounds per minute. One horsepower is equivalent to 33,000 foot-pounds per minute, or the power required to lift 550 pounds by one foot in one second, and is equivalent to about 746 watts. Other units include dBm, a relative logarithmic measure with 1 milliwatt as reference; food calories per hour (often referred to as kilocalories per hour); BTU per hour (BTU/h); and tons of refrigeration (12,000 BTU/h).
M/S
m/s is the symbol for metre per second, a unit of both speed and velocity.
International System of Units
The derived units in the SI are formed by powers, products or quotients of the base units and are unlimited in number. Derived units are associated with derived quantities; for example, velocity is a quantity that is derived from the base quantities of time and length, and thus the SI derived unit is metre per second (symbol m/s). The dimensions of derived units can be expressed in terms of the dimensions of the base units.
Tachymeter (watch)
A counter-intuitive method of measuring speeds slower than can be directly indicated was published by Michael Bilow on the web site TimeZone.com: "Because the tachymeter scale is calibrated quite simply as divisors of 3600, it is easy to use the 0-60 segment as a reference scale for the reciprocals. That is, since something which traverses the unit distance in 45 seconds is going 80 units per hour, then it follows mathematically that something which traverses the unit distance in 80 seconds is going 45 units per hour. Since something which traverses the unit distance in 30 seconds is going 120 units per hour, then it follows that something which traverses the unit distance in 120 seconds is going 30 units per hour... Using this method, it is fairly simple to read by inspection nearly any slowly moving object down to the range of 8-12 units per hour." The method was later explained in detail: "It is not intuitive to use the tachymeter scale in this way because there will, in all likelihood, not be any hands pointing to the important places on it. Unlike the conventional use of the tachymeter scale to measure things which take 60 seconds or less to traverse the unit distance, you must mentally move from the number of seconds measured on the chronograph to the same number on the tachymeter scale, and only then read the number opposite on the dial. The ability of the tachymeter scale to be used 'backwards' like this, freely interchanging time and rate, is a direct consequence of the fact that it is created as a table of reciprocals in the first place."
Kinetic energy
where formula_2 is the mass and formula_3 is the speed (or the velocity) of the body. In SI units, mass is measured in kilograms, speed in metres per second, and the resulting kinetic energy is in joules.
Velocity-addition formula
The proof as given is highly formal. There are other more involved proofs that may be more enlightening, such as the one below.
Since a relativistic transformation rotates space and time into each other much as geometric rotations in the plane rotate the - and -axes, it is convenient to use the same units for space and time, otherwise a unit conversion factor appears throughout relativistic formulae, being the speed of light. In a system where lengths and times are measured in the same units, the speed of light is dimensionless and equal to . A velocity is then expressed as fraction of the speed of light.
Work (physics)
For moving objects, the quantity of work/time (power) is integrated along the trajectory of the point of application of the force. Thus, at any instant, the rate of the work done by a force (measured in joules/second, or watts) is the scalar product of the force (a vector), and the velocity vector of the point of application. This scalar product of force and velocity is known as instantaneous power. Just as velocities may be integrated over time to obtain a total distance, by the fundamental theorem of calculus, the total work along a path is similarly the time-integral of instantaneous power applied along the trajectory of the point of application.
Velocity
Since the derivative of the position with respect to time gives the change in position (in metres) divided by the change in time (in seconds), velocity is measured in metres per second (m/s). Although the concept of an instantaneous velocity might at first seem counter-intuitive, it may be thought of as the velocity that the object would continue to travel at if it stopped accelerating at that moment.
Metrication in the United States
Meteorology extensively uses both metric and customary units. The METAR reporting system in use in the United States differs from the one used by the World Meteorological Organization (WMO) in that wind speeds are delivered in knots (instead of meters per second), water-equivalent precipitation in hundredths of an inch, and altimeter setting in the customary inches of mercury (instead of hectopascals). Furthermore, temperatures are reported in whole degrees Fahrenheit but are converted to Celsius when encoded in METAR reports. Most synoptic observation charts are expressed in SI units, except for the aforementioned wind speeds, which must be converted for the purposes of calculation. Upper-air temperature charts typically use the Celsius scale (the freezing line is of great importance at lower levels of the atmosphere in determining precipitation type) while potential temperature charts use the kelvin; both of these are in line with international usage. Atmospheric vorticity is typically measured in full rotations per 100,000 seconds (a rough metrication of the day) but does not use the SI-recommended radian.
Velocity
Escape velocity is the minimum speed a ballistic object needs to escape from a massive body such as Earth. It represents the kinetic energy that, when added to the object's gravitational potential energy, (which is always negative) is equal to zero. The general formula for the escape velocity of an object at a distance "r" from the center of a planet with mass "M" is
where "G" is the Gravitational constant and "g" is the Gravitational acceleration. The escape velocity from Earth's surface is about 11 200 m/s, and is irrespective of the direction of the object. This makes "escape velocity" somewhat of a misnomer, as the more correct term would be "escape speed": any object attaining a velocity of that magnitude, irrespective of atmosphere, will leave the vicinity of the base body as long as it doesn't intersect with something in its path.
Velocity
In polar coordinates, a two-dimensional velocity is described by a radial velocity, defined as the component of velocity away from or toward the origin (also known as "velocity made good"), and an angular velocity, which is the rate of rotation about the origin (with positive quantities representing counter-clockwise rotation and negative quantities representing clockwise rotation, in a right-handed coordinate system).
Knot (unit)
The knot () is a unit of speed equal to one nautical mile per hour, exactly (approximately ). The ISO standard symbol for the knot is . The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE); is also common, especially in aviation where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.
Proper velocity
In relativity, proper velocity, also known as celerity, is an alternative to velocity for measuring motion. Whereas velocity relative to an observer is distance per unit time where both distance and time are measured by the observer, proper velocity relative to an observer divides observer-measured distance by the time elapsed on the clocks of the traveling object. Proper velocity equals velocity at low speeds. Proper velocity at high speeds, moreover, retains many of the properties that velocity loses in relativity compared with Newtonian theory.
Gravimetry
An instrument used to measure gravity is known as a gravimeter, or gravinometer. For a small body, general relativity predicts gravitational effects indistinguishable from the effects of acceleration by the equivalence principle. Thus, gravimeters can be regarded as special-purpose accelerometers. Many weighing scales may be regarded as simple gravimeters. In one common form, a spring is used to counteract the force of gravity pulling on an object. The change in length of the spring may be calibrated to the force required to balance the gravitational pull. The resulting measurement may be made in units of force (such as the newton), but is more commonly made in units of gals.
Tachometer
A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common. 
The word comes from Greek "ταχος" ("tachos" "speed") and "metron" ("measure"). Essentially the words tachometer and speedometer have identical meaning: a device that measures speed. It is by arbitrary convention that in the automotive world one is used for engine and the other for vehicle speed. In formal engineering nomenclature, more precise terms are used to distinguish the two.
Speed of light
In 2011, the CGPM stated its intention to redefine all seven SI base units using what it calls "the explicit-constant formulation", where each "unit is defined indirectly by specifying explicitly an exact value for a well-recognized fundamental constant", as was done for the speed of light. It proposed a new, but completely equivalent, wording of the metre's definition: "The metre, symbol m, is the unit of length; its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly when it is expressed in the SI unit ." This is one of the proposed changes to be incorporated in the next revision of the SI also termed the "New SI".
Velocity
Speed describes only how fast an object is moving, whereas velocity gives both how fast it is and in which direction the object is moving. If a car is said to travel at 60 km/h, its speed has been specified. However, if the car is said to move at 60 km/h to the north, its velocity has now been specified.
SI base unit
The International System of Units (SI) defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the meter for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.
Revolutions per minute
According to the International System of Units (SI), rpm is not a unit. This is because the word "revolution" is a semantic annotation rather than a unit. The annotation is instead done as a subscript of the formula sign if needed. Because of the measured physical quantity, the formula sign has to be for (rotational) frequency and or for angular velocity. The corresponding basic SI derived unit is s or Hz. When measuring angular speed, the unit radians per second is used.
Equivalent airspeed
formula_16 and formula_17 are airspeeds and can be measured in knots, km/h, mph or any other appropriate unit.
Newton's laws of motion
The first law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant. Velocity is a vector quantity which expresses both the object's speed and the direction of its motion; therefore, the statement that the object's velocity is constant is a statement that both its speed and the direction of its motion are constant.
Gal (unit)
The gal (symbol: Gal), sometimes called galileo after Galileo Galilei, is a unit of acceleration used extensively in the science of gravimetry. The gal is defined as 1 centimeter per second squared (1 cm/s). The milligal (mGal) and microgal (µGal) refer respectively to one thousandth and one millionth of a gal.
Metric system
Today, the International system of units consists of 7 base units and innumerable coherent derived units including 22 with special names. The last new derived unit, the "katal" for catalytic activity, was added in 1999. Some of the base units are now realised in terms of invariant constants of physics. As a consequence, the speed of light has now become an exactly defined constant, and defines the metre as of the distance light travels in a second. The kilogram remains defined by a man-made artefact of platinum-iridium, and it is deteriorating. The range of decimal prefixes has been extended to those for 10, "yotta", and 10, "yocto", which are unfamiliar because nothing in our everyday lives is that big or that small.
Coherence (units of measurement)
A definition of a physical quantity is a statement that determines the ratio of any two instances of the quantity. The specification of the value of any constant factor is not a part of the definition since it does not affect the ratio. The definition of velocity above satisfies this requirement since it implies that "v"/"v" = ("d"/"d")/("t"/"t"); thus if the ratios of distances and times are determined, then so is the ratio of velocities. A definition of a unit of a physical quantity is a statement that determines the ratio of any instance of the quantity to the unit. This ratio is the numerical value of the quantity or the number of units contained in the quantity. The definition of the meter per second above satisfies this requirement since it, together with the definition of velocity, implies that "v"/mps = ("d"/m)/("t"/s); thus if the ratios of distance and time to their units are determined, then so is the ratio of velocity to its unit. The definition, by itself, is inadequate since it only determines the ratio in one specific case; it may be thought of as exhibiting a specimen of the unit.
Astronomical unit
The metre is defined to be a unit of proper length, but the SI definition does not specify the metric tensor to be used in determining it. Indeed, the International Committee for Weights and Measures (CIPM) notes that "its definition applies only within a spatial extent sufficiently small that the effects of the non-uniformity of the gravitational field can be ignored". As such, the metre is undefined for the purposes of measuring distances within the Solar System. The 1976 definition of the astronomical unit was incomplete because it did not specify the frame of reference in which time is to be measured, but proved practical for the calculation of ephemerides: a fuller definition that is consistent with general relativity was proposed, and "vigorous debate" ensued until August 2012 when the IAU adopted the current definition of 1 astronomical unit = metres.
Glossary of video terms
The standard unit for measuring frequency. One hertz (abbreviated Hz) equals one cycle per second. It is commonly used to measure sound waves, light waves and radio waves.
Speed of light
An astronomical unit (AU) is approximately the average distance between the Earth and Sun. It was redefined in 2012 as exactly . Previously the AU was not based on the International System of Units but in terms of the gravitational force exerted by the Sun in the framework of classical mechanics. The current definition uses the recommended value in metres for the previous definition of the astronomical unit, which was determined by measurement. This redefinition is analogous to that of the metre, and likewise has the effect of fixing the speed of light to an exact value in astronomical units per second (via the exact speed of light in metres per second).
Metric system
The metre, ampere, candela, and mole are all defined in terms of other base units. For example, the speed of light is defined as 299,792,458 metres per second, and the metre is derived from that constant and the definition of a second. As a result, in dimensional analysis, they remain wholly separate concepts.
Metric system
While there are numerous named derived units of the metric system, such as watt and lumen, other common quantities such as velocity and acceleration do not have their own unit, but are defined in terms of existing base and derived units such as metres per second for velocity.
Weight
In United States customary units, the pound can be either a unit of force or a unit of mass. Related units used in some distinct, separate subsystems of units include the poundal and the slug. The poundal is defined as the force necessary to accelerate an object of one-pound "mass" at 1 ft/s, and is equivalent to about 1/32.2 of a pound-"force". The slug is defined as the amount of mass that accelerates at 1 ft/s when one pound-force is exerted on it, and is equivalent to about 32.2 pounds (mass).
Length
In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light. The centimetre and the kilometre, derived from the metre, are also commonly used units. In U.S. customary units, English or Imperial system of units, commonly used units of length are the inch, the foot, the yard, and the mile.
Proper velocity
In flat spacetime, proper velocity is the ratio between distance traveled relative to a reference map frame (used to define simultaneity) and proper time τ elapsed on the clocks of the traveling object. It equals the object's momentum p divided by its rest mass "m", and is made up of the space-like components of the object's four-vector velocity. William Shurcliff's monograph mentioned its early use in the Sears and Brehme text. Fraundorf has explored its pedagogical value while Ungar, Baylis and Hestenes have examined its relevance from group theory and geometric algebra perspectives. Proper velocity is sometimes referred to as celerity.
Metre per second squared
The metre per second squared is the unit of acceleration in the International System of Units (SI). As a derived unit, it is composed from the SI base units of length, the metre, and time, the second. Its symbol is written in several forms as m/s, m·s or m s, or less commonly, as m/s/s.
Speed
Speed has the dimensions of distance divided by time. The SI unit of speed is the metre per second, but the most common unit of speed in everyday usage is the kilometre per hour or, in the US and the UK, miles per hour. For air and marine travel the knot is commonly used.
Specific force
Specific force (also called g-force and mass-specific force) is measured in meters/second² (m·s) which is the units for acceleration. Thus, specific force is not actually a force, but a type of acceleration. However, the (mass-)specific force is not a coordinate-acceleration, but rather a proper acceleration, which is the acceleration relative to free-fall. Forces, specific forces, and proper accelerations are the same in all reference frames, but coordinate accelerations are frame-dependent. For free bodies, the specific force is the cause of, and a measure of, the body's proper acceleration.
Flow measurement
For liquids, various units are used depending upon the application and industry, but might include gallons (U.S. or imperial) per minute, liters per second, bushels per minute or, when describing river flows, cumecs (cubic meters per second) or acre-feet per day. In oceanography a common unit to measure volume transport (volume of water transported by a current for example) is a sverdrup (Sv) equivalent to 10 m/s.
Units of measurement in France
During the early part of the twentieth century, the French introduced their own units of power - the "poncelet", which was defined as being the power required to raise a mass of 100 kg against standard gravity with a velocity of 1 m/s giving a value of 980.665 W. However, many other European countries defined their units of power (the "Pferdestärke" in Germany, the "paardekracht" in the Netherlands and the "cavallo vapore" in Italy) using 75 kg rather than 100 kg, which gave a value of 735.49875 W (about 0.985 HP). Eventually, the "poncelet" was replaced with the "cheval vapeur", which was identical to equivalent units of measure in neighbouring countries. In 1977, these units, along with the "stere" and the "livre" (and amongst others, the German "pfund") were proscribed by EEC Directive 71/354/EEC
Newton (unit)
Newton's second law of motion states that , where is the force applied, is the mass of the object receiving the force, and is the acceleration of the object. The newton is therefore:
where the following symbols are used for the units: N for newton, kg for kilogram, m for metre, and s for second.
Miles per hour
Miles per hour (abbreviated mph, MPH or mi/h) is an imperial and United States customary unit of speed expressing the number of statute miles covered in one hour. It is used in the United States, United Kingdom and many Commonwealth nations, particularly in the Caribbean region.
Velocity
and
Force
In physics, a force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a push or a pull. A force has both magnitude and direction, making it a vector quantity. It is measured in the SI unit of newtons and represented by the symbol F.
Power (physics)
In physics, power is the rate of doing work or transferring heat, the amount of energy transferred or converted per unit time. Having no direction, it is a scalar quantity. In the International System of Units, the unit of power is the joule per second (J/s), known as the watt in honour of James Watt, the eighteenth-century developer of the condenser steam engine. Another common and traditional measure is horsepower (comparing to the power of a horse). Being the rate of work, the equation for power can be written:
Atomic units
Both Planck units and au are derived from certain fundamental properties of the physical world, and are free of anthropocentric considerations. It should be kept in mind that au were designed for atomic-scale calculations in the present-day universe, while Planck units are more suitable for quantum gravity and early-universe cosmology. Both au and Planck units normalize the reduced Planck constant. Beyond this, Planck units normalize to 1 the two fundamental constants of general relativity and cosmology: the gravitational constant "G" and the speed of light in a vacuum, "c". Atomic units, by contrast, normalize to 1 the mass and charge of the electron, and, as a result, the speed of light in atomic units is a large value, formula_21. The orbital velocity of an electron around a small atom is of the order of 1 in atomic units, so the discrepancy between the velocity units in the two systems reflects the fact that electrons orbit small atoms much slower than the speed of light (around 2 orders of magnitude slower).
List of unusual units of measurement
MERU (Milli Earth Rate Unit), an angular velocity equal to 1/1000 of Earth's rotation rate: 1 MERU = 0.015 degrees/hour ≈ 0.072921 microradian/second. Sometimes used to measure the angular drift rate of an inertial navigation system.
International System of Units
The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, and one physical artefact. The artefact is the international prototype kilogram, certified in 1889, and consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units entirely in terms of constants of nature, scheduled to be put into effect on 20 May 2019.
Velocimetry
In general, velocity measurements are made in the Lagrangian or Eulerian frames of reference (see Lagrangian and Eulerian coordinates). Lagrangian methods assign a velocity to a volume of fluid at a given time, whereas Eulerian methods assign a velocity to a volume of the measurement domain at a given time. A classic example of the distinction is particle tracking velocimetry, where the idea is to find the velocity of individual flow tracer particles (Lagrangian) and particle image velocimetry, where the objective is to find the average velocity within a sub-region of the field of view (Eulerian).
Wind gradient
In common usage, wind gradient, more specifically wind speed gradient
or wind velocity gradient,
or alternatively shear wind,
is the vertical gradient of the mean horizontal wind speed in the lower atmosphere. It is the rate of increase of wind strength with unit increase in height above ground level. In metric units, it is often measured in units of meters per second of speed, per kilometer of height (m/s/km), which reduces to the standard unit of shear rate, inverse seconds (s).
Gurley Precision Instruments
The Gurley unit is a measure of the stiffness of a material. Since the measurement device holds a piece of material vertically and tests the force required to deflect the material a given amount. One Gurley unit is equivalent to one milligram of force
, however the Gurley unit also implies measurement with this apparatus. A related unit, the Taber, is highly correlated but uses a different apparatus (manufactured by Taber Industries) for performing measurements.
Unit of length
A unit of length refers to any discrete, pre-established length or distance having a constant magnitude which is used as a reference or convention to express linear dimension. The most common units in modern use are U.S. customary units in the United States and metric units elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.
Velocity
where we may identify
Kilometre
The kilometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: km; or ) or kilometer (American spelling) is a unit of length in the metric system, equal to one thousand metres (kilo- being the SI prefix for ). It is now the measurement unit used officially for expressing distances between geographical places on land in most of the world; notable exceptions are the United States and the road network of the United Kingdom where the statute mile is the official unit used.
Faster-than-light
The speed of light is a dimensional quantity and so cannot be measured. Measurable quantities in physics are, without exception, dimensionless, although they are often constructed as ratios of dimensional quantities. For example, when the height of a mountain is measured, what is really measured is the ratio of its height to the length of a meter stick. The conventional SI system of units is based on seven basic dimensional quantities, namely distance, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity. These units are defined to be independent and so cannot be described in terms of each other. As an alternative to using a particular system of units, one can reduce all measurements to dimensionless quantities expressed in terms of ratios between the quantities being measured and various fundamental constants such as Newton's constant, the speed of light and Planck's constant; physicists can define at least 26 dimensionless constants which can be expressed in terms of these sorts of ratios and which are currently thought to be independent of one another. By manipulating the basic dimensional constants one can also construct the Planck time, Planck length and Planck energy which make a good system of units for expressing dimensional measurements, known as Planck units.
G-force
The unit of measure of acceleration in the International System of Units (SI) is m/s. However, to distinguish acceleration relative to free fall from simple acceleration (rate of change of velocity), the unit g (or g) is often used. One "g" is the acceleration due to gravity at the Earth's surface and is the standard gravity (symbol: "g"), defined as  metres per second squared, or equivalently  newtons of force per kilogram of mass. Note that the "unit definition" does not vary with location—the g-force when standing on the moon is about 0.181 g.
Metrication
Air and sea transportation commonly use the nautical mile. This is about one minute of arc of latitude along any meridian arc and it is precisely defined as 1852 metres (about 1.151 statute miles). It is not an SI unit (although it is accepted for use in the SI by the BIPM). The prime unit of speed or velocity for maritime and air navigation remains the knot (nautical mile per hour).
Four-velocity
The value of the magnitude of an object's four-velocity, i.e. the quantity obtained by applying the metric tensor to the four-velocity , that is , is always equal to , where is the speed of light. Whether the plus or minus sign applies depends on the choice of metric signature. For an object at rest its four-velocity is parallel to the direction of the time coordinate with . A four-velocity is thus the normalized future-directed timelike tangent vector to a world line, and is a contravariant vector. Though it is a vector, addition of two four-velocities does not yield a four-velocity: the space of four-velocities is not itself a vector space.
International Practical Shooting Confederation
The official unit used for the power factor is the imperial unit "kilo grain feet per second" (kgr·ft/s). "Grain feet per second" (gr·ft/s) can be obtained by measuring the "mass" in grain (gr) (equal to pound), and "velocity" in feet per second (ft/s), but since their product yields a very large number it is common to multiply by a factor of , obtaining the power factor in ""kilo" grain feet per second" instead.
Motion graphs and derivatives
Where formula_13 is the velocity, measured in formula_14, and formula_3 is the time measured in seconds. This slope therefore defines the average acceleration over the interval, and reducing the interval infinitesimally gives formula_16, the instantaneous acceleration at time formula_3, or the derivative of the velocity with respect to time (or the second derivative of the position with respect to time). The units of this slope or derivative are in meters per second per second (formula_18, usually termed "meters per second-squared"), and so, therefore, is the acceleration.
Water metering
A velocity-type meter measures the velocity of flow through a meter of a known internal capacity. The speed of the flow can then be converted into volume of flow to determine the usage. There are several types of meters that measure water flow velocity, including jet meters (single-jet and multi-jet), turbine meters, propeller meters and mag meters. Most velocity-based meters have an adjustment vane for calibrating the meter to the required accuracy.
Velocity (software development)
The following terminology is used in velocity tracking.Velocity always involves counting the number of units of work completed in a certain interval. However, units of work can be measured in many different units.
Speed
Italian physicist Galileo Galilei is usually credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time. In equation form, that is
Velocimetry
Velocimetry is the measurement of the velocity of fluids, as often used to solve fluid dynamics problems, or to study fluid networks, as well as in industrial and process control applications, or in the creation of new kinds of fluid flow sensors. Methods of velocimetry include particle image velocimetry and particle tracking velocimetry, Molecular tagging velocimetry, laser-based interferometry, ultrasonic Doppler methods, Doppler sensors, and new signal processing methodologies. Spatial reconstruction of fluid streamtubes using long exposure imaging of tracer can be applied for streamlines imaging velocimetry, high resolution frame rate free velocimetry of stationary flows. Temporal integration of velocimetric information can be used to totalize fluid flow. For measuring velocity and length on moving surfaces, laser surface velocimeters are used.
Foot per second
The foot per second (plural feet per second) is a unit of both speed (scalar) and velocity (vector quantity, which includes direction). It expresses the distance in feet (ft) traveled or displaced, divided by the time in seconds (s, or "sec"). The corresponding unit in the International System of Units (SI) is the metre per second.
Momentum
The unit of momentum is the product of the units of mass and velocity. In SI units, if the mass is in kilograms and the velocity is in meters per second then the momentum is in kilogram meters per second (kg⋅m/s). In cgs units, if the mass is in grams and the velocity in centimeters per second, then the momentum is in gram centimeters per second (g⋅cm/s).
Specific impulse
The amount of propellant is normally measured either in units of mass or weight. If mass is used, specific impulse is an impulse per unit mass, which dimensional analysis shows to have units of speed, and so specific impulses are often measured in meters per second and are often termed effective exhaust velocity. However, if propellant weight is used, an impulse divided by a force (weight) turns out to be a unit of time, and so specific impulses are measured in seconds. These two formulations are both widely used and differ from each other by a factor of "g", the dimensioned constant of gravitational acceleration at the surface of the Earth.
Geometrized unit system
Any velocity can be interpreted as the slope of a curve; in geometric units, slopes are evidently dimensionless ratios. Physical quantities which can be identified with dimensionless ratios include the components of the electromagnetic potential four-vector and the electromagnetic current four-vector.
Velocity
The kinetic energy of a moving object is dependent on its velocity and is given by the equation
ignoring special relativity, where "E" is the kinetic energy and "m" is the mass. Kinetic energy is a scalar quantity as it depends on the square of the velocity, however a related quantity, momentum, is a vector and defined by 
In special relativity, the dimensionless Lorentz factor appears frequently, and is given by 
where γ is the Lorentz factor and "c" is the speed of light.
Velocity
The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion (e.g. to the north). Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.
Faster-than-light
Special relativity postulate that the speed of light in vacuum is invariant in inertial frames. That is, it will be the same from any frame of reference moving at a constant speed. The equations do not specify any particular value for the speed of the light, which is an experimentally determined quantity for a fixed unit of length. Since 1983, the SI unit of length (the meter) has been defined using the speed of light.
Metrication in the United States
The 2000, 2003, and 2004 editions of the Manual on Uniform Traffic Control Devices were published using both metric and American Customary Units. Metric signs, including speed signs in kilometers per hour, were displayed in the manual even though they were not normally used on the roads and there was no definite plan to go metric. For visual distinction, regulatory and advisory metric speed signs have black circles around the numbers followed by "km/h" and other regulatory metric signs have yellow plates reading "METRIC" in black on top of the main signs. Metric signs and metric measurements were removed for the 2009 edition and replaced with an appendix of metric conversion tables. This is in marked contrast to the position in the (largely metric) United Kingdom, where metric road signs are prohibited by law (except for those denoting widths and height restrictions, which include both metric and imperial units).
List of unusual units of measurement
It is common in particle physics, where mass and energy are often interchanged, to use eV/c, where eV (electronvolt) is the kinetic energy of an electron accelerated over one volt ( joules), c is the speed of light in a vacuum (from E = mc). This definition is intuitive for a linear particle accelerator when accelerating electrons.
List of unusual units of measurement
The light-nanosecond was popularized as a unit of distance by Grace Hopper as the distance which a photon could travel in one billionth of a second (roughly 30 cm or one foot): "The speed of light is one foot per nanosecond." In her speaking engagements, she was well known for passing out light-nanoseconds of wire to the audience, and contrasting it with light-microseconds (a coil of wire 1,000 times as long) and light-picoseconds (the size of ground black pepper). Over the course of her life, she found many uses for this visual aid, including demonstrating the waste of sub-optimal programming, illustrating advances in computer speed, and simply giving young scientists and policy makers the ability to conceptualize the magnitude of very large and small numbers.
Knot (unit)
Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out. Knots placed at a distance of 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.
Thermal velocity
The thermal velocity or thermal speed is a typical velocity of the thermal motion of particles which make up a gas, liquid, etc. Thus, indirectly, thermal velocity is a measure of temperature. Technically speaking it is a measure of the width of the peak in the Maxwell–Boltzmann particle velocity distribution. Note that in the strictest sense "thermal velocity" is not a velocity, since "velocity" usually describes a vector rather than simply a scalar speed.
Velocity
where
The "magnitude of the radial velocity" is the dot product of the velocity vector and the unit vector in the direction of the displacement.
where
The "magnitude of the transverse velocity" is that of the cross product of the unit vector in the direction of the displacement and the velocity vector. It is also the product of the angular speed formula_27 and the magnitude of the displacement.
such that
History of the metric system
In 1960, the CGPM launched the International System of Units (in French the "Système international d'unités" or SI) which had six "base units": the metre, kilogram, second, ampere, degree Kelvin (subsequently renamed the "kelvin") and candela; as well as 16 further units derived from the base units. A seventh base unit, the mole, and six additional derived units were added in succeeding years through the close of the twentieth century. During this period, the metre was redefined in terms of the speed of light, and the second was redefined in terms of the microwave frequency of a cesium atomic clock. Since the end of the 20th century, an effort has been undertaken to redefine the ampere, kilogram, mole and kelvin in terms of invariant constants of physics.
Bubnoff unit
The Bubnoff unit (abbreviated B) is a unit of speed equal to 1 m / 10 a. In other words, 1 B is equal to 1 meter in 1,000,000 years, 1 millimeter in 1,000 years, or one micrometer per year. It was defined in 1969.
Speed of light
In 1983 the 17th CGPM found that wavelengths from frequency measurements and a given value for the speed of light are more reproducible than the previous standard. They kept the 1967 definition of second, so the caesium hyperfine frequency would now determine both the second and the metre. To do this, they redefined the metre as: "The metre is the length of the path travelled by light in vacuum during a time interval of 1/ of a second." As a result of this definition, the value of the speed of light in vacuum is exactly and has become a defined constant in the SI system of units. Improved experimental techniques that prior to 1983 would have measured the speed of light, no longer affect the known value of the speed of light in SI units, but instead allow a more precise realization of the metre by more accurately measuring the wavelength of Krypton-86 and other light sources.
Kilogram
The stability of the IPK is crucial because the kilogram underpins much of the SI system of measurement as it is currently defined and structured. For instance, the newton is defined as the force necessary to accelerate one kilogram at one metre per second squared. If the mass of the IPK were to change slightly then the newton would also change proportionally. In turn, the pascal, the SI unit of pressure, is defined in terms of the newton. This chain of dependency follows to many other SI units of measure. For instance, the joule, the SI unit of energy, is defined as that expended when a force of one newton acts through one metre. Next to be affected is the SI unit of power, the watt, which is one joule per second. The ampere too is defined relative to the newton.
Natural units
Natural units are most commonly used by "setting the units to one". For example, many natural unit systems include the equation in the unit-system definition, where is the speed of light. If a velocity is half the speed of light, then as and , hence . The equation means "the velocity has the value one-half when measured in Planck units", or "the velocity is one-half the Planck unit of velocity".
Metre per second
The metre per second (American English: meter per second) is an SI derived unit of both speed (scalar) and velocity (vector quantity which specifies both magnitude and a specific direction), defined by distance in metres divided by time in seconds.
Tachymeter (watch)
A tachymeter-equipped watch can be used to measure distance by timing the travel over the distance while the speed is held constant. The tachymeter scale is rotated to align with the second hand at the start of the length to be measured. When the second hand reaches the point on the scale where the speed indicated equals the speed of the vehicle, one unit of distance (miles if speed is miles per hour, kilometres if kilometres per hour, etc.) has been covered. For example, if you travel at a constant 80 mph (or at 80 km/h), then the distance travelled while the second hand sweeps to "80" (45 seconds) will be exactly 1 mile (or 1 kilometre at 80 km/h).
Second
A second is part of other units, such as frequency measured in hertz (inverse seconds or second), speed (meters per second) and acceleration (meters per second squared). The metric system unit becquerel, a measure of radioactive decay, is measured in inverse seconds. The meter is defined in terms of the speed of light and the second; definitions of the metric base units ampere and candela also depend on the second. Of the 22 named derived units of the SI, only three: degree Celsius, radian, and steradian, do not depend on the second. Many derivative units for everyday things are reported in terms of larger units of time, not seconds, such as clock time in hours and minutes, velocity of a car in miles per hour or kilometers per hour, kilowatt hours of electricity usage, and speed of a turntable in rotations per minute.
Gravimeter
Gravimeters display their measurements in units of gals (cm/s), nanometers per second squared, and parts per million, parts per billion, or parts per trillion of the average vertical acceleration with respect to the earth. Some newer units are pm/s2 (picometers per second squared), fm/s2 (femto), am/s2 (atto) for very sensitive instruments.
Centimetre–gram–second system of units
In this table, "c" = 29,979,245,800 is the numeric value of the speed of light in vacuum when expressed in units of centimetres per second. The symbol "↔" is used instead of "=" as a reminder that the SI and CGS units are "corresponding" but not "equal" because they have incompatible dimensions. For example, according to the next-to-last row of the table, if a capacitor has a capacitance of 1 F in SI, then it has a capacitance of (10 "c") cm in ESU; "but" it is usually incorrect to replace "1 F" with "(10 "c") cm" within an equation or formula. (This warning is a special aspect of electromagnetism units in CGS. By contrast, for example, it is "always" correct to replace "1 m" with "100 cm" within an equation or formula.)
Linear motion
Speed is the absolute value of velocity i.e. speed is always positive. The unit of speed is metre per second.
If formula_17 is the speed then,
Kinematics
The velocity of a particle is a vector quantity that describes the direction of motion and the magnitude of the motion of particle. More mathematically, the rate of change of the position vector of a point, with respect to time is the velocity of the point. Consider the ratio formed by dividing the difference of two positions of a particle by the time interval. This ratio is called the average velocity over that time interval and is defined as
Velocity=displacement/time taken
where ΔP is the change in the position vector over the time interval Δ"t".
Planck constant
It is currently planned to redefine certain of the SI base units in terms of fundamental physical constants. This has already been done for the metre, which since 1983 has been defined in terms of a fixed value of the speed of light. The most urgent unit on the list for redefinition is the kilogram, whose value has been fixed for all science (since 1889) by the mass of a small cylinder of platinum–iridium alloy kept in a vault just outside Paris. While nobody knows if the mass of the International Prototype Kilogram has changed since 1889 – the value 1 kg of its mass expressed in kilograms is by definition unchanged and therein lies one of the problems – it is known that over such a timescale the many similar Pt–Ir alloy cylinders kept in national laboratories around the world have changed their relative masses by several tens of parts per billion, however carefully they are stored. A change of several tens of micrograms in one kilogram is equivalent to the current uncertainty in the value of the Planck constant in SI units.
Metrication in the United States
U.S. Federal Motor Vehicle Safety Standard 101, which governs vehicle controls and displays, permits speedometers to display miles per hour (MPH) or both MPH and km/h. In practice, most U.S.-market vehicles have mile odometers and dual-labeled speedometers with miles-per-hour as the primary calibration. Some 2000s era Buicks have a single speed gauge with a button on the instrument panel to switch the scale between MPH and km/h. Other vehicles have digital speedometers which can be set to read out speeds in either MPH or km/h.
Velocity
Although velocity is defined as the rate of change of position, it is often common to start with an expression for an object's acceleration. As seen by the three green tangent lines in the figure, an object's instantaneous acceleration at a point in time is the slope of the line tangent to the curve of a graph at that point. In other words, acceleration is defined as the derivative of velocity with respect to time:
Foot per second squared
The foot per second squared (plural "feet per second squared") is a unit of acceleration. It expresses change in velocity expressed in units of feet per second (ft/s) divided by time in seconds (s) (or the distance in feet (ft) traveled or displaced, divided by the time in seconds (s) squared). The corresponding unit in the International System of Units (SI) is the metre per second squared.
Physical quantity
A physical quantity is a physical property of a material that can be quantified by measurement. A physical quantity can be expressed as the combination of a magnitude expressed by a number – usually a real number – and a unit: formula_1 where formula_2 is the magnitude and formula_3 is the unit. For example, (the mass of the neutron), or (the speed of light). The same physical quantity formula_4 can be represented equivalently in many unit systems, "i.e." formula_5.
Curvature
Another way to understand the curvature is physical. Suppose that a particle moves along the curve with unit speed. Taking the time as the parameter for , this provides a natural parametrization for the curve. The unit tangent vector (which is also the velocity vector, since the particle is moving with unit speed) also depends on time. The curvature is then the magnitude of the rate of change of . Symbolically,
This is the magnitude of the acceleration of the particle and the vector is the acceleration vector. Geometrically, the curvature measures how fast the unit tangent vector to the curve rotates. If a curve keeps close to the same direction, the unit tangent vector changes very little and the curvature is small; where the curve undergoes a tight turn, the curvature is large.
Force
The SI unit of force is the newton (symbol N), which is the force required to accelerate a one kilogram mass at a rate of one meter per second squared, or . The corresponding CGS unit is the dyne, the force required to accelerate a one gram mass by one centimeter per second squared, or . A newton is thus equal to 100,000 dynes.
Accelerometer
An accelerometer is a device that measures proper acceleration. Proper acceleration, being the acceleration (or rate of change of velocity) of a body in its own instantaneous rest frame, is not the same as coordinate acceleration, being the acceleration in a fixed coordinate system. For example, an accelerometer at rest on the surface of the Earth will measure an acceleration due to Earth's gravity, straight upwards (by definition) of g ≈ 9.81 m/s. By contrast, accelerometers in free fall (falling toward the center of the Earth at a rate of about 9.81 m/s) will measure zero.
Particle velocity
Particle velocity is the velocity of a particle (real or imagined) in a medium as it transmits a wave. The SI unit of particle velocity is the metre per second (m/s). In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string.
Speed of light
After centuries of increasingly precise measurements, in 1975 the speed of light was known to be with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/ of a second.
Velocity (software development)
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.
Revolutions per minute
Formally, hertz (Hz) and radian per second (rad/s) are two different names for the same SI unit, s. However, they are used for two different but proportional ISQ quantities: frequency and angular frequency (angular speed, magnitude of angular velocity). The conversion between a frequency (measured in hertz) and an angular velocity (measured in radians per second) are: