velocity

• n.

• 速度

関

rate、speed

　

WordNet

1. be worthy of or have a certain rating; "This bond rates highly"
2. amount of a charge or payment relative to some basis; "a 10-minute phone call at that rate would cost $5" (同)charge per unit
3. a magnitude or frequency relative to a time unit; "they traveled at a rate of 55 miles per hour"; "the rate of change was faster than expected"
4. a quantity or amount or measure considered as a proportion of another quantity or amount or measure; "the literacy rate"; "the retention rate"; "the dropout rate"
5. assign a rank or rating to; "how would you rank these students?"; "The restaurant is rated highly in the food guide" (同)rank, range, order, grade, place
6. estimate the value of; "How would you rate his chances to become President?"; "Gold was rated highly among the Romans" (同)value
7. travel at an excessive or illegal velocity; "I got a ticket for speeding"
8. distance travelled per unit time (同)velocity
9. changing location rapidly (同)speeding, hurrying
10. a rate (usually rapid) at which something happens; "the project advanced with gratifying speed" (同)swiftness, fastness
11. the constant maximum velocity reached by a body falling through the atmosphere under the attraction of gravity
12. velocity along the line of sight toward or away from the observer

PrepTutorEJDIC

1. 〈C〉速度
2. 〈C〉『割合』,『率』 / 〈C〉《the~,a~》『速度』(speed),進度 / 〈C〉『値段』,『相場』;料金 / 〈U〉等級(class),(…)等 / 《複数形ぃ》《英》地方税 / (…の金額に)…‘の'『値段を決める』,見積もる《+『名』+『at』+『名』》 / …‘を'『評価する』,みなす / 《米話》…‘に'値する,‘の'価値がある / 『評価される』;みなされる
3. …‘を'どなりつける
4. 〈U〉(運動・動作の)『速いこと』,速さ・〈C〉『速度』,速力(velocity) ・〈C〉(自動車などの)変速ギア・〈U〉〈C〉(写真で) / (フィルムの)感度,感光性 / レンズの明るさ・シャッター速度・覚醒(かくせい)剤,興奮剤 / ・『急ぐ』,疾走する・《通例現在分詞形で》制限速度以上で運転する,反則の速度で走る・〈仕事・車など〉‘の'速度を上げる

Wikipedia preview

出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2019/07/24 16:36:05」(JST)

wiki ja

[Wiki ja表示]

Velocityには、以下の意味がある。

• 速度の英語名
• Apache Velocity - Apache Software FoundationがJavaで開発しているオープンソースのテンプレートエンジン
• velocity - 高知県にて発行されている季刊誌の名前。ベロシティ編集部が発行しており、3月、6月、9月、12月に発行されている。
• Velo-City - カナダ、トロントで推進されている中空自転車道計画のこと。
• Velocity - オーストラリアの格安航空会社、ヴァージン・ブルーのマイレージプログラムの名称。

このページは曖昧さ回避のためのページです。一つの語句が複数の意味・職能を有する場合の水先案内のために、異なる用法を一覧にしてあります。お探しの用語に一番近い記事を選んで下さい。このページへリンクしているページを見つけたら、リンクを適切な項目に張り替えて下さい。

wiki en

[Wiki en表示]

This article is about velocity in physics. For other uses, see Velocity (disambiguation).

Velocity
As a change of direction occurs while the racing cars turn on the curved track, their velocity is not constant.
Common symbols	v, v, v→
Other units	mph, ft/s
In SI base units	m/s
Dimension	L T−1

Part of a series of articles about
Classical mechanics
$$ F → = m a → {\displaystyle {\vec {F}}=m{\vec {a}}} Second law of motion
History Timeline
Branches Applied Celestial Continuum Dynamics Kinematics Kinetics Statics Statistical
Fundamentals Acceleration Angular momentum Couple D'Alembert's principle Energy kinetic potential Force Frame of reference Inertial frame of reference Impulse Inertia / Moment of inertia Mass Mechanical power Mechanical work Moment Momentum Space Speed Time Torque Velocity Virtual work
Formulations Newton's laws of motion Analytical mechanics Lagrangian mechanics Hamiltonian mechanics Routhian mechanics Hamilton–Jacobi equation Appell's equation of motion Udwadia–Kalaba equation Koopman–von Neumann mechanics
Core topics Damping (ratio) Displacement Equations of motion Euler's laws of motion Fictitious force Friction Harmonic oscillator Inertial / Non-inertial reference frame Mechanics of planar particle motion Motion (linear) Newton's law of universal gravitation Newton's laws of motion Relative velocity Rigid body dynamics Euler's equations Simple harmonic motion Vibration
Rotation Circular motion Rotating reference frame Centripetal force Centrifugal force reactive Coriolis force Pendulum Tangential speed Rotational speed Angular acceleration / displacement / frequency / velocity
Scientists Galileo Huygens Newton Kepler Horrocks Halley Euler d'Alembert Clairaut Lagrange Laplace Hamilton Poisson Daniel Bernoulli Johann Bernoulli Cauchy

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. 60 km/h to the north). Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.

Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, being a coherent derived unit whose quantity is measured in the SI (metric system) as metres per second (m/s) or as the SI base unit of (m⋅s⁻¹). For example, "5 metres per second" is a scalar, whereas "5 metres per second east" is a vector. If there is a change in speed, direction or both, then the object has a changing velocity and is said to be undergoing an acceleration.

Constant velocity vs acceleration

To have a constant velocity, an object must have a constant speed in a constant direction. Constant direction constrains the object to motion in a straight path thus, a constant velocity means motion in a straight line at a constant speed.

For example, a car moving at a constant 20 kilometres per hour in a circular path has a constant speed, but does not have a constant velocity because its direction changes. Hence, the car is considered to be undergoing an acceleration.

Difference between speed and velocity

Main article: Speed

Kinematic quantities of a classical particle: mass m, position r, velocity v, acceleration a.

Speed, the scalar magnitude of a velocity vector, denotes only how fast an object is moving.

Equation of motion

Main article: Equation of motion

Average 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, v(t), over some time period Δt. Average velocity can be calculated as:

${\boldsymbol {\bar {v}}}={\frac {\Delta {\boldsymbol {x}}}{\Delta {\mathit {t}}}}.$

The average velocity is always less than or equal to the average speed of an object. This can be seen by realizing that while distance is always strictly increasing, displacement can increase or decrease in magnitude as well as change direction.

In terms of a displacement-time (x vs. t) graph, the instantaneous velocity (or, simply, velocity) can be thought of as the slope of the tangent line to the curve at any point, and the average velocity as the slope of the secant line between two points with t coordinates equal to the boundaries of the time period for the average velocity.

The average velocity is the same as the velocity averaged over time – that is to say, its time-weighted average, which may be calculated as the time integral of the velocity:

${\boldsymbol {\bar {v}}}={1 \over t_{1}-t_{0}}\int _{t_{0}}^{t_{1}}{\boldsymbol {v}}(t)\ dt,$

where we may identify

$\Delta {\boldsymbol {x}}=\int _{t_{0}}^{t_{1}}{\boldsymbol {v}}(t)\ dt$

and

$\Delta t=t_{1}-t_{0}.$

Instantaneous velocity

Example of a velocity vs. time graph, and the relationship between velocity v on the y-axis, acceleration a (the three green tangent lines represent the values for acceleration at different points along the curve) and displacement s (the yellow area under the curve.)

If we consider v as velocity and x as the displacement (change in position) vector, then we can express the (instantaneous) velocity of a particle or object, at any particular time t, as the derivative of the position with respect to time:

${\boldsymbol {v}}=\lim _{{\Delta t}\to 0}{\frac {\Delta {\boldsymbol {x}}}{\Delta t}}={\frac {d{\boldsymbol {x}}}{d{\mathit {t}}}}.$

From this derivative equation, in the one-dimensional case it can be seen that the area under a velocity vs. time (v vs. t graph) is the displacement, x. In calculus terms, the integral of the velocity function v(t) is the displacement function x(t). In the figure, this corresponds to the yellow area under the curve labeled s (s being an alternative notation for displacement).

${\boldsymbol {x}}=\int {\boldsymbol {v}}\ d{\mathit {t}}.$

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.

Relationship to acceleration

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 v(t) graph at that point. In other words, acceleration is defined as the derivative of velocity with respect to time:

${\boldsymbol {a}}={\frac {d{\boldsymbol {v}}}{d{\mathit {t}}}}.$

From there, we can obtain an expression for velocity as the area under an a(t) acceleration vs. time graph. As above, this is done using the concept of the integral:

${\boldsymbol {v}}=\int {\boldsymbol {a}}\ d{\mathit {t}}.$

Constant acceleration

In the special case of constant acceleration, velocity can be studied using the suvat equations. By considering a as being equal to some arbitrary constant vector, it is trivial to show that

${\boldsymbol {v}}={\boldsymbol {u}}+{\boldsymbol {a}}t$

with v as the velocity at time t and u as the velocity at time t = 0. By combining this equation with the suvat equation x = ut + at²/2, it is possible to relate the displacement and the average velocity by

${\boldsymbol {x}}={\frac {({\boldsymbol {u}}+{\boldsymbol {v}})}{2}}{\mathit {t}}={\boldsymbol {\bar {v}}}{\mathit {t}}$.

It is also possible to derive an expression for the velocity independent of time, known as the Torricelli equation, as follows:

$v^{2}={\boldsymbol {v}}\cdot {\boldsymbol {v}}=({\boldsymbol {u}}+{\boldsymbol {a}}t)\cdot ({\boldsymbol {u}}+{\boldsymbol {a}}t)=u^{2}+2t({\boldsymbol {a}}\cdot {\boldsymbol {u}})+a^{2}t^{2}$

$(2{\boldsymbol {a}})\cdot {\boldsymbol {x}}=(2{\boldsymbol {a}})\cdot ({\boldsymbol {u}}t+{\frac {1}{2}}{\boldsymbol {a}}t^{2})=2t({\boldsymbol {a}}\cdot {\boldsymbol {u}})+a^{2}t^{2}=v^{2}-u^{2}$

$\therefore v^{2}=u^{2}+2({\boldsymbol {a}}\cdot {\boldsymbol {x}})$

where v = |v| etc.

The above equations are valid for both Newtonian mechanics and special relativity. Where Newtonian mechanics and special relativity differ is in how different observers would describe the same situation. In particular, in Newtonian mechanics, all observers agree on the value of t and the transformation rules for position create a situation in which all non-accelerating observers would describe the acceleration of an object with the same values. Neither is true for special relativity. In other words, only relative velocity can be calculated.

Quantities that are dependent on velocity

The kinetic energy of a moving object is dependent on its velocity and is given by the equation

$E_{\text{k}}={\tfrac {1}{2}}mv^{2}$

ignoring special relativity, where E_k 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

${\boldsymbol {p}}=m{\boldsymbol {v}}$

In special relativity, the dimensionless Lorentz factor appears frequently, and is given by

$\gamma ={\frac {1}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}$

where γ is the Lorentz factor and c is the speed of light.

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

$v_{\text{e}}={\sqrt {\frac {2GM}{r}}}={\sqrt {2gr}},$

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.

Relative velocity

Main article: Relative velocity

Relative velocity is a measurement of velocity between two objects as determined in a single coordinate system. Relative velocity is fundamental in both classical and modern physics, since many systems in physics deal with the relative motion of two or more particles. In Newtonian mechanics, the relative velocity is independent of the chosen inertial reference frame. This is not the case anymore with special relativity in which velocities depend on the choice of reference frame.

If an object A is moving with velocity vector v and an object B with velocity vector w, then the velocity of object A relative to object B is defined as the difference of the two velocity vectors:

${\boldsymbol {v}}_{A{\text{ relative to }}B}={\boldsymbol {v}}-{\boldsymbol {w}}$

Similarly, the relative velocity of object B moving with velocity w, relative to object A moving with velocity v is:

${\boldsymbol {v}}_{B{\text{ relative to }}A}={\boldsymbol {w}}-{\boldsymbol {v}}$

Usually, the inertial frame chosen is that in which the latter of the two mentioned objects is in rest.

Scalar velocities

In the one-dimensional case, the velocities are scalars and the equation is either:

$\,v_{rel}=v-(-w)$, if the two objects are moving in opposite directions, or:

$\,v_{rel}=v-(+w)$, if the two objects are moving in the same direction.

Polar coordinates

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).

The radial and angular velocities can be derived from the Cartesian velocity and displacement vectors by decomposing the velocity vector into radial and transverse components. The transverse velocity is the component of velocity along a circle centered at the origin.

${\boldsymbol {v}}={\boldsymbol {v}}_{T}+{\boldsymbol {v}}_{R}$

where

${\boldsymbol {v}}_{T}$ is the transverse velocity

${\boldsymbol {v}}_{R}$ is the radial velocity.

The magnitude of the radial velocity is the dot product of the velocity vector and the unit vector in the direction of the displacement.

$v_{R}={\frac {{\boldsymbol {v}}\cdot {\boldsymbol {r}}}{\left|{\boldsymbol {r}}\right|}}$

where

${\boldsymbol {r}}$ is displacement.

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 $\omega$ and the magnitude of the displacement.

$v_{T}={\frac {|{\boldsymbol {r}}\times {\boldsymbol {v}}|}{|{\boldsymbol {r}}|}}=\omega |{\boldsymbol {r}}|$

such that

$\omega ={\frac {|{\boldsymbol {r}}\times {\boldsymbol {v}}|}{|{\boldsymbol {r}}|^{2}}}.$

Angular momentum in scalar form is the mass times the distance to the origin times the transverse velocity, or equivalently, the mass times the distance squared times the angular speed. The sign convention for angular momentum is the same as that for angular velocity.

$L=mrv_{T}=mr^{2}\omega \,$

where

$m\,$ is mass

$r=\|{\boldsymbol {r}}\|.$

The expression $mr^{2}$ is known as moment of inertia. If forces are in the radial direction only with an inverse square dependence, as in the case of a gravitational orbit, angular momentum is constant, and transverse speed is inversely proportional to the distance, angular speed is inversely proportional to the distance squared, and the rate at which area is swept out is constant. These relations are known as Kepler's laws of planetary motion.

See also

• Four-velocity (relativistic version of velocity for Minkowski spacetime)
• Group velocity
• Hypervelocity
• Phase velocity
• Proper velocity (in relativity, using traveler time instead of observer time)
• Rapidity (a version of velocity additive at relativistic speeds)
• Terminal velocity
• Velocity vs. time graph

References

• Robert Resnick and Jearl Walker, Fundamentals of Physics, Wiley; 7 Sub edition (June 16, 2004). ISBN 0-471-23231-9.

External links

• physicsabout.com, Speed and Velocity
• Velocity and Acceleration
• Introduction to Mechanisms (Carnegie Mellon University)

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Related Pictures

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★リンクテーブル★

リンク元	「speed」「速度」
拡張検索	「proximal isovelocity surface area」「blood flow velocity」「mean spectral velocity」

「speed」

　　[★]

• n.

• 速度、スピード

関

rate、velocity

　 　 　 　 　

「速度」

　　[★]

英

velocity

関

速さ

「proximal isovelocity surface area」

　　[★]

同

PISA, 近位部等流速表面

「blood flow velocity」

　　[★]

• 血流速度

関

blood flow rate

「mean spectral velocity」

　　[★] 平均スペクトル速度