# Dictionary Definition

deceleration n : a decrease in speed; "the deceleration of the arms race" [syn: slowing, retardation] [ant: acceleration]

# User Contributed Dictionary

## English

### Pronunciation

• /dɪˌsɛləˈɹeɪʃən/, /dI%sEl@"reIS@n/
• Rhymes: -eɪʃǝn

### Noun

deceleration (countable and uncountable; plural decelerations)
1. The act or process of decelerating.
The rocket is now in deceleration.
2. The amount by which a speed or velocity decreases (and so a scalar quantity or a vector quantity).
The brakes produce a deceleration of 10 metres per second per second.

# Extensive Definition

In physics, acceleration is defined as the rate of change of velocity, or as the second derivative of position (with respect to time). It is then a vector quantity with dimension length/time². In SI units, acceleration is measured in meters/second² (m·s-2). The term "acceleration" generally refers to the rate of change in instantaneous velocity. (velocity is speed and direction)
In common speech, the term acceleration is only used for an increase in speed; a decrease in speed is called deceleration. In physics, any increase or decrease in speed is referred to as acceleration and similarly, motion in a circle at constant speed is also an acceleration, since the direction component of the velocity is changing. See also Newton's Laws of Motion.

## Relation to relativity

After completing his theory of special relativity, Albert Einstein realized that forces felt by objects undergoing constant proper acceleration are indistinguishable from those in a gravitational field. This was the basis for his development of general relativity, a relativistic theory of gravity. This is also the basis for the popular Twin paradox, which asks why one twin ages less when moving away from his sibling at near light-speed and then returning, since the non-aging twin can say that it is the other twin that was moving. General relativity solved the "why does only one object feel accelerated?" problem which had plagued philosophers and scientists since Newton's time (and caused Newton to endorse absolute space). In special relativity, only inertial frames of reference (non-accelerated frames) can be used and are equivalent; general relativity considers all frames, even accelerated ones, to be equivalent. (The path from these considerations to the full theory of general relativity is traced in the Introduction to general relativity.)

## Formula

The formula for the average acceleration over a time period \Delta t is
\mathbf=\frac
where
\mathbf(t+\Delta t) is the final velocity
\mathbf(t) is the initial velocity
\mathbf is the initial time and \Delta \mathbf is the change in time
The formula for the instantaneous acceleration at time t is
\mathbf(t)=\lim_\frac=\frac
Thus acceleration is the first derivative of velocity. One should note that the expression (Final position - Initial Position) / (Total time taken) is the average velocity, and the limit as the time interval approaches zero is the instantaneous velocity. Therefore, velocity is the first derivative of position, making acceleration the second.
One should also note that the average and instantaneous accelerations over a time period \Delta t=t_1-t_0 are related through the Mean Value Theorem for Integrals:
\bar\int_^\mathrmt=\int_^\mathbf(t)\mathrmt
Putting it all together means:
\mathbf = \frac = \frac
where
\mathbf is acceleration
\mathbf is velocity
\mathbf is position
is time
deceleration in Afrikaans: Versnelling
deceleration in Arabic: تسارع
deceleration in Asturian: Aceleración
deceleration in Bengali: ত্বরণ
deceleration in Min Nan: Ka-sok-tō͘
deceleration in Belarusian: Паскарэнне
deceleration in Bosnian: Ubrzanje
deceleration in Bulgarian: Ускорение
deceleration in Catalan: Acceleració
deceleration in Czech: Zrychlení
deceleration in Danish: Acceleration
deceleration in German: Beschleunigung
deceleration in Estonian: Kiirendus
deceleration in Modern Greek (1453-): Επιτάχυνση
deceleration in Spanish: Aceleración
deceleration in Esperanto: Akcelo
deceleration in Basque: Azelerazio
deceleration in Persian: شتاب
deceleration in French: Accélération
deceleration in Irish: Luasghéarú
deceleration in Manx: Bieauaghey
deceleration in Galician: Aceleración
deceleration in Hakka Chinese: Kâ-suk-thu
deceleration in Korean: 가속도
deceleration in Croatian: Ubrzanje
deceleration in Ido: Acelero
deceleration in Indonesian: Percepatan
deceleration in Icelandic: Hröðun
deceleration in Italian: Accelerazione
deceleration in Hebrew: תאוצה
deceleration in Latin: Acceleratio
deceleration in Latvian: Paātrinājums
deceleration in Lithuanian: Pagreitis
deceleration in Hungarian: Gyorsulás
deceleration in Malayalam: ത്വരണം
deceleration in Malay (macrolanguage): Pecutan
deceleration in Mongolian: Хурдатгал
deceleration in Dutch: Versnelling (natuurkunde)
deceleration in Japanese: 加速度
deceleration in Norwegian: Akselerasjon
deceleration in Norwegian Nynorsk: Akselerasjon
deceleration in Novial: Akseleratione
deceleration in Polish: Przyspieszenie
deceleration in Portuguese: Aceleração
deceleration in Quechua: P'ikwachiy
deceleration in Russian: Ускорение
deceleration in Sicilian: Accilirazzioni
deceleration in Simple English: Acceleration
deceleration in Slovak: Zrýchlenie
deceleration in Slovenian: Pospešek
deceleration in Serbian: Убрзање
deceleration in Serbo-Croatian: Ubrzanje
deceleration in Finnish: Kiihtyvyys
deceleration in Swedish: Acceleration
deceleration in Telugu: త్వరణము
deceleration in Thai: ความเร่ง
deceleration in Vietnamese: Gia tốc
deceleration in Turkish: İvme
deceleration in Ukrainian: Прискорення
deceleration in Chinese: 加速度

# Synonyms, Antonyms and Related Words

arrest, arrestation, catabasis, check, collapse, constraint, control, cooling, cooling down, cooling off, crash, curb, curtailment, declension, decline, decline and fall, decrescendo, delay, detention, diminuendo, dive, downtrend, downturn, drag, drop, dwindling, ease-off, ease-up, ebb, fall, flagging, hindrance, holdup, inhibition, injunction, interdict, lag, lapse, legal restraint, letdown, letup, minus acceleration, monopoly, obstruction, plunge, prohibition, protection, protectionism, protective tariff, rationing, rein, remission, restraint, restraint of trade, retardation, retardment, retreat, retrenchment, self-control, setback, slack-up, slackening, slowdown, slowing, slowing down, slowup, slump, subsidence, tariff wall, thought control, wane