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

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.

Antonyms

• (act, amount): acceleration

Related terms

• decelerate

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

See also

• Uniform Acceleration
• Angular acceleration
• Coordinate vs. physical acceleration
• Derivatives of position
• Equations of Motion
• Proper Acceleration

External links

• Acceleration and Free Fall - a chapter from an online textbook
• Trajectories and Radius, Velocity, Acceleration on Project PHYSNET (ERROR - PAGE MOVED)
• Science aid: Movement
• Physics Classroom: Acceleration
• Science.dirbix: Acceleration
• Acceleration Calculator
• Motion Characteristics for Circular Motion

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