In physics, and more specifically kinematics, acceleration is the change in velocity over time.^{[1]} Because velocity is a vector, it can change in two ways: a change in magnitude and/or a change in direction. In one dimension, i.e. a line, acceleration is the rate at which something speeds up or slows down. However, as a vector quantity, acceleration is also the rate at which direction changes.^{[2]}^{[3]} Acceleration has the dimensions L T^{−2}. In SI units, acceleration is measured in metres per second squared (m/s^{2}).
In common speech, the term acceleration commonly is used for an increase in speed (the magnitude of velocity); a decrease in speed is called deceleration. In physics, a change in the direction of velocity also is an acceleration: for rotary motion, the change in direction of velocity results in centripetal (toward the center) acceleration; where as the rate of change of speed is a tangential acceleration.
In classical mechanics, for a body with constant mass, the acceleration of the body is proportional to the resultant (total) force acting on it (Newton's second law):
where F is the resultant force acting on the body, m is the mass of the body, and a is its acceleration.
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Average acceleration is the change in velocity (Δv) divided by the change in time (Δt). Instantaneous acceleration is the acceleration at a specific point in time.
The velocity of a particle moving on a curved path as a function of time can be written as:
with v(t) equal to the speed of travel along the path, and
a unit vector tangent to the path pointing in the direction of motion at the chosen moment in time. Taking into account both the changing speed v(t) and the changing direction of u_{t}, the acceleration of a particle moving on a curved path on a planar surface can be written using the chain rule of differentiation as:
where u_{n} is the unit (outward) normal vector to the particle's trajectory, and R is its instantaneous radius of curvature based upon the osculating circle at time t. These components are called the tangential acceleration and the radial acceleration, respectively. The negative of the radial acceleration is the centripetal acceleration, which points inward, toward the center of curvature.
Extension of this approach to threedimensional space curves that cannot be contained on a planar surface leads to the FrenetSerret formulas.^{[4]}^{[5]}
After completing his theory of special relativity, Albert Einstein realized that forces felt by objects undergoing constant proper acceleration are actually feeling themselves being accelerated, so that, for example, a car's acceleration forwards would result in the driver feeling a slight pressure between herself and her seat. In the case of gravity, which Einstein concluded is not actually a force, this is not the case; acceleration due to gravity is not felt by an object in freefall. This was the basis for his development of general relativity, a relativistic theory of gravity.

Acceleration is a measure of how fast velocity changes. Acceleration is the change of velocity divided by the change of time. Acceleration is a vector, and therefore includes both a size and a direction.
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Acceleration is the rate of change of the velocity of an object. Acceleration $\backslash mathbfTemplate:A$ can be found by using:
\mathbfTemplate:A = {\mathbf{v_1}  \mathbf{v_0} \over { t_1  t_0 }}
where
Sometimes the change in velocity $\backslash mathbf\{v\_1\}\; \; \backslash mathbf\{v\_0\}$ is written as Δ$\backslash mathbf\{v\}$. Sometimes the change in time $\{\; t\_1\; \; t\_0\; \}$ is written as Δt.
In difficult situations, the acceleration can be calculated using mathematics: in calculus, acceleration is the derivative of the velocity (with respect to time), $\backslash mathbf\{a\}\; =\; \backslash frac\{\backslash mathrm\{d\}\backslash mathbf\{v\}\}\{\backslash mathrm\{d\}t\}$.
Acceleration has its own units of measurement. For example, if velocity is measured in meters per second, and if time is measured in seconds, then acceleration is measured in meters per second squared (m/s^{2}).
Acceleration can be positive or negative. When the acceleration is negative (but the velocity does not change direction), it is sometimes called deceleration. For example, when a car brakes it decelerates. Physicists usually only use the word "acceleration".
There are rules for how things move. These rules are called "laws of motion". Isaac Newton is the scientist who first wrote down the main laws of motion. According to Newton's Second Law of Motion, the force something needs to accelerate an object depends on the object's mass (the amount of "stuff" the object is made from or how "heavy" it is). The formula of Newton's Second Law of Motion is $\backslash mathbf\{F\}\; =\; m\; \backslash mathbf\{a\}$, where $\backslash mathbf\{a\}$ is the acceleration, $\backslash mathbf\{F\}$ is the force, and $m$ the mass. This formula is very wellknown, and it is very important in physics. Newton's Second Law of Motion, in short "Newton's Second Law", is often one of the first things that physics students learn.krc:Терклениу
