In physics, acceleration is the rate of change of velocity over time.^{[1]} In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity.^{[2]}^{[3]} Acceleration has the dimensions L T ^{−2}. In SI units, acceleration is measured in meters per second per second (m/s^{2}).
Proper acceleration, the acceleration of a body relative to a freefall condition, is measured by an instrument called an accelerometer.
In common speech, the term acceleration 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 net 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 and instantaneous acceleration
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 which is for a very short interval of time as Δt approaches zero.
Tangential and centripetal acceleration
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:
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