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'''Acceleration''' is the rate of change of an object's [[velocity]].<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref>. More precisely, it is defined as the derivative of [[velocity]] over time, and is a vector. For an object to undergo an acceleration, a [[force]] needs to be exerted on the object. An example is a falling object on [[Earth]], which is subject to a [[gravity|gravitational force]]. The resulting acceleration ''g'' is independent of the mass of the object, and is approximately 9.81 [[meter]]s per [[second]] per [[second]] near the Earth's surface<ref>Marcelo Alonso and Edward J. Finn, ''Fundamental University Physics'', Addison-Wesley.</ref>.
According to Newton's Second Law of Motion, calculation of acceleration is done with the formula <math>\vec F=m \vec a</math>, where F=force and is measured in Newtons, m=mass and is measured in kilograms, and a=acceleration and is measured in meters per second squared. Using the formula we can find that <math> \vec a=\frac{\vec F}{m}</math>.
In the case of straight trajectory, if an object's acceleration and [[velocity]] have the same sign, the object is gaining [[speed]]. If acceleration and velocity have different signs, the object is losing speed. If [[velocity]] is [[zero]], acceleration is not necessarily zero. If acceleration is zero, velocity is not necessarily zero.
In case of a curvilinear trajectory, there is an acceleration, even if speed is constant.
==References==
<references/>
[[Category:Physics]]
[[Category:Mechanics ]]