Changes

'''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]] squared 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 the [[force]], measured in Newtons, m=mass and is the [[mass]], measured in kilograms, and a=acceleration and is the acceleration, measured in meters per second squared. Using the formula we can find see that <math> \vec a=\frac{\vec F}{m}</math>. This formula is only correct when the mass is constant.

In the case of straight trajectory, if an object's acceleration and [[velocity]] have the same direction, the object is gaining [[speed]]. If the acceleration and velocity have opposite directions, the object is losing speed. If the acceleration is zero, then the object is either at rest or travelling in a straight line at constand constant speed. This is Newton's first law that the [[velocity]] of an object is constant if the '''net''' [[force]] acting on it is zero.

In case of a curvilinear trajectory, there is an acceleration, even if the speed of the object is constant. This is because the ''direction'' of the object's velocity changes if the path is curved. And whenever there is a change in [[velocity]], there is an acceleration, since acceleration is the change in velocity with respect to time. An example of this is [[circular motion]], where the speed of a particle is constant, but it is accelerating towards the centre of its circular trajectory.