Acceleration is the rate at which the velocity of an object changes with time. Acceleration is a vector quantity and may involve a change in an object's speed or direction or both. Deceleration is considered to be negative acceleration.
For a body moving in a straight line with constant acceleration a, from initial speed u to final speed v, the acceleration is given by
a = (v - u)/t = (v2 - u2)/2s
where t is the time taken and s is the distance covered.
If the acceleration is not constant it is given by dv/dt = d 2s/dt 2.
Linear acceleration occurs when the rate of motion (speed) along a straight line changes. Centripetal acceleration occurs in circular motion at constant angular velocity as the direction of motion changes. Angular acceleration occurs in circular motion when the rate of rotation changes.
Acceleration is the result of a (net) force (F) applied to an object (of mass m). We can therefore express acceleration in terms of an applied force through this equation derived from Newton's second law: a = F/m.
The SI unit for acceleration is meter per second squared (m/s2).