# PlanetPhysics/Acceleration

The *acceleration* of an object is the time derivative of its velocity. Like velocity, acceleration can therefore be considered either as a vector quantity or as a scalar quantity. Acceleration is usually denoted by the symbol , by (the time derivative of the velocity) or by (the second time derivative of the position). We can write the definition of acceleration (in vector form) as follows:

The SI unit of acceleration is (metres per second per second, or metres per second squared). Another unit of acceleration is , defined as ; this is approximately the acceleration due to gravity at the surface of the Earth at a latitude of .

In addition to acceleration as the time derivative (instantaneous rate of change) of velocity, the *average acceleration* , or the change of velocity over a specified period of time , can also be defined:

In classical mechanics, acceleration is caused by forces. If a total force acts on an object with constant mass , the object undergoes an acceleration as described by Newton's second law:
In contrast to velocity, which depends on the observer's system of reference, acceleration can be called an *absolute* quantity, in the sense that two observers moving with constant velocity with respect to each other perceive the same acceleration.