Summary Acceleration is the rate of change of velocity with time. As a vector, it has magnitude only. Average acceleration is acceleration measured over a non-zero time interval. Instantaneous acceleration is the limit of average acceleration as the time interval approaches zero. In the language of calculus, instantaneous acceleration is also the first derivative of velocity with respect to time or the second derivative of displacement with respect to time.
Negative acceleration is acceleration in the direction opposite the positive vectors.
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