Although it is common to assign deceleration a negative value, negative acceleration does not automatically imply deceleration. It won't matter which you chose as long as you are consistent throughout a problem. Don't learn any rules for assigning signs to particular directions and don't let anyone tell you that a certain direction must be positive or negative. 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. Acceleration Centrifuge Training Human Centrifuge Ejection Site , Kevin Coyne Human Systems Wing , Brooks Air Force Base, US Air Force Acceleration Protection Advanced Technology Anti-G Suit Stapp, John Paul , National Aviation Hall of Fame cycling Effectiveness of Bicycle Helmets , Michael Henderson, Motor Accidents Authority of New South Wales galileo Dialogue Concerning Two New Sciences . This occurs 7 minutes 40 seconds into the mission.