An object that is moving in a straight line is accelerating if its velocity (sometimes incorrectly referred to as speed) is increasing or decreasing during a given period of time. Acceleration (a) can be either positive or negative depending on whether the velocity is increasing (+a) or decreasing (-a). An automobile's motion can help explain linear acceleration. The speedometer measures the velocity. If the auto starts from rest and accelerates to 60 MPH in 10 seconds, what is the acceleration? The auto's velocity changed 60 MPH in 10 seconds. Therefore, its acceleration is 60 MPH/10 s = +6 mi/hr/s. That means its acceleration changed six miles per hour every second it was moving. Notice there are one distance unit and two time units in the answer. If the auto had started at 60 MPH and then stopped in 10 seconds after the brakes were applied, the acceleration would be = -6 mi/hr/s. If this automobile changes direction while moving at this constant acceleration, it will have a different acceleration because the new vector will be different from the original vector. The mathematics of vectors is quite complex.