This is Matt riding his skateboard down the hill.  Now, because he is riding down a hill and because the Earth has gravity, his velocity is not constant.  In contrast to most of the examples in class, where position is directly proportional to time and the velocity is constant, Matt's velocity is actually increasing.  If we were to make a position vs time graph for Matt riding down the hill, we would see an exponential curve.  This means his velocity is increasing.  Therefore, he is accelerating.  We could find his acceleration by the change in velocity over the change in time and it would be measured in meters per second per second.