This is, for example, what happens when an object goes around in a circle with a constant speed (the direction of the velocity vector changes). If the length of the velocity vector (speed) is constant, it is still possible that the direction of the velocity vector changes with time, and thus, that the acceleration vector is non-zero. This means that if the velocity vector changes with time, then the acceleration vector is non-zero. Recall, the acceleration vector is defined as the time derivative of the velocity vector ( Equation 4.1.4). One key difference with one dimensional motion is that, in two dimensions, it is possible to have an acceleration even when the speed is constant. Assistant Professor (Physics) at Queen's University.
