We can define all these concepts (velocity, momentum, potential) in one dimensionas well as in three dimensions. Let us look at the example for a barrier
We can't find a solution for less than 0 (no solution for ). For energy less then the particles can move left or right from the barrier, with constant velocity, but will make a hard bounce at the barrier (sign of is not determined from energy). For energies higher than particles can move from one side to the other, but will move slower if they are above the barrier.
