"Slope of velocity" implies acceleration. The vector is thevelocity vector, and is theslope of the tangentof the parametric curve at time t. The magnitude of velocity . If we want to calculate acceleration, then . In general, velocity and acceleration are not orthogonal (perpendicular), but theunit vectorof velocity and its derivative (the component of acceleration perpendicular to velocity) is. In this case, the product of the slopes would be -1 (true for any perpendicular slopes).