When we talk about a curved path, the direction of movement at an instant is determined along the tangent of the curve at that instant. While moving on such a path, the direction of movement changes at every point on the curve. Hence, the quantities that change with motion (direction of motion) are velocity and acceleration and those that remain constant during motion are speed and magnitude of acceleration. We can also say that speed and magnitude of acceleration are scalars whereas the velocity and acceleration are vectors.