We say that velocity is the time rate of change of position, or the derivative of position with respect to time.
Acceleration is the rate of change of velocity with time.
Let us assume the point is moving with uniform speed v around the circle of radius r. Its position vector is shown at two points separated by a small time interval, during which it has revolved through an angle . The velocity of the point, although remaining of constant magnitude, has also rotated through the same angle. In the case of the triangle with the position vectors r, the small side is the distance the point moved in the small time interval. Note that the second law implies the first one, for if the force is zero, the second law says that the rate of change of momentum is zero, which is the same thing as saying that the momentum is constant.