import math


def calc_drag_force(v: float, d: float, Cd=0.5):
    """ F_drag = 0.5 * Cd * ρ * v^2 * A

    https://www.grc.nasa.gov/www/k-12/airplane/drageq.html

    :param v: Velocity
    :param d: Diameter
    :param Cd: Drag coefficient
    :return: The drag force
    """
    direction = 1
    if v > 0:
        direction = -1
    return direction * 0.5 * Cd * 1.205 * (v ** 2) * ((d / 2) ** 2 * math.pi)