--- /dev/null
+/*
+ * Copyright © 2014 Keith Packard <keithp@keithp.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; version 2 of the License.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+#include "ao-fakeflight.h"
+
+/* Density of dry air in kg/m³ for a given pressure and temperature. */
+double
+density_air(double pressure, /* Pa */
+ double temperature) /* °C */
+{
+ return pressure / (GAS_CONSTANT * (temperature + 273.15));
+}
+
+/* Area of a circle */
+double
+area_circle(double diameter)
+{
+ double radius = diameter / 2;
+ return M_PI * radius * radius;
+}
+
+/* Force due to drag (N) */
+double
+force_drag(double speed, /* m/s */
+ double rho, /* kg/m³ */
+ double cd, /* unitless */
+ double area) /* m² */
+{
+ return 0.5 * rho * (speed * speed) * cd * area;
+}
+
+/* Force due to gravity (N) */
+double
+force_gravity(double mass, /* kg */
+ double altitude) /* m */
+{
+ double distance = altitude + EARTH_RADIUS;
+ return GRAVITATIONAL_CONSTANT * EARTH_MASS * mass / (distance * distance);
+}