Added the gravity vector method for polygons (kinda blindly copied and

pasted it from the ball version, hope it works)

gravity.cc

@@ -1,6 +1,7 @@
 #include "gravity.h"
 
 #include "game.h"
+#include "polygons.h"
 
 #include <cmath>
 #include <gtk/gtk.h>
@@ -82,6 +83,19 @@ void gravity_change(double dx, double dy) {
     }
 }
 
+vec2d gravity_vector(const polygon* p) {
+    if (constant_field) {
+        return g;
+    } else {
+        vec2d b_c = vec2d{width / 2.0, height / 2.0} - p->centroid();
+        double r2 = vec2d::dot(b_c, b_c);
+        if (r2 < g_r * g_r)
+            return vec2d{0, 0};
+        else
+            return g_g / r2 / sqrt(r2) * b_c;
+    }
+}
+
 vec2d gravity_vector(const ball* b) {
     if (constant_field) {
         return g;