add moments of inertia

geom/src/aabb.rs

@@ -88,6 +88,13 @@ impl AABB {
         }
     }
 
+    #[inline]
+    /// Calculates the moment of inertia of the AABB around its center.
+    pub fn moment_of_inertia(&self, mass: f32) -> f32 {
+        let Vec2 { x: w, y: h } = self.size();
+        mass * (w * w + h * h) / 12.0
+    }
+
     #[inline]
     pub fn size(&self) -> Vec2 {
         self.ur - self.ll

geom/src/circle.rs

@@ -8,9 +8,15 @@ pub struct Circle {
 }
 
 impl Circle {
+    #[inline]
     pub fn contains(&self, p: Vec2) -> bool {
         self.center.is_close(p, self.radius)
     }
+
+    #[inline]
+    pub fn moment_of_inertia(&self, mass: f32) -> f32 {
+        mass * self.radius * self.radius * 0.5
+    }
 }
 
 impl Circle {

geom/src/obb.rs

@@ -55,6 +55,15 @@ impl OBB {
         ]
     }
 
+    #[inline]
+    /// Calculates the moment of inertia of the OBB around its center.
+    pub fn moment_of_inertia(&self, mass: f32) -> f32 {
+        let [a, b] = self.axis();
+        let a = a.mag2();
+        let b = b.mag2();
+        mass * (a + b) / 12.0
+    }
+
     #[inline]
     pub fn center(&self) -> Vec2 {
         (self.corners[2] + self.corners[0]) * 0.5