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diff --git a/src/main/scala/sims/dynamics/RegularPolygon.scala b/src/main/scala/sims/dynamics/RegularPolygon.scala
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+++ b/src/main/scala/sims/dynamics/RegularPolygon.scala
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+/*    _____ _ __  ________    ___                                      *\
+**   / ___/(_)  |/  / ___/   |__ \  Simple Mechanics Simulator 2       **
+**   \__ \/ / /|_/ /\__ \    __/ /  copyright (c) 2011 Jakob Odersky   **
+**  ___/ / / /  / /___/ /   / __/                                      **
+** /____/_/_/  /_//____/   /____/                                      **
+\*                                                                     */
+
+package sims.dynamics
+
+import scala.math._
+import sims.collision.ConvexPolygon
+import sims.math._
+
+/**A regular polygon with <code>n</code> sides whose excircle has a radius <code>radius</code>.
+  * @define shape regular polygon
+  * 
+  * @param n number of sides.
+  * @param radius radius of the excircle
+  * @param density density of this regular polygon
+  * @throws IllegalArgumentException if <code>n</code> is smaller than 3 */
+case class RegularPolygon(n: Int, radius: Double) extends ConvexPolygon with Shape {
+  require(n >= 3, "A polygon must have at least 3 sides.")
+  
+  /**Height of one of the constituting triangles.*/
+  private val h: Double = radius * cos(Pi / n)
+  /**Half width of one of the constituting triangles.*/
+  private val b: Double = radius * sin(Pi / n)
+  
+  def halfDiags = (for (i <- (0 until n).toArray) yield (Vector2D(0, radius) rotate (2 * Pi * i / n))) map (_ rotate rotation)
+  
+  def vertices = for (h <- halfDiags) yield position + h
+  
+  val area = n * h * b / 2
+  
+  /**Moment of inertia of one of the constituting triangles about the center of this polygon.*/
+  private val Ic: Double = density * b * (3 * b + 16) * pow(h, 4) / 54
+  
+  val inertia = n * Ic
+}