moving the vector examples and adding comments

java/examples/Topics/Simulate/GravitationalAttraction3D/Sun.pde

@@ -0,0 +1,39 @@
+// Gravitational Attraction (3D) 
+// Daniel Shiffman <http://www.shiffman.net>
+
+// A class for an attractive body in our world
+
+class Sun {
+  float mass;         // Mass, tied to size
+  PVector location;   // Location
+  float G;            // Universal gravitational constant (arbitrary value)
+
+  Sun() {
+    location = new PVector(0,0);
+    mass = 20;
+    G = 0.4;
+  }
+
+
+  PVector attract(Planet m) {
+    PVector force = PVector.sub(location,m.location);    // Calculate direction of force
+    float d = force.mag();                               // Distance between objects
+    d = constrain(d,5.0,25.0);                    // Limiting the distance to eliminate "extreme" results for very close or very far objects
+    force.normalize();                                   // Normalize vector (distance doesn't matter here, we just want this vector for direction)
+    float strength = (G * mass * m.mass) / (d * d);      // Calculate gravitional force magnitude
+    force.mult(strength);                                // Get force vector --> magnitude * direction
+    return force;
+  }
+
+  // Draw Sun
+  void display() {
+    stroke(255);
+    noFill();
+    pushMatrix();
+    translate(location.x,location.y,location.z);
+    sphere(mass*2);
+    popMatrix();
+  }
+}
+
+