Add Physics#gravitate to calculate gravity

Author: una-ada

js/main.js

@@ -42,4 +42,3 @@ model.scene.push(titan);
 view.loop();
 physics.loop();
 console.log(model);
-window.setTimeout(physics.stop, 100);

js/modules/Physics.js

@@ -38,6 +38,21 @@ export default class Physics {
       (obj) => obj.position.add(obj.velocity.copy().scale(Physics.timeScale))
     );
   }
+  /**
+   * Calculate gravitational acceleration between two Celestial instances
+   * @param {Celestial} m_1 Celestial to apply acceleration to.
+   * @param {Celestial} m_2 Celestial applying the acceleration.
+   * @returns {Vector} Gravitational acceleration Vector.
+   */
+  gravitate(m_1, m_2) {
+    // Vector between m_1 and m_2
+    const between = m_1.position.vectorTo(m_2.position),
+      // Distance between m_1 and m_2
+      r = between.magnitude;
+    // Make `between` a unit vector
+    between.magnitude = 1;
+    return between.scale((Physics.G * m_2.mass) / r ** 2);
+  }
 
   /*----- Running Methods ----------------------------------------------------*/
   /** Calculate physics on a set interval. */

js/modules/Utils.js

@@ -37,7 +37,7 @@ export class Point {
    * @returns {Point} The updated point.
    */
   add(addend) {
-    if(typeof addend === "number"){
+    if (typeof addend === "number") {
       this.x += addend;
       this.y += addend;
     } else {
@@ -52,7 +52,7 @@ export class Point {
    * @returns {Point} The updated point.
    */
   subtract(subtrahend) {
-    if(typeof subtrahend === "number"){
+    if (typeof subtrahend === "number") {
       this.x -= subtrahend;
       this.y -= subtrahend;
     } else {
@@ -80,7 +80,7 @@ export class Point {
   }
   /**
    * Get a Vector between two points.
-   * @param {Point} point 
+   * @param {Point} point
    * @returns {Vector} A Vector between this Point and another Point.
    */
   vectorTo(point) {