Revert "remove negligible vector projection"

This reverts commit 6123c02581.

sketch.js

@@ -38,18 +38,34 @@ function move_charges() {
       const acceleration_mag = 1 / displacement.mag() * 0.005;
       let ai = displacement.copy().normalize().mult(acceleration_mag);
       let aj = p5.Vector.mult(ai, -1);
-      charges[i].acceleration.add(ai);
+      let ai_norm = project_onto_plane(ai, charges[i].position);
-      charges[j].acceleration.add(aj);
+
+      let aj_norm = project_onto_plane(aj, charges[j].position);
+
+      charges[i].acceleration.add(ai_norm);
+      charges[j].acceleration.add(aj_norm);
     }
   }
   for (let i = 0; i < charges.length; i += 1) {
     let charge = charges[i];
     charge.velocity = charge.velocity.add(charge.acceleration);
+    //charge.velocity = project_onto_plane(charge.velocity, charge.position);
     charge.position = charge.position.add(charge.velocity);
     charge.position.normalize();
   }
 }
 
+function project_onto_unit_vector(v, unit_vector) {
+  let size = p5.Vector.dot(v, unit_vector);
+  return p5.Vector.mult(unit_vector, size);
+}
+
+/// Project `v` onto the plane normal to `unit_vector`
+function project_onto_plane(v, unit_vector) {
+  let v_proj = project_onto_unit_vector(v, unit_vector);
+  return v.sub(v_proj)
+}
+
 function make_charges(n) {
   charges = [];
   for (let i = 0; i < n; i += 1) {