diff --git a/sketch.js b/sketch.js
index a04f4ac..d0a1b95 100644
--- a/sketch.js
+++ b/sketch.js
@@ -70,7 +70,7 @@ function draw() {
   camera.centerZ = 0;
 
   make_lights();
-  if (physics) move_charges(charges);
+  if (physics) move_charges(charges, 8e-4);
 
   draw_charges(sphere_radius);
   if (skeleton) draw_skeleton(sphere_radius);
diff --git a/thomson-problem.js b/thomson-problem.js
index bb7466c..d13cc31 100644
--- a/thomson-problem.js
+++ b/thomson-problem.js
@@ -1,4 +1,4 @@
-function move_charges(charges) {
+function move_charges(charges, force_constant) {
   for (let charge of charges) {
     charge.acceleration.setMag(0);
   }
@@ -8,15 +8,21 @@ function move_charges(charges) {
         charges[i].position,
         charges[j].position,
       );
-      let acceleration_mag = 1 / displacement.mag() * 0.001;
+      const force_mag = 1 / displacement.magSq() * force_constant;
+      // XXX possible extension: divide by charge's mass
+      const ai_mag = force_mag;
+      const aj_mag = force_mag;
       let ai;
-      if (acceleration_mag === Infinity) {
+      let aj;
+      if (force_mag === Infinity) {
         ai = p5.Vector.random3D();
+        aj = p5.Vector.random3D();
       } else {
         ai = displacement.copy().normalize();
+        aj = ai.copy().mult(-1);
       }
-      ai.mult(acceleration_mag);
-      let aj = p5.Vector.mult(ai, -1);
+      ai.mult(ai_mag);
+      aj.mult(aj_mag);
       project_onto_plane(ai, charges[i].position);
       project_onto_plane(aj, charges[j].position);
       charges[i].acceleration.add(ai);