public VerletParticle3D clearVelocity() {
   prev.set(this);
   return this;
 }
 public VerletParticle3D setPreviousPosition(Vec3D p) {
   prev.set(p);
   return this;
 }
 protected void applyForce() {
   temp.set(this);
   addSelf(sub(prev).addSelf(force.scale(weight)));
   prev.set(temp);
   force.clear();
 }