protected void applyForce() {
   temp.set(this);
   addSelf(sub(prev).addSelf(force.scale(weight)));
   prev.set(temp);
   force.clear();
 }
 /** @param force the force to set */
 public void setForce(Vec3D force) {
   this.force = force;
   scaledForce = force.scale(timeStep);
 }