function move_particles(particles, force_constant) {
  for (let particle of particles) {
    particle.acceleration.setMag(0);
  }
  for (let i = 0; i < particles.length; i += 1) {
    for (let j = 0; j < i; j += 1) {
      const displacement = p5.Vector.sub(
        particles[i].position,
        particles[j].position,
      );
      const force_mag = (
        particles[i].charge * particles[j].charge
        / displacement.magSq()
        * force_constant
      );
      // XXX possible extension: divide by charge's mass
      const ai_mag = force_mag;
      const aj_mag = force_mag;
      let ai;
      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(ai_mag);
        aj.mult(aj_mag);
      }
      project_onto_plane(ai, particles[i].position);
      project_onto_plane(aj, particles[j].position);
      particles[i].acceleration.add(ai);
      particles[j].acceleration.add(aj);
    }
  }
  for (let particle of particles) {
    particle.velocity = particle.velocity.add(particle.acceleration);
    particle.position = particle.position.add(particle.velocity);
    particle.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`
/// Mutates `v`
function project_onto_plane(v, unit_vector) {
  let v_proj = project_onto_unit_vector(v, unit_vector);
  v.sub(v_proj)
}