velocity.js

var velocity = function (el, header) {

  var wWidth = $(window).width(),
      spacing = $(header).height(),
      offset = $(el).offset();

  //  Create consistent velocity!
  //  Perfect velocity = 2550px per 3000milliseconds = .85 you know, since v = d/t

  var current = window.pageYOffset;
  //  get current position
  var movement = offset.top - spacing;
  //  set distance for movement from the top minus space for header
  var distance = current - movement;
  //  get distance from current position
  distance = Math.abs(distance);
  //  can't use negative numbers
  var speed = distance / 1.105;
  //  1.105 is the velocity!

  return {
    speed: speed,
    movement: movement
  };
};