What is Weight? Every time you step onto a scale, you're weighing yourself. Not quite a revolutionary statement, you might think to yourself, but in reality, you're measuring a force, not how massive your body is. When you weigh yourself, what you're measuring is the earth's pull on your body, which varies depending on certain situations. Weight and the Gravitational Force Every object with mass exerts a force on every other object with mass - it's called the gravitational force. Basically, the force is proportional to the product of the masses of the two objects - in other words: That makes sense, for the most part - as the mass of an object increases, the force between it and the other object, in this case the earth, should increase along with it. What isn't quite as intuitive is that as the mass of the other object increases, the earth, the force increases along with that too. What this means is that if the earth were twice as heavy you would weigh twice as much. There's another factor to the gravitational force - the separation between the two objects. When you're standing on the surface of the earth, you'll never notice any difference between your weight at sea level and your weight at 10,000 ft. (3 km) above sea level. A person who weighs 150 lb. (68.03 kg) on the surface of the earth would weigh approximately 149.92 lb. (68 kg) at 10,000 ft. above sea level - certainly no more than your weight would vary during the course of the day. So What Does This Mean? Basically, the implications of the way gravitational attraction works is that the mass of the planet and the distance from the center of the planet matter in the amount that the plane weighs. While on earth, the changes in altitude hardly change the weight of the plane (Varying less than 0.2% in an average flight), on the moon, the plane would weigh approximately one-sixth as much, therefore requiring only one-sixth as much lift. Additionally, since the moon is smaller in radius than the earth, a plane's weight could vary as much as 6% during a flight. The Implications of Weight For every pound that a plane weighs, the wings (or whatever apparatus used) must provide one pound of lift at the speed the plane flies at. As you learned in the lift section, the amount of lift a wing can provide is dependent on certain features of the wing and the motion and orientation of the wing. Therefore, it is incumbent upon the airplane designer to try to make the plane as light as possible, while still retaining the maximum structural integrity.