- To understand how zero gravity works, you first have to understand Newton's third law of motion. Newton's third law tells us that if one object pushes or pulls on another object, that other object pushes or pulls back equally in the opposite direction. We know this is so because when you stand still, the force of gravity doesn't pull you through the floor. Instead, the floor pushes back the exact amount the gravity is pulling.
- Such a state of affairs is described by Newton's first law, the law of inertia, which says an object stays at rest or in motion in a straight line at a constant speed unless something acts upon it. In other words, to change the balanced state of affairs, some force has to unbalance it. Newton's second law describes what happens if a force comes along to overcome the inertia of an object: the object moves in the same direction as the force.
- If you are standing at rest, say on an elevator going at a constant speed, the force opposing gravity--we'll call it counter gravity--is equal to the force gravity exerts on you. That force is your mass multiplied by acceleration due to gravity. (Gravity is accelerating you when it pulls.) We'll call that force 1 gravity unit. To make it even easier, we'll call gravity "g." So the counter gravity preventing you from falling through the floor is equal to 1 g. If the floor of the elevator had a built-in scale, you'd read your normal weight on it.
- Let's say, though, that the elevator started accelerating upward. Gravity is still pulling you down and the counter gravity force is still pushing you up, but now another force--the elevator acceleration--is pushing up, too. If that second force happens to be accelerating up at a rate equal to the acceleration of gravity, there are now 2 g forces pushing you up: the original 1 g counter gravity and the 1 g of the elevator. And, of course, there's another force that seems to be resisting, pushing you down with equal force. The resisting force is called the inertial force. Now that the g forces against you have doubled in the elevator, your weight on that built-in scale has doubled, too.
- Let's take the opposite scenario, one in which the elevator starts going down with an acceleration equal to gravity's. The elevator's force is 1 g going down, while counter gravity's force is still 1 g pushing up. They've canceled each other out and the built-in scale is reading zero for your weight. Including the inertial force, which pushes up, in the opposite direction of the acceleration, there are no g forces pushing or pulling you in any direction. You are floating.
- By itself, a zero gravity environment wouldn't be suitable long-term human living. Bones, for instance, need to bear weight to remain healthy, the reason exercise with weights is helpful in preventing osteoporosis.
