Patty Knox, age 12, of Galveston, Texas, for her question:
How much force is one gravity?
When our astronauts leave and return, millions of young space agers hold their breath and worry about extra gravities. We know that the spacemen are feeling the stress of great weight. When the stress period is over, we relax and start to wonder about the meaning of two, three or more units of gravity.
The laws of gravity are the traffic regulations that keep the celestial objects of the universe in their proper places. A specific quota of this cosmic pulling force is a built in feature of every speck of matter, every blazing star and wheeling galaxy. In order to measure this cosmic force, we need to invent a basic measuring unit. Then we can estimate its far flung operations in terms such as three gravity units or three million gravity units.
It seemed logical to look for our gravity unit close at home and thoughtful earthling sensibly decided to base it on the gravity quota of their planet. One unit of gravity is the amount of force that the earth exerts upon each and every object within its reach. It makes a pound of sugar weigh a pound. This force decreases at a fixed ratio with distance from the planet's center, but above the surface it is still strong enough to hug down the atmosphere. It also pulls down falling objects and keeps us from falling up and off the earth.
We can pin down the gravity unit more precisely when we estimate its force in action. It takes energy to travel and a falling body is pulled down by the force of one gravity. Falling, like all forms of traveling, involves time and distance. The force in one gravity unit can be estimated by the rate at which a falling body is pulled down to the ground. This is complicated because the rate of falling speeds up, or accelerates, on the way down. There is, however, a fixed rate of acceleration for falling objects. This ratio gives us the exact measure of the force in one gravity unit.
A falling body accelerates at 32 feet per second per second. In the first second it is pulled down at a rate of 32 feet a second, accelerating as it goes. In the next second its speed accelerates another 32 feet per second to 64 feet per second. In the third second it accelerates another 32 feet per second to 96 feet per second. The fall continues to accelerate at the rate of 32 feet second by second. The force that powers this operation is one unit of gravity. A spacecraft increasing its speed by 96 feet per second per second is accelerating at three gravities. A man inside it feels the pressure of three times more weight than he feels upon the solid ground.
The effects of gravity on our bodies are very strange. At one unit of gravity, on the surface of the earth, we just feel our normal weight. Extra Gs, or gravity units, make us feel more weight. But inside a spacecraft out in orbit, all this changes. Here the earth's gravitational force has freedom to exert itself equally on the craft and the crew. This odd state is called free fall. When in orbit, we lose the effects of the normal one G and experience a rubbery state called weightlessness.