Re: G force in space?

Hi Lynton,

"G-forces" have a confusing name. The forces you're talking about are unrelated to gravity! Let me explain, by talking a bit about accelerations.

The idea of gravitational acceleration on the ground, is that the Earth is pulling on you, trying to make you accelerate downwards at 9.8 meters per second per second. (When you step off of a ladder, you speed increases by 9.8 m/s every second. If you were in the air for a second, you'd be traveling downward at 9.8 m/s by the end.) Your feet, however, exert forces on the ground - enough force to try to accelerate you upwards at 9.8 meters per second per second. That's what you feel as "weight" on the ground: your feet, or your bottom, or or your back, resisting the gravitational acceleration of the earth. Sometimes it's easier to think about these forces if you imagine yourself standing on a big spring; the force of gravity on your body is counteracted by the upward force of the spring.

But anything that tries to accelerate you (or decelerate, same thing) requires it to push on your feet or bottom or whatver. When you're in a car that's accelerating hard, the back of the seat has to push on you to make you accelerate with it. If the car is turning hard, the seat or door or whatever has to push on you sideways to make your body move in the same way the car moves - remember that in the absence of these forces, "an object in motion tends to stay in motion in a straight line". These are what we sometimes call G-forces; there's a G-force acting on you from behind when the car accelerates, and from the side when it turns, and from the front when it brakes. And there's always the chair pushing up on your bottom to counteract gravity itself. Very different phenomena, but from your perspective they all feel like different strengths of gravity acting in all different directions.

Put the car in space; now, there's no acceleration due to gravity pulling you down into your seat. But if your car (with rockets on the back!) accelerates or turns, it still has to push on your body so that you keep up. Those pushing forces are the G-forces. So in a rocket moving in a straight line in space (and by Einstein's theory, any "gravitational orbit" counts as a straight line!), yeah, you could float forever in the middle of the cockpit. But the rocket turns suddenly - your body "doesn't notice" and keeps going in a straight line - oof! and the wall crashes into you and pushes you wherever it wants. That feels like a G-force. If the acceleration due to the wall was 9.8 m/s^2, you could "stand" on the wall and it would feel like you were standing on the Earth, since the force on your feet would be the same!

Really, if you think about any object in enough detail, you can find all of the forces acting on it - forces to counteract gravity, to cause various accelerations, to overcome friction - and you can figure out how the object will move using vector addition; see any intro physics textbook for more on this.

Sincerely,

-Ben Monreal