Here's a neat toy: it's a vector calculator!
So, add in your forces, say, 1G down (that's Earth's gravitational pull. At orbit, it's slightly less than that, but let's call it 1.) by clicking one square up from the center. Now you can add other vectors, cuz if you don't, that's the answer: you fall to Earth, eventually. So let's say we're in orbit. Our centrifugal force is pulling us away, and it's a stable orbit, so it has to be the same, but in the opposite direction, up. So click one square up from the center.
If you want, you can calculate by hitting the "add 1,2" button. Unsurprisingly, those are balanced forces, so they're zero. If you're off by a little bit and the y component of the sum vector is negative, your orbit is decaying. I hope you've got a ballute. If it's positive, you're going to a higher orbit or perhaps leaving orbit altogether. I hope you have a way to get home.
Now, let's add a shock to the side: a 6G dodge. Click 6 boxes to the left. Now sum all three vectors. You'll note that only the dodge — the black arrow — mattered. It's because the orbit and the gravity of Earth balanced out.
So let's do something a little more complex.
First, we know that, if we're in orbit, the forces equal out until we accelerate in a direction, so let's just ignore that.
Now, let's say that we're losing orbit. As we've seen above, it doesn't take much — the upwars thrust it just has to be less than 1G to fall out of orbit. So let's add in that basic falling acceleration: let's call it .5G down. It doesn't really matter as long as it's less than 1.
Now, here's the thing: we're hitting the atmosphere all of a sudden with great force. Even though our acceleration's really small, at some point in the past we've gotten going really fast, because you need to be in order to orbit, about 7 km a second
, that is 420 km/minute, or 25,200 km/hr. And you're going to come to a complete stop, standing at your target, in just minutes, so you have to slow down really fast. That's the 3-6 G. Because in Gundam we're doing really exciting ZOINKS, let's say it's a 6G entry — really steep, fast deceleration. That's the force of the atmosphere pushing up against our MS as it's falling, so click 6 boxes up.
And, oh, hey, we're fighting, right? So we're going to add in a 6G maneuver, let's say to the right.
Now have it calculate all three vectors. You see how little the actual pull of gravity matters compared to all the extreme maneuvering going on? Look at those total forces. I just did this now: the forces on my MS, Haro, and my body sum to 7.9G (that's the "magnitude" in the sum box, which is the length of the black arrow) at those moments when I'm doing my maneuvers. That means that my body weighs 1080 lbs, except in extreme moments of lateral maneuvering, during which I weigh 1422 lbs.
Try it with a couple of different maneuvers by hitting "clear" (not "clear all" which will get rid of both gravity and re-entry forces). Remember that, if you maneuver down, you're speeding up your descent, which means you'll be unable to stop in time and wind up being a smoldering crater. So only maneuver left or right, but watch the numbers change as you maneuver upward, too, slowing your descent.
That's some serious business, there. Just doodling around now, I found that an upward, diagonal dodge increased the force on my poor body to 9.8G. That means I weigh almost a ton. My head, which usually weighs something like 14 lbs now weighs 137 lbs. If my jaw weighs a pound normally, right now it weighs almost 10. I probably can't keep it from just bobbling around. My arms, usually about 15 pounds together, now weigh some 140. My heart has to pump blood that weighs almost 10 times as much. And so forth.