What is a g force? A simple answer
What is a g force? It is a way of saying how heavy something feels right now, compared to normal.
That is genuinely all it is. One g is normal — what you feel sitting still reading this. Two g means everything feels twice as heavy as usual. Half a g means half as heavy.
It is a comparison, not a force
The name is a bit unfair, because a g force is not really a force. It is a ratio. Scientists would rather call it acceleration, but “he pulled 6g” is what everybody says, so we are stuck with it.
The useful thing is that it works on everything at once. At 3g your arms feel three times heavier, your head feels three times heavier, and the blood in your body weighs three times more too. That last one turns out to matter a great deal.
What different amounts feel like
| g force | What it feels like |
|---|---|
| 0g | Floating. Astronauts on a space station. |
| 1g | Normal. Standing on the ground. |
| 1.5g | A fast lift starting upwards, or a gentle plane turn. |
| 2–3g | A good rollercoaster loop. You are pressed hard into the seat. |
| 4–6g | Hard to lift your arms. Vision starts to grey at the edges. |
| 9g | The limit for fighter pilots, and only with a special suit. |
Why you feel it in a turn
Here is the part that surprises people. You feel g force whenever your speed or your direction changes. Turning counts, even at a steady speed.
To turn, a plane has to push itself sideways into the curve. To pull out of a dive, it has to push itself upwards. Both mean the wings are working harder than they do in level flight — and everything inside, including the pilot, gets pressed down into the seats.
The tighter the turn, the more g. This is exactly the same reason you slide across the back seat of a car going round a roundabout too fast.
Why pilots black out
Your heart is used to pumping blood up to your brain against normal gravity. At 5g the blood weighs five times as much and your heart cannot lift it that high.
So the blood drains out of your head. First your colour vision goes, then your sight narrows to a tunnel, then goes grey, then black — while you are still awake and can still hear. Pilots call that grey-out. Push further and you pass out completely. That is G-LOC, and it has killed people.
Fighter pilots fight it with a g-suit, which squeezes their legs and belly to stop blood pooling down there, and with a straining technique: tensing every muscle and breathing in short hard bursts.
Negative g — being pushed up out of the seat — is worse, and humans handle much less of it. Too much and blood is forced into your head instead, which is called a red-out.
Where you have already felt it
- A rollercoaster loop is 3–4g at the bottom
- A lift starting upwards is about 1.2g
- Sneezing briefly puts around 3g through your head
- A car braking hard is roughly 1g forwards
Feel the trade in a game
Skimloft is a free browser game about a plane with no engine, where height is the only fuel you have.
Pulling up hard is how you turn speed back into height — and it is the same move that generates g in a real aircraft. Do it gently and the wing works beautifully. Do it greedily out of a fast dive and you ask the wing for more than it can give, and it stops working altogether. That is a stall, and in the game it is always your own doing.
Quick answers
What does 1g mean? Normal weight. What you feel standing still.
How many g can a human take? About 5g before vision goes, around 9g with a g-suit and training, and only for a short time.
Is g force a real force? Not exactly — it is a comparison to normal weight. But everyone calls it one.
Do you feel g force in a turn? Yes. Changing direction counts, even at a steady speed.
More: how high planes fly, and the parts of an airplane and what each one does.