How It Works

G-force explained: How acceleration can knock you out


When you’re hurtling down the steel track of a roller coaster, it might seem that your stomach is climbing into your throat, and your eyes are squishing deep into your skull.

Several forces are at play when you feel that way. Earth is constantly pulling down on every one of us. It has a great deal of mass, and that gives it a large gravitational field. And when we take a sharp turn on a fast ride, blast off in a rocket, or slam on the brakes, we’re thrown around by forces far stronger than Earth’s gravity. But why?

Engineers rate those experiences with numbers called g- forces, to explain how strong they are. One g is the amount of force that Earth’s gravitational field exerts on your body when you are standing still on the ground. Every particle that makes up our planet is tugging on you simultaneously. Each one of those pulls is quite weak, but combined they are strong enough to keep your feet on the ground. Five g acceleration, something that race car drivers regularly experience, is five times as intense.

Any time that an object changes its velocity faster than gravity can change it, the forces will be greater than one g. At zero g, you would feel weightless. And past 100g, you’re almost certainly dead. Forces that intense can crush bones and squash organs.

Gravity is not the only source of g-forces. They take hold whenever a vehicle, like a car or a plane, suddenly changes its velocity. Speed up, slow down, or make a turn, and your velocity will change. The faster it happens, the more force you will experience.

Understanding g-force

bugatti veyron
Accelerating in a Bugatti Veyron will create 1.2gs

To find out how many gs you experienced during an intense acceleration, take your maximum speed, divide it by the time it took to hit that rate, and then divide by 9.81m/s2. The resulting number is how many gs you experienced.

For example, put the pedal to the metal in a Bugatti Veyron and you will go from 0-100kph in 2.3 seconds. 100kph is 28m/s, 28 / 2.3 = 12m/s2, 12 / 9.8 = 1.2g.

Acceleration can knock you out

Pilots train to resist g-force

If you are riding in a jet while it is making sharp turns, the blood in your head may rush out into your lower body.

As the plane turns, all of the fluids in your body will act as if they were in a centrifuge, moving toward your feet, or whatever part of your body is on the outer edge of the turn. When that happens, your eyes will not get enough oxygen and you may experience a greyout, a sudden loss of colour vision, or a full blackout, temporary blindness. Accelerate harder and you will lose consciousness as blood retreats from your brain, depriving it of oxygen.

Some people will experience these effects below 5g, but seasoned fighter pilots can take a bit more because they are very physically fit. They train themselves to resist the forces, and wear special suits that squeeze blood up into their heads.

Discover exactly how Red Arrows and Blue Angels pilots cope with g-force whilst completing their death-defying stunts in Issue 75 of How It Works magazine. It’s available from all good retailers, or you can order it online from the ImagineShop. If you have a tablet or smartphone, you can also download the digital version onto your iOS or Android device. To make sure you never miss an issue of How It Works magazine, make sure you subscribe today!


  • Steve Harris

    Well, gravity is not really a source of g-force (which is really an acceleration, not a force). You cannot feel gravity (except as tides) so when only gravity acts, there is no g-force. An example is the vomit comet, where gravity certainly acts, but there is zero-g force (zero-g) and people are weightless. The same is true in the ISS space station (the tides are “microgravity”). Gravity still operates, but you can’t feel it up there as g-force, and you can’t feel it anywhere.

    What causes g-force is a mechanical push from a wall, floor, or some physical object. The g-force you feel in your chair, for example, is from the chair, not from gravity. Take away the chair and while you fall, gravity works, but you are in zero-g. No g-force.