The human body isn’t built for speed. If you go really fast, physics kicks in and you start to experience g forces. If those get strong enough they can crush you like a bug.

Mark Greenfield, a pilot and an aerobatics instructor at Ultimate High at Kemble Airfield in Britain, explains g force to SCOPE. “G force is a term that we use to describe the acceleration and the force resulting from the acceleration of the body,” he says.

So g force is like the force we feel when we are subject to the acceleration of gravity. The g stands for gravity and the scale starts at 1-g. We might feel a small amount of g force when going around a corner in a car.

”When we go around corners there’s a bigger acceleration. The higher the acceleration the more force you feel,” says Mark “So at 3-g, or three times the acceleration due to gravity, you’ll feel three times as heavy.” Roller coasters do around 2-g or 3-g, and some airplanes can do 10-g.

G force effects

So what happens the body when it experiences g force?

”The whole body is subject to the same acceleration,” says Mark. “The blood is effectively pulled down to the lower half of the body. You get a pooling of blood in the lower half of the body, and you get a reduction of blood pressure in the upper half of the body.”

“This is potentially dangerous, because the blood transports oxygen to the brain. And if there’s not enough blood there, the brain isn’t getting enough oxygen.”

“Your internal organs are under the same acceleration, so your kidneys, your lungs, your stomach, is also all being pulled down on the same acceleration,” he continues. “Your body might send you a message that the body’s reaching its own limits. It’ll do that through the eyes.”

“You’ll get stars in front of your eyes, you might get black and white vision, and you might start losing your peripheral vision. If you keep pulling g when that happens, you get what we call g-LOC,” says Mark. “That’s g-induced loss of consciousness, and you’ll go to sleep. It’s not a problem, as long as you wake up again fairly shortly afterwards.”

Braving the skies

SCOPE presenter Kathriona is going to see if she can survive some high-altitude g force. First, she finds out what she can do to withstand the g.

“You’re looking to stop the blood from going into the bottom half of the body,” says Mark. “To do that, you strain your muscles and that constrains all of the blood transport systems. So the muscles you might strain are your calves, your thighs, your butt and your stomach. Without being too indelicate, it’s like you’re trying really hard to take a poo when you’re constipated. You have to make the noise too!

“As the g comes on, you do the strain movement. That will give you about a g and a half’s worth of protection. Up there, it’s going to be helpful, he says.”

Kathriona and Mark set off in the plane and fly 400kms an hour. When they hit 4.5g’s, she feels like a huge invisible weight is crushing her. She does the strain as hard as she can and then feels her guts pushing back into place. Thankfully, she lands safely on the ground after the thrilling ride.

The things SCOPE presenters do for science!

Learn more:

Learn more about gravity

Check out how gravity works

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