1

My face is sliding backward!

Many modern roller coasters begin with a quick launch – 0 to 120 mph in less than four seconds – rather than the ka-chunk, ka-chunk of a chain slowly towing a car uphill. The force of forward acceleration, linear G-force
(G for gravity), pulls your cheeks toward your ears and pushes you back in your seat.

2

I weigh 1,000 pounds!

At the end of a steep drop, you feel as if your weight has quadrupled and you're being flattened like a cartoon character. You may even get lightheaded as blood rushes toward your feet. That is positive G-force. We walk around every day in 1G (one times the force of gravity), so anything more than that feels oppressive. Some coasters top 4 or 5G, but only briefly, because a sustained force of even 2 or 3G – what you'd feel during a space shuttle launch, for instance – makes it tough for the heart to pump blood to the brain.

3

I'm upside down!

Many tracks have loops – “inversions” in coaster-speak – but you rarely feel as though you'll fall. Inertia helps keep you moving while centripetal force, which means “center seeking,” helps you round the loop smoothly and keeps you in your seat. Nearly all loops are tear-shaped “clothoids” because true circles require too much acceleration to get a car all the way around. Forces are so well balanced that you probably wouldn't fall out of most roller coasters even without the harness, said Larry Giles, vice president of design and engineering at Busch Gardens in Williamsburg – but he also said he wouldn't try that himself.

4

My spleen is in my throat

As you crest a hill and the car starts to descend, you feel as if you, or perhaps your stomach and spleen, might fly out of your seat. Coaster designers love to upend your innards, which really do float around in your body when momentarily freed from gravity. That's called negative G-force in roller coaster terminology, although it is most likely not really negative but somewhat less than our normal 1G. Coaster designers avoid truly negative G-force, because humans can tolerate very little of the violent upward rush of blood, which can cause brains to bleed and eyeballs to explode.

5

My heart is pounding!

Thanks largely to fear and adrenaline, even a healthy person's heart can race and beat irregularly while riding a roller coaster, a German study found. In fact, pulses leaped most during slow climbs. Scary visual cues can cause the physical reaction: dangling before a drop, avoiding a collision by an eyelash or seeing a giant creature leap at your car. Busch Gardens' Griffon has a 90-degree dive, six inversions and several near misses. ''But if you closed your eyes, it's almost like sitting in your rocking chair,'' Giles said.

6

Ooooof. My shoulder!

You round a turn and are knocked into the far side of your seat. You've been jolted by lateral G-force. On some coasters, the jostle is part of the ambiance. But too much lateral force can cause injuries such as whiplash and broken blood vessels in the brain. Banking or tilting the track on the turns smooths the ride and reduces lateral force.

7

I might be sick.

Even people who don't tend to get motion sickness can become queasy after all the pitching, dipping and diving. Sometimes it's an inner ear issue, but usually the problem is farther south. “Gravity keeps everything in your stomach,” said Carolina Ilie, a physicist at SUNY Oswego who has done projects on roller coasters. “Moving around can cause a valve to open between the esophagus and the stomach.”