In the Tour de France, speed starts as a state of mind.

Physics tells us that going fast on a bicycle depends, in addition to force and mass, on two elements: inertia and balance. Spinning wheels want to keep spinning. The rider wants to stay upright.

But science can't entirely explain a sprint finish at the Tour de France, where dozens of cyclists are squeezed shoulder to shoulder and tire to tire, barreling down the straightaway to throw their wheels over the finish line. Nor can science account for the riders' whiplash flight down mountain roads at speeds nearing 65 mph.

There is no room for mistakes on a descent in the Pyrenees or the Alps, where the Tour travels next week. The riders will be screaming down twisty hairpin turns, with no guardrails and no shoulders -- just cliff to one side and sheer drop to the other. On this kind of road you either pick the right line and angle of attack, or your Tour ends in a broken heap.

In sprints and descents, what powers the swift is just one thing.

Nerve.

Muscle power and the incline of the road factor into how fast the riders go. But more important than either of those is what ordinary mortals lack: fearlessness and a profound hunger for an adrenaline rush.

"Personally, I love it," said Australian sprinter Stuart O'Grady. "You're pushing yourself and your bike and everything on the limit. It's just total concentration, every second. Guys are locking it up in front of you, hitting the brakes, dodging, speeding home, sprinting about; you're coming into corners. It's full-on concentration every second. It's a huge rush."

O'Grady, a member of the French Cofidis team, won a tightly contested sprint in the fifth stage last week. On a particularly nasty day of cold wind and heavy rain in northern France, O'Grady burst across the finish line after a spate of unsuccessful attacks from his rivals.

Sprinting is an exercise of power and intimidation on a 15-pound machine going 40 miles an hour on the flat. There's a sprinter mentality in many sports, an in-your-face aggressiveness. Think of Maurice Greene's arrogance, running the 100-meter dash, or swimmer Amy Van Dyken spitting into a rival's lane. Cycling sprinters are no different. Squinting into the sunlight just before the start of the 11th stage Thursday into the sloping region known as the Massif Central, the freckled, sunburned O'Grady speaks softly about the thrill of the chase. But his sentiments are no less clear: He thrives on the mass competition of a sprint, the mano a mano battle. In sprinting, the killer instinct gets free rein.

"It's just all feeling," O'Grady said. "When to brake, when not to brake. When to push. When to put your elbows out. When to defend your position."

Sprinting is also a social thing -- you've got your rivals close at hand -- which makes the more solitary experience of a downhill ride far less fun.

"Oh, a mountain descent is different," said O'Grady, with a trace of disdain. "You're on your own."

The sprinters' somewhat blockier build is ill-suited for the mountain climbing that started in earnest with Friday's stage into the Pyrenees. (Blocky is a relative thing here -- O'Grady has not an ounce of body fat but is noticeably broader in the shoulders than the toothpick-armed climbers.) But for the bike riders who specialize in descents, who are light and aerodynamic and get their rush from gravity-assisted swooping, the upcoming stages offer nothing but thrills.

"I'm never afraid on descents, so that makes it easy," said French eminence Richard Virenque, regarded as one of cycling's fastest descenders. He has a jockey's diminutive build and a mule's capacity to work. He's at the other end of the speed spectrum from the pack-animal sprinters. Virenque, who rides for Quick-Step-Davitamon, is a noted loner who excels at the long-distance solo run. Wednesday, on the French national holiday of Bastille Day, he won the 10th stage -- and the ecstatic accolades of his countrymen -- with a 200-kilometer breakaway across the hills of the Auvergne.

But the steeply raked mountain roads are Virenque's true home. "I love speed, so for me it's a pleasure. When I take the turns, it's very fluid. I have no apprehensions.

"Sure, I use the brakes," he said, "but only at the right moment, and I use a supple touch. It's not . . . " He screws up his face in a faux panic attack.

Does he ever think about falling, or even dying? "No, no, no, no. The ones who think about that are the ones who are unsure, the ones who are afraid." Yet the descents have claimed Tour riders over the years. Fabio Casartelli crashed in the Pyrenees in 1995, breaking his skull open on a stone (a day that saw Virenque take another day-long breakaway to win the stage). His death shocked the cycling world -- he was Italy's Olympic champion -- and moved his fellow riders to slow down.

The day after his death, the race became a funeral march. "In muted tribute, the entire [group] rode the stage in a bunch, at a pedestrian crawl, a punishing ride in torrid heat," writes Graeme Fife in his artfully detailed book, "Tour de France: The History, the Legend, the Riders." Casartelli's Motorola team led the cortege. Among its members was the young American Lance Armstrong, who later won a stage in tribute to his friend -- an early sign of the greatness to come from this rider, though cancer would interrupt his ascent the next year.

Can you learn not to flinch at the road's oncoming twists, and not to blink when the crowd of cyclists around you starts to stampede for the finish? Snug your body down to the frame, keep your shoulders and elbows low, push your rear end back off the saddle. Look ahead to the line of the next curve. Agility and flow are key. Find the rhythm of the road.

Axel Merckx, son of Eddy Merckx, the Belgian five-time Tour winner whom many consider to be the greatest cyclist ever, said his father taught him a few simple things about descending: Trust. And use good equipment. He knows whereof he speaks: The elder Merckx lends his name to a manufacturer of high-end bikes.

"You trust your bike, trust your components and trust yourself," said the younger Merckx, whose Lotto-Domo team is outfitted with his father's bikes. "And don't overdo it. Try to stay as relaxed as you can because if you're way too stressed and way too nervous, that's when you'll make some mistakes."

"Mistakes," of course, include killing or crippling yourself.

If the riders themselves have no fear, those who watch them do. After exchanging a quick kiss, Antje Bakker watched her husband, Marc Lotz, ride off to join his Rabobank teammate at the starting line. This Tour is a comeback for him: Last year, in a sprint for the finish of the first stage, he was caught in a crash that felled 30 riders. Lotz broke his jaw and an eye socket.

"That's why I'm always afraid," Bakker said. And with the mountains to come, she grows increasingly apprehensive. Lotz loves speed.

Francis Van Londersele, one of the team managers of Cofidis, said he also gets nervous. As with all the directeurs sportifs, his job is to follow the pack in his car and issue orders through the earpiece each rider wears. Yet even as he's urging his riders to attack, he's afraid for them, he said. Afraid of a fall, afraid of a poor choice of trajectory that could end in disaster.

"We don't speak often enough about the risks the riders take," he said. "But that's part of the bike racer's job."

Riders near Col de Neronne speed through the 10th stage of the Tour de France on Wednesday. This time, they're lucky enough to have a guardrail.Overall leader Thomas Voeckler of France, right, rides with the pack during Thursday's stage of the Tour de France. Behind them is a sign that warns of the steep grade.Riders weave their way downhill Wednesday in the 10th stage of the Tour de France. The road from Limoges to Saint-Flour took its toll on Credit Agricole rider Sebastien Hinault, right.