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What makes a racecar spontaneously rip a 360 backflip? A perfect storm of hills and tailgating, that’s what. In this case, driver Yannick Dalmas, racing for Team Porsche in the 1998 Petit LeMans at Road Atlanta, was drafting the car in front of him while zooming over a rise. As he crested the hill, the car’s suspension pulled up, allowing more air to flow under the car and creating lift. Simultaneously, the draft from the car in front of him interrupted the airflow over the nose of Yannick’s car, sapping the much-needed downforce that kept the car in contact with the pavement. Without that downforce, there was nothing to stop the car’s nose from continuing upward once it started. After that, it’s pure physics opera: The nose of the car leaves the draft zone and enters the airstream, which accelerates the lift and pushes the nose backward while the weight of the rear-mounted engine continues its forward momentum. Voilà! A fantastic, white-knuckled twirl that—luckily—sustained enough momentum to end upright. Must have been an awesome ride. (Dalmas walked away uninjured.) —Martha Harbison

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The BBC’s clever automotive show Top Gear recently staged its own vehicular version of the Winter Olympics. The high point—pardon the pun—was when they launched a rocket-powered Mini off a ski jump. Despite the extra kick provided by the rockets, the Mini failed to match the distance of a real Olympic ski jumper. Why?

Once an object leaves the ground, we can forget about everything but four simple forces: (1) lift, which opposes (2) gravity, and (3) thrust, which opposes (4) drag. In an airplane, the engines produce enough thrust to overcome the drag created by the airframe punching a hole in the sky at hundreds of miles per hour, while the wings create enough lift to fight gravity and keep the plane aloft.

Our example is a bit simpler. A ski jumper lacks thrust, and, as we see in the video, even the Mini’s rockets are largely exhausted by the time it runs out of ramp. So we can ignore that component. Drag is important, but uninteresting, and ultimately less critical than the other two forces: lift and gravity.

Ask 100 scientists and engineers what causes lift, and most of them will probably give you some version of the nonsense the rest of us learned in school: high pressure below a wing, low pressure above. Wrong! This is a typical consequence of lift, but it’s not the cause. What creates lift, as deftly explained here by the folks who put the first “A” in NASA, is what they call “turning” the air. As air passes beneath a wing, the wing pushes that air down. By Newton’s third law (the one about every action having an equal and opposite reaction), the air must also push back up on the wing. This push is lift.

What does all this have to do with our Mini? Well, a stocky car on skis isn’t pushing air in any one direction, it’s just pushing it out of the way. That means it isn’t producing any lift. A ski jumper, on the other hand, positions her body and skis in a very precise way so as to maximize a net downward push of air. She pushes down so that the air might push back up.

But we must subtract from this push the persistent force of gravity. Fair enough. Fortunately for our jumper, the force of gravity is proportional to an object’s mass, and so the Earth pulls her down with a force less than a tenth the magnitude of the Mini’s. So our jumper’s net acceleration will be her lift (which is small but important) minus her gravity, while the Mini’s net acceleration will be its lift (which is zero) minus its gravity (which is an order of magnitude higher than the jumpers). Result: Even though the Mini might take off at a higher speed, it drops so much faster than the skier that their jump distances can’t compare. —Michael Moyer

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In this clip, we watch in open-mouthed wonder as 7-foot-6-inch leviathan Yao Ming becomes the property of 5-foot-9-inch Nate Robinson. Yao, whose defender had left him to guard the ball, receives a pass and leaves his feet for what should have been an easy one-foot jumper. But Robinson flies in from the weak side, takes a strong two-footed leap, and smacks the shot out of Yao’s hands (and back into his face) just as he shoots. Yao doubles over and brings his hands to his face, covering not only his injury but his deep sense of shame.

Before analyzing the physics of this maneuver, it’s tempting to assume the following things: Robinson, who gives up 21 inches to Yao, seems to be an immeasurably more talented athlete who plays with more energy and shows more heart. He certainly has a superior vertical leap (measure the height of Robinson’s shoes relative to Yao’s leg in this clip). But Robinson is not just 21 inches shorter than Yao. At 180 pounds, he’s 130 pounds lighter than Yao’s 310. Every time Robinson jumps, he’s moving less weight, and less weight takes less energy.

Just how much less energy? Let’s figure out how Yao’s and Robinson’s vertical leaps would compare if each expended the same amount of energy. The energy of a jump—and hence the work that must go into jumping—is proportional to both the jumper’s weight and how high he gets off the ground. Since we know that Yao weighs 58.1 percent more than Robinson does (180 divided by 310 equals 0.581), we can calculate that Yao’s vertical leap should be only 58.1 percent of Robinson’s.

Although updated numbers are hard to come by, Robinson’s vertical was measured to be 42 inches when he was drafted, and Yao’s as around two feet (a note to the viewer: two-foot vertical not on display in this video). Robinson can jump twice as high as Yao, so we can conclude that Yao would have to work twice as hard to reach the same height.

The lesson: Apply the same amount of energy to a smaller body and that body will jump higher every time. That, and Yao should dunk when he’s a foot away from the basket. —Michael Moyer

Related:

Flight of the Pole Dancer

Shake, Shake Chinook

Crane Overboard!

Goodbye, Moto

Stick That Landing

Ordinarily, driving is pretty straightforward: You just point the wheels and go. But piloting an aircraft is trickier, because you not only must deal with complexities like the potential for traffic above and below the plane, but your roadway—the air—moves. Until it’s time to land, of course. Seamlessly transitioning from sky to asphalt is the most difficult thing a pilot regularly has to execute, especially when winds are strong and blowing from side to side (as in the crosswind landing featured in this video). But it’s easy enough to understand what a pilot should do in such circumstances, even if you’re too freaked out to ever in a million years attempt to do it yourself. All you need are vectors.

A vector describes how something moves; picture it as an arrow. The vector’s length describes how fast the thing is moving, and its direction tells you which way it’s going. If you threw a baseball straight up in the air, the vector that described its movement would start out long—the ball’s going fast—and pointed toward the sky. Then the vector would shorten as the ball slowed and, at the top of its arc, would flip downward and grow long again as the ball fell.

If an object is moving in or on a medium that’s also moving—a person on a moving sidewalk, a swimmer in water, a plane in the sky—you figure out how the two will move together by taking the vector for the object and the vector for the medium and joining them together head-to-tail.

In our example, the wind is whipping from left to right, so its vector points that way. For the plane to move straight ahead, its vector must cancel out the left-to-right vector of the wind. That means it has to point a little to the left, or into the wind.

Of course, once the plane hits the ground, it had better be pointing in the direction it’s moving. That’s why the pilot has to straighten the plane out at the last second. If he did it any earlier, the wind would start to pull the plane to the right; if he did it any later, the plane would hit the tarmac sideways and flip over onto its wing. And you thought parallel parking hard. —Michael Moyer

Related:

Flight of the Pole Dancer

Shake, Shake Chinook

Crane Overboard!

Goodbye, Moto

Though A-Team reruns would have you believe otherwise, vehicles that crash in real life aren’t immediately and inexorably consumed by giant explosions. Any movie geek knows this. Gasoline doesn’t explode—it burns, just like wood—except in the uncommon environment of an internal combustion engine. Yet our unlucky racer’s motorcycle blows up with such vigor, you’d think Michael Bay placed the explosive charges there himself. So what gives?

The answer lies in the way the bike tumbles across the racetrack. Take a close look at how it flips before conflagration. The first time the bike bounces off the ground, the force seems to knock the cap off the gas tank. As the bike flips again, you can see racing fuel spray out of the top of the tank in great arcs, billowing through the air along with the dirt and gravel kicked up by the skid. This, as they say, is a bad sign.

Gasoline, like every other fuel, needs oxygen to burn. Ordinarily, if you were to set a match to a pool of gasoline, only its surface would burn, because only its surface would be in contact with the oxygen in air. But as it’s injected into your engine, the gasoline is atomized (imagine a tiny gasoline spritzer set on “mist”) in order to thoroughly mix the fuel with air before your spark plug ignites the combination. Since every bit of nearby fuel is now surrounded by oxygen, this flame spreads almost instantaneously through the combustion chamber until everything is alight.

But in the case of the motorcycle explosion, the bike’s acrobatics did the work of atomizing the gasoline. Once a spark ignited the little droplets, the whole thing went up in a bang. So a word to the wise: If you’re going to have a catastrophic accident in a motorcycle race, try to keep your gas cap on. —Michael Moyer

Related:

Flight of the Pole Dancer

Shake, Shake Chinook

Crane Overboard!

Physics has given us a great many simple principles that make it easier to understand what’s going on in the world, some better-known than others. To wit: Every action has an equal and opposite reaction; what goes up must come down—both classics, for good reason. And the blingiest of the axioms, E=mc², is particularly useful for understanding why a fistful of plutonium can cause such a big bang. Less famous but far more important on a day-to-day basis if you’re an SUV designer, a high jumper or—as in the present case—a crane operator, is the principle that any object will behave as if all its weight is concentrated at its center of mass.

Finding an object’s center of mass is fairly simple. It’s the point at which half the mass is above the center and half below, half is on the right and half on the left, and half is in front and half in back. If you stand straight up with your arms at your sides, your center of mass is a little below your bellybutton (unless you’re J. Lo). But here’s the important part: If your center of mass is not above your feet, you’re going to fall over. The same principle works for a crane. If the center of mass of the total system—crane plus whatever it’s carrying—moves to one side of the crane’s base, the crane will tip.

As our crane lifts the bus out of the water, trouble is a-brewin’. The water itself is holding up the partially submerged bus. (Remember Archimedes? No? Here: Water pushes up on an object with a force equal to the weight of the water being displaced—this is the reason things feel lighter in water.) As the bus leaves the river, the crane takes on more of its weight until the center of mass shifts so far away from the crane’s arm that suddenly there’s a tip, a splash and the call for a bigger crane. —Michael Moyer

Related:

Flight of the Pole Dancer

Shake, Shake Chinook

Everything has a beat. A rhythm. A frequency at which it likes to shake. You can rock most objects off-beat for as long and hard as you like, and not much will happen (see: the career of John Mayer). But start to push and pull in time with the natural frequency—the “resonant” frequency—of the object in question, and it will quite literally start to fall apart, much like the helicopter in the video below.

I always understood resonant frequencies best by thinking of the old-timey toy the paddleball. This uniquely solitary time-waster—Minesweeper for the Greatest Generation—consists of a bouncy red ball attached by elastic string to a small wooden paddle. Success comes when you hit the ball, the elastic pulls it back to the paddle, and you hit it again. And again and again and again. You quickly notice that there’s only one frequency that works, only one rhythm that prevents you from flailing wildly at the stupid little red ball. This is the paddle’s resonant frequency, and in this case, it’s a good thing.

Not so when dealing with bridges, skyscrapers or helicopters, however. Shake these at their resonant frequency, and the back-and-forth motion spells trouble. Each push adds more and more energy to the object—energy that, if not dissipated, starts to wreak havoc. That’s what happens with our Chinook. The rotating blades begin to shake the airframe at its resonant frequency, and physics takes care of the rest: Because the blades are unable to dissipate the excess energy, the convulsions rend them from the fuselage.

According to PopSci’s aviation expert, Bill Sweetman, helicopters are prone to resonant effects, which is why resonance ground testing (as seen in this video) is a standard part of chopper R&D. If both blades in a twin-rotor helicopter share the same heavy vibration and the engine mounts aren’t rock-solid, the energy generated can actually make the motors start moving around the engine mounts, and the next thing you know, that bird’s goose is cooked.

Sweetman also offered up this anecdotal tidbit: “Little-known fact: Charles Kaman, a U.S. heli designer who was also a bluegrass guitar player, combined his knowledge of acoustics and fiberglass (used in rotor blades) to create the Ovation guitar series.” Cue Patsy Cline’s “I Fall to Pieces”. . .  —Michael Moyer

Related:
The Breakdown: Flight of the Pole Dancer
The Full-Tilt Flying Machine

For a side view of the same test, click here.