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
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.
