字幕表 動画を再生する 英語字幕をプリント I see a 10 meter drop into a pool and think I’m going to die. Divers see a 10 meter drop and think, “How can I bend physics to make this awesome?” Greetings mathletes, HOST here for DNews. The summer games are upon us! The best the world has to offer have gathered to play just about every sport under the sun. And while each sport has its own unique appeal, from a physics perspective there may be none more interesting than diving. Divers are spinning, flipping, twisting water ninjas that have bent the laws of physics to their whim with nary a splash. How do they do it? Let’s take it from the top. Be it from a springboard 3 meters up or a solid platform a dizzying 10 meters up, the initial leap is all-important. First off when the diver jumps they have to make sure they’re not going to end up where they started; on a concrete platform 3 stories in the air and decidedly dry. Failing to clear the platform or board is a big no-no, so the diver has to give themselves some forward velocity, meaning they will travel in an arc until they hit the water. The diver also wants to get as high as possible, because more time in the air means more tricks they can pull off. The final dimension determined at takeoff is the diver’s rotation. If they’re going to spin and flip, they need to start that from the jump. But they can increase the speed they’re spinning as they fall by drawing their body into a compact ball. This tuck position reduces what’s called their “moment of inertia,” the body's tendency to resist angular acceleration. When the moment of inertia goes down, their rotational speed goes up to conserve their rotational momentum, more commonly known as angular momentum. The result is divers can squeeze four and a half flips into a second and a half of fall time. The other commonly used position for increasing spin speed is called the pike, but unlike the tuck the legs are kept straight, meaning the diver has less control over rotational speed, and the dives have a higher difficulty. Of course all the flips and spins in Rio won’t get a diver a good score if they finish with a belly flop. Diver’s have to spot the water to know when they’ll splash down, no easy feat when they’re twisting and rotating and traveling at 51 kilometers per hour after falling 10 meters. When it’s time to get wet, the diver’s goal is to enter the water with as little splash as possible. If they do it right, they’ll part the water with a sound like ripping paper, known as a rip entry. To achieve this the diver has to punch a hole in the water and enter it as straight as possible. Remember though that because angular momentum is conserved, they’re still spinning when they straighten out, just more slowly. Their body still rotates underwater, but masterful divers will hit the surface in such a way that they look like they’re going straight in. If they can pull off the rip entry after flipping out more than a teenager’s mom, they stand a chance of making a splash at the summer games. Concepts from physics are hiding everywhere in sports. To learn how they’re crucial to throwing a wicked curveball, check out Julian’s video here. What’s your favorite physics-al activity?