For his 50th birthday next month, Will Smith will bungee-jump from a helicopter over the Grand Canyon. It’s a dramatic stunt that strikes fear into the hearts of fans (or at least some Verge staffers; you know who you are) who wonder whether it’s possible he’ll spring directly up into the helicopter’s blades.
That’s not going to happen. Will Smith isn’t the first one to bungee-jump from a helicopter, and no one — not Smith, not the non-famous adrenaline junkies who enjoy bungee jumping — would do it if it meant being shredded. But why is it safe?
Will Smith is protected by one of the most fundamental laws of physics, called the conservation of energy, explains Chad Orzel, a Union College physicist and writer. The conservation of energy means that energy cannot be created or destroyed, just transformed. Will Smith has a certain amount of potential energy from being in a high place. As he falls to a lower place, as the cord stretches, and he goes up and down, the energy will change into different forms, and that’s what’ll make him bounce. But you can’t end up with more energy than you began with, so you can’t end up higher than the point where you jumped. “There’s no way for him to raise his total energy as he’s falling, which is what would have to happen for him to go up and be in danger from helicopter blades,” Orzel says.
The sample principle is often used in introductory physics demonstrations, like in the video below. One man holds a ball, suspended from the ceiling at an angle, right by his chin and then lets it go. If he releases the ball from a certain height, it can never go to a higher height than it began (unless he gives it extra force by pushing it, instead of merely releasing it from an angle).
In fact, it’s very unlikely that Will Smith will even get back to the original height he jumped from, let alone even higher. You almost always lose energy due to sources like air resistance and friction.
However, there is one interesting complication to bungee-jumping off a helicopter, as opposed to a fixed point like a bridge. “You could imagine raising the helicopter up higher, which would increase the stretch into the cord and something weird might happen,” says Orzel. “That would be a really strange thing to do and not a terribly good idea.” Don’t do it, Will.
Update 8/15/18 5:45 p.n.: The original version of this post included a physics video by Walter Lewin, the MIT physics professor who was stripped of his emeritus title after numerous sexual harassment charges. We have replaced the video.