Experiencing turbulence on a flight is worrying enough, so it certainly doesn’t help to look outside your window and see the plane’s wing bouncing up and down like it’s made of plastic. After observing such oscillation on a recent flight, one WIRED writer decided to dig deeper into the physics behind the phenomenon.

By analyzing a video he shot using his iPhone, he was able to determine that the wing of the Boeing 737 he was aboard reached an oscillation amplitude of 10 centimeters (nearly 4 inches). The amount of time it took for the wing to move from one minimum position to the next was about 0.3 seconds.

While all that wobbliness may seem like cause for panic, the flexibility of a plane’s wings is actually a sign of safety. The Federal Aviation Administration requires that all airplanes are able to withstand 150 percent of the maximum expected load for 4 seconds. According to CBS MoneyWatch, that means a plane’s wings can survive turbulence 50 percent stronger than the worst that’s ever been encountered before breaking. In order to absorb all that force, the wings are built like giant springs. If they were rigid and unyielding, it would take a lot less wind power for them to snap off—not something you want happening at 30,000 feet.

As for why the wings respond to turbulence by bouncing up and down, it’s simply a matter of physics. If an aircraft is flying at a constant speed and altitude, the net force pushing it up and down would amount to zero. If the plane moves into an area with higher air density (or experiences a similar atmospheric change), this results in more lift than there was before. This causes the plane to temporarily accelerate upward, and the wings to bend up farther. When the plane moves back to a place with lower air density the lift is reduced, causing the wings to bend back down. Sudden changes in lift force, which is what goes on during periods of turbulence, are what bring about the oscillation.

So next time you see your plane’s wing wobbling during a bumpy flight, remember that it’s just a product of basic physics. And if that doesn’t do much to comfort you, maybe try shutting the window shade.

[h/t: WIRED]