Physics of the Bottle Flip Challenge

One of the biggest trends of 2016 was the water bottle flip challenge, and almost everyone was trying their hand at it. But, there is much more to it than just skill and luck, and-even though the trend seriously died out-I’ll be teaching you a few cheats that are going make you flip! Or at the very least give you the ability to become the mic-dropping finale at your school’s talent show. But in all seriousness, of all the trends to hit big in 2016, bottle flipping was perhaps the most surprising. Sure, we expect the collective mind of the internet to dredge up long forgotten childhood shows, ironically appreciate terrible movies, or recreate popular dance moves; but flipping bottles of water so that they can land correctly? It’s something almost anyone can do, watch, and appreciate. You see it happen and you understand it regardless of how old you are or what language you speak. But, for those of you who many not be familiar, here is a quick summary. Bottle flipping entails taking a bottle and tossing it so that it twirls through the air like a gymnast, and sticks a 10/10 landing. Now, despite what Know Your Meme-a website that dedicates itself to documenting online phenomena and memes-might say, the origins go back as far as 2007 to a video released by a skateboarder named Ben Daleman. However, it was Michael Senatore’s epic flip during his school talent show last year that really caused people to flip out over the flip. As elegantly simple as bottle flipping might be, scientifically, it’s a completely different story. Involving everything from elasticity of collisions, to fluid dynamics, to momentum, to laws of gravity, the science behind the bottle flip is quite amazing. But, once you understand that science, you’ll actually have an unfair advantage in your next bottle flipping tournament, at your next school talent show, your next visit to grandma’s house, or whenever you might just want to impress someone.
Now, without further ado: The Scientific Cheat Code That’ll Get You the Upper Hand in the World of Bottle Ballistics. Before you start pitching your old plastic around, your flip can either be made or lost by the very first choice that you make-the bottle. There are hundreds, if not thousands of different brands of bottled water on the market. As well, almost all of them have different bottle shapes, heights, even thicknesses of plastic, and picking the right one can make the difference between becoming a flipping champ, or a flipping chump.
So if you think of bottle flipping as a video game, then think of your choice of bottle as the difficulty level. The typical bottle most people are using rounded bottle, thin plastic, like Poland Springs, those are effectively the normal setting. Not too hard, not to easy, just a good sweet spot to judge other bottles. But what in particular makes this one the ideal, middle candidate? Well, let’s take two other bottles and it’ll all become clear. A water cooler jug, and a bottle of Coca-Cola. Now, the Coca-Cola bottle is the bottle flipping equivalent of hard mode, and the reason for that is surface area. Notice that the bottom isn’t round like the other two, but rather has multiple prongs that give the bottle added stability. Now, under normal usage, that’s a great shape, but when flipping bottles, it is a completely different story.    
You see, in physics, the energy of being in motion is called kinetic energy, and the word elasticity is a measure of how much kinetic energy remains as kinetic energy after two objects combine. To give you an example, let’s look at anime. Goku punches Krillin in the face. Goku’s fist has a bunch of kinetic, or movement energy. That then is transferred into Krillin’s face and body launching him off for miles. So, one would say that the elasticity of that collision of fist to face is really high. The kinetic energy of Goku’s fist stays as movement energy as Krillin flies through the air. So that’s an elastic collision. Alternatively, there are inelastic collisions, where the kinetic energy is transformed into deforming the material, or where energy is lost in other ways, like heat and sound. The Goku punch example isn’t perfectly elastic, because some of his punch energy is lost deforming Krillin’s face. A good example of an inelastic collision is two balls of clay thrown together. They stick, there is no bounce, all the kinetic energy of the two balls moving is lost as they deform into one larger ball. Now, knowing all that, look at our bottle flip. It’s plastic hitting a table, so the collision would be mostly elastic, where the kinetic energy of the bottle falling is preserved. But, the table isn’t going to move so that movement energy has to go somewhere, and that’s back into the bottle-which is causing it to bounce. The plastic of the bottle compresses ever so slightly, and then springs back into position, causing it to rebound from the table and potentially costing you your glorious moment at the talent show. Yet, as that may be, the force isn’t going to cause a ton of bounce when it’s spread out over a wide surface such as the rounded bottom of your average water bottle. But what about the prongs of the Coca-Cola bottle? That force is moved to the very small area that’s actually hitting the table, causing the bottle to rebound higher and thus making it harder to get that perfect landing. Whereas on the other end of the spectrum are the large jugs of water-which are the equivalent of flipping easy mode. Part of it is the huge surface area of the base, but to see why these are the clear choice for any inexperienced flippers we need to talk about the other elephant in the room-finding the perfect water level.
Empty water bottles weigh practically nothing, averaging in at 10 grams, and if you have an empty water bottle just sitting on a table, its center of mass-the point on the bottle where all the mass averages out-is right around the middle. But water is a lot heavier than plastic, and when you put something like water into a bottle, the center of mass is going to move. In this case it moves downward. There’s more mass on the bottom of the bottle than on the top, and thus the place where the mass averages out is going to end up a lower. Now, why is that important? Because the center of mass sometimes goes by another name, the center of gravity, and gravity-as the largest obstacle between you and that perfect bottle flip-pulls from the center of gravity. Therefore, the lower the center of gravity is, the less likely that bottle is going to tip over when you flip it. Thus, allowing you to walk off the high school stage head held high. But if you overfill the bottle with water now the center of gravity is much higher, the point at which all the mass has averaged out has crept back upwards. When that happens and it’s tipped to the left, the center of mass is now to the left of the tipping point, and gravity will pull the bottle down, causing it to fall, and causing you to take the walk of shame, or move to another school district. This is why getting just the right amount of water is tricky and essential. You need to put in enough water in the bottle so that the center of mass goes about as far down as it can get without inadvertently allowing that center of gravity to creep back upwards by overfilling. So mathematically the ideal sweet spot is going to be filling up that bottle to the one-third mark. It’s there that the center of mass is going to be about as low as it can go. Those wide, short squat, water cooler bottles are the easiest ones to flip because they’re the ones where keeping the center of gravity low is a cinch. It doesn’t take a lot of water to weigh them down-getting their center of mass low-and all in all these jugs are just super stable. Now obviously you’re not going to shove a water cooler jug in your backpack to pull out during lunch hour bottle flipping tournaments. So what bottle do you choose? Well, in between these jugs and our standard round-bottomed ones is everything from Smart Water-being tall and narrow-to Fiji water-being made from a firmer plastic and with a wider base, and shorter height. The fact that Fiji water is short means that you’re going to have to be a lot more precise in your water level measurements making it much harder to land the flip; therefore, making Smart Water a little more incentivizing.
Lastly, and arguably the most important part of each flip: the throw. Now, before we talk about throwing bottles, let’s talk hammers. Grab a hammer and toss it in the air! Eh, actually, no. It’s dangerous and you’ll put a hole in the floor. Instead, let’s visit history’s most famous hammer tossers-The Hammer Bros. Notice how the hammer twirls all lopsided? Well, it’s because of our old friend-the center of mass. Objects in free fall that have any rotation will always rotate around their center of mass. Hammers, which traditionally have wooden handles and steel heads, have a hugely lopsided center of mass and rotate around that. But the cool part is, for as chaotic as that movement looks, if you trace the trajectory of the center of mass it will always follow a perfect arc. But here’s the thing, for as complicated as throwing a hammer may look, a water bottle is even more complicated because of fluid dynamics. When you flip the bottle into the air, the center of mass is shifting as the mass of the water moves around in the bottle. As it does this, the water robs the bottle of angular momentum-or spin. This becomes most dramatic just after the bottle flips, when the water sloshes around and goes from being in the bottom of the bottle to the top. It adjusts to gravity, which simultaneously helps push the bottle down, allowing for that signature straight-up landing. How this knowledge helps you is that most newbie bottle flippers include too much horizontal velocity. You see, if the bottle is moving forward too fast when it lands, the bottle’s movement is suddenly stopped, but the water inside the bottle still wants to move forward. And it does, which takes all the mass with it. So instead of acting like a stabilizing force inside the bottle, the water actually helps knock the bottle over. The key here is to try to use as much force upward while also putting that spin on the bottle. The best way to do this is to make sure the bottle is parallel with the ground when you release it. This sends the center of mass straight upward, and since you’re holding onto the other end, the bottle with naturally rotates as you let go.
So, there you have it, the steps to a prefect flip. Choose a bottle with a firmer plastic and a wide base like a Fiji bottle, fill that bottle to about a third full, double-checking the exact level by doing that 45 degree fall test to measure where the center of gravity is at, and then practice your toss so that you release it in parallel with the ground, and try to minimize its forward momentum. But if you want even more of an edge, consider swapping out the water in the bottle for Mercury. As the heaviest liquid at room temperature, that should lower the center of mass to amazing depths while still maintaining all the benefits of fluid dynamics. Just, fair warnings, don’t drink it, or touch it, or do anything with it really because it’s toxic. Between Mercury and hammer tossing, explaining the science behind bottle flipping can be filled with bad ideas.