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The complex physics of that viral water bottle trick, explained

The viral video of a high school student flipping a water bottle so that it lands straight upright on a table has set the internet ablaze.

With the pressure of his peers looking on, Michael Senatore, a senior at Ardrey Kell High School in Charlotte, North Carolina, flipped a third-full Deer Park water bottle through the air, landing it perfectly vertical on a table.

The theatrics, the focus, the precision, and the reaction of the audience help make this seemingly pedestrian feat extraordinary. But what really makes it special is the physics behind the now-famed flip — which are actually pretty complex.

"This is not your average undergraduate physics problem," Nathaniel Stern, an assistant professor of physics and astronomy at Northwestern University, said. "It's much more complicated."

Stern and I spoke by phone this morning about Senatore's feat. He explained to me that what separates a typical bottle toss from a perfect landing is just the right amount of water combined with a practiced and thought-through technique.

Angular momentum + fluid dynamics = AMAZING

"The flip," as it will henceforth be known, all begins with a flick of the wrist, generating a specific force called angular momentum, Stern said.

Angular momentum is very similar to the classical sense of momentum, where an object moving will not change its motion unless acted upon by another force. With angular momentum, the same concept is applied to rotation.

"If you just lob it, that's not good enough," Stern said. (At this point, I could hear a water bottle crashing against a table over the phone.) "He releases it from the cap, so that as he tosses it out of his hand the bottle already naturally wants to be spinning. If you toss it from the body, then you're not imparting that same force that causes it to spin."

Once Senatore gets the bottle spinning through the air, en route to its final destination, the water inside the bottle then needs to react to that angular momentum.

Liquids are unlike solids in that their molecules are able to move freely and independently from one another. That characteristic leads into another important concept, fluid dynamics — how the water moves inside the bottle.

When the bottle is released, it pushes on the water and starts trying to transfer its angular momentum to the liquid, according to Stern. But the water has more mass than the bottle — and it doesn't want to rotate because, as a fluid, it can slosh around unevenly the inside of the bottle. So as this angular momentum is transferred to the water, the rate of rotation slows down.

The water bottle will land squarely on the table when the angular rotation slows enough that the water is at the bottom of the bottle and the bottle itself is straight upright. Having that instance play out just before it hits the table takes careful timing and perhaps a little practice.

How to execute "the flip"

According to the Charlotte Observer, Senatore has put a lot of work into perfecting his craft. The 18-year-old said he practices when he gets bored at both school and work.

"All I wanted to do was flip a bottle," Senatore said humbly.

But little did he know he would inspire many to recreate "the flip."

Here are some tips to make it happen, according to Stern:

1) "I would find a water bottle and fill it to about a third."

This is vitally important. Stern says there is a "sweet spot" when it comes to filling up the water bottle. If there is too much water, spinning the bottle will be like spinning any solid object, because there is no room for the water to move around. Too little water means there will not be enough mass to slow down the angular momentum and make the bottle land consistently. Spinning the bottle just a third full allows it to spin easier and allows the water to adjust easier to the angular momentum because it can move around.

2) "Make sure every time you throw, you throw it the same way, so you understand that if you want to change it a little bit, you know what you need to change."

"Consistency is key," Stern said. This is a human trick after all, so while a machine may be able to accomplish "the flip" time after time, variations in how humans toss the bottle will greatly impact the result. Tossing the bottle too fast or slightly off axis could result in a fallen water bottle. Staying consistent allows tossers to pinpoint small, specific adjustments.

3) "You have to impart spin."

Without the spin, the trick is nothing. The spin is essential for landing the bottle upright.

With those keys in mind, give "the flip" a shot. Just remember to make sure the cap is closed tightly.

Watch Vox try "the flip", live on Facebook

Flipping a water bottle, with physics.

Gravity, angular momentum and fluid dynamics all play a critical role in flipping a bottle of water to land upright.

Posted by Vox on Thursday, May 26, 2016

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