Oh, Flip!

Yanhao strides into the classroom, wielding a half-full (optimism ftw) plastic bottle of water by the neck. Positioning himself at the front of the room, he takes up a stance of utter concentration and determination, eyes directly pointedly at a spot on the front desk. With a single intake of breath, the room falls into silence as it anticipates the magnitude of the feat it is about to witness. The teacher clasps cupped hands over their mouth, barely stifling a shocked gasp. As Yanhao shuts his eyes, one singular bead of sweat trickles down the length of his neck. In one fell swoop of his arm, he chucks the bottle up into the air. It pirouettes, landing elegantly on its base with barely a wobble. The silence explodes into madness.

Of course, this was only attempt #1003.

Bottle flipping has become an internet sensation in recent months, following an outstanding performance at a high school talent show. Wannabe viral videos from all over the globe have been attempting to recreate the stunt with increased levels of difficulty and complexity. The popularity of this particular activity, appropriately coined “Water bottle flipping” has even been given its own Wikipedia article. Last week, myself and Yanhao decided to take on the challenge ourselves. It turns out that it’s not as hard as you’d might expect to achieve… especially when you know the science behind it.

If I kick a football (can’t remember the last time I did that), the football which I have put into motion will tend to stay in motion at a constant velocity, if we ignore all the finickity resistive forces. This is because the momentum of the ball is conserved. The only way for the ball to stop moving would be for it to transfer its momentum to another object, say, a goal. One – nil. This is essentially what force is, the transfer of momentum.

Now, imagine if I try to kick the ball but just skim the edge, setting it spinning around a single point (not hard to imagine, I know). Just as with our linear example above, this ball will keep spinning on the same axis at the same velocity as long as we don’t disturb it. This is because the angular momentum is conserved.

When Yanhao flicks the bottle, he imparts some angular momentum into the bottle, which makes it want to spin around the point where he swung it. The water inside the bottle begins by performing a similar sort of circular motion, but since it is free to flow inside the bottle, it sloshes around having been acted on by gravitational attraction of the Earth. The angular momentum of the light bottle transfers to the heavy water, which slows down the spinning of the bottle, until it’s practically unspinning by the point it is upright in mid-air. The water then proceeds to fall back to the bottom of the bottle, causing the bottle to settle on a surface below.

Cheeky tip: a bottle filled about a third of the way up is more likely to land successfully. This is due to the centre of mass of the water being at a point where it won’t cause the bottle to be unstable if it tips too much from side to side. There’s also not too little water to anchor the bottle down when it lands.

The science makes it sound so simple… On the flip side (ah?) – I actually had nothing to say I just really wanted to get that pun in. Sorry.

Harvey