Key concepts
Physics
Mass
Gravity
Angular momentum

Introduction
The bottle-flipping craze might be dying down, but it isn’t too late to investigate the physics of this internet sensation. Even if you’ve never heard of it, give this project a try—not only can you impress your friends with a fun new trick, but you’ll also be able to explain the science behind it!

Background
“Bottle flipping” took the internet by storm in 2016. If you haven’t seen it yet, check out the links in the “More to explore” section below or search for it on YouTube or your favorite social network; you’re bound to find a few videos. The process involves flipping a partially filled water bottle into the air so it lands upright. This might seem like a very simple concept but the physics behind it are actually quite complex—and it takes some practice to master the feat!

To understand the physics of bottle-flipping, first you need to understand angular momentum. An object's angular momentum depends on its angular velocity (how fast it is spinning) and its moment of inertia (how much its mass is spread out from a central point). When no external torque acts on an object, its angular momentum must be conserved. The classic example of this is a spinning ice skater. If she is first spinning with her arms extended, she has a high moment of inertia (her mass is spread out, away from the center of her body). If she pulls her arms in tightly, her moment of inertia decreases. In order for her angular momentum to stay the same, her angular velocity must increase so she spins faster. You can observe this for yourself in a spinning office chair (see the link in More to explore).

What does that have to do with bottle-flipping? Imagine throwing a rigid object, such as a coin. Gravity will pull the coin back down to the ground. Because the object is solid, the distribution of its mass does not change as it flies and spins through the air, and its moment of inertia and angular velocity remain the same. That makes it very difficult to predict whether the coin will land heads or tails because it keeps spinning as it falls. A water bottle is different, however. It contains liquid water, which is free to slosh around inside the bottle changing the distribution of mass. Just like an ice skater spreading out or pulling in her arms, this changes the bottle's moment of inertia and therefore its angular velocity (because the total angular momentum must stay the same). You can exploit this fact to make it easier to successfully flip a bottle. How? Try this activity to find out!

Materials

• Plastic water bottle
• Tap water

Preparation

• If you have never tried bottle-flipping before, you should practice before you start this project. You want your technique to remain consistent (for example, how high you throw the bottle, how far you throw it horizontally and how fast you spin it) throughout the activity.
• Fill a plastic water bottle about one quarter to one third full with water and put the cap on tightly.
• Hold the bottle loosely by the neck, and toss it forward (so the bottom rotates away from you).
• Try to throw the bottle so that it does one complete flip and lands upright without falling over. This can take a lot of practice!
• If you get frustrated, at least try to observe which side the bottle initially lands on (top, bottom or side), even if it falls over after that. Can you get the bottle to consistently land on its bottom?

Procedure

• Once you have practiced your bottle-flipping method, try it 10 times in a row. Remember to keep your technique as consistent as possible. How many times can you get the bottle to land upright?
• Now try it 10 times with an empty bottle. Can you still get the bottle to land upright?
• Now try it 10 times with a completely full bottle. Can you still get the bottle to land upright?
• Try to see if you can find the optimal amount of water in the bottle. What if the bottle is one half or three quarters? What amount of water gives you the best success rate?
• Extra: Put some water bottles filled with different amounts of water in the freezer overnight (make sure they are sitting upright). Try flipping them the next day. Is it easier or harder to successfully flip bottles with ice instead of liquid water inside them?
• Extra: Try throwing the bottle different distances and heights—and vary how much you spin it. Is it easier to get the bottle to land upright if you throw it across the room or so it lands just in front of you? What if you try to land it on a table instead of the floor? What if you try to get it to complete two flips instead of one?
• Extra: Try landing the bottle on different surfaces, such as carpet, wood floors, tile, etcetera. Is it easier to land the bottle upright on some surfaces than others?
• Extra: Try the activity with different size or shape bottles. Do some work better than others? Do you have a favorite type of bottle?

Observations and results
Although results may vary slightly depending on an individual's technique, you probably found you had the most success with a bottle roughly one quarter to one third full of water. It was very difficult (maybe almost impossible) to successfully flip either an empty or completely full bottle.

The explanation for this phenomenon depends on angular momentum, which you’ll remember must be conserved when no outside torque acts on an object, and depends on moment of inertia and angular velocity. When a water bottle is spinning through the air, no torque is exerted on it (neglecting air resistance). Also remember the moment of inertia of a rigid object, such as an empty water bottle, does not change as it spins. The empty bottle’s angular velocity, therefore, stays the same as it flies through the air just like a spinning coin. That makes it very hard to control the bottle's descent and difficult to get it to land upright. The same also applies to the completely full bottle. Even though it is full of liquid water, there is no room for the water to slosh around, so the distribution of mass within the bottle remains the same, and its angular velocity stays constant.

All that changes when you use a partially filled bottle of water. Initially the water’s mass is concentrated at the bottom of the bottle. When you toss the bottle, there is room for the water to slosh around. It spreads out along the bottle’s length, increasing the moment of inertia and decreasing the angular velocity (conserving angular momentum). The bottle’s spinning slows down as it flies through the air—making it possible, if timed properly, to get the bottle to land upright. If that process is hard to visualize, see the references in More to explore for some excellent diagrams.

If you try the same trick with ice, even though the bottle is filled the same amount, it doesn't work because the solid ice cannot slosh around.

Cleanup
Don't forget to recycle your bottle when you are done with it!

More to explore
The Complex Physics of That Viral Water Bottle Trick, Explained, from Vox
The Water Bottle Flip, from Institute of Physics
Water Bottle Flipping Physics (pdf), University of Twente, from Cornell University Library arXiv
Swiveling Science: Conserving Momentum in a Spinning Chair, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies