The Physics of Bottle Flipping

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Summary

Key Concepts

Angular momentum, mass, gravity

Credits

Ben Finio, PhD, Science Buddies

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, you’ll be able to explain the science behind it!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

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, and 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 both 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 link in the Explore More section).

What does that have to do with bottle flipping? Imagine throwing a rigid object, like a coin. Gravity will pull the coin back down to the ground. Since the object is solid, the distribution of its mass does not change as it flies and spins through the air. Its moment of inertia and angular velocity stay the same. That makes it very difficult to predict whether the coin will land heads or tails, since it keeps spinning as it falls. However, a water bottle is different. 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, therefore its angular velocity (since 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 project 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 (e.g. how high you throw the bottle, how far you throw it horizontally, and how fast you spin it) throughout the experiment.
Fill a plastic water bottle about 1/4 to 1/3 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?

Instructions

Once you have practiced your bottle-flipping method, try it ten 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 ten times with an empty bottle. Can you still get the bottle to land upright?
Now try it ten times with a completely full bottle. Can you still get the bottle to land upright?
Experiment to see if you can find the optimal amount of water in the bottle. What if the bottle is 1/2 or 3/4 full? 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: experiment with the distance and height you throw the bottle, and 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 on the floor? What if you try to get it to complete two flips instead of one?

Extra: try landing the bottle on different surfaces like carpet, wood floors, tile, etc. Is it easier to land the bottle upright on some surfaces than others?

Extra: try the experiment with different size or shape bottles. Do some work better than others? Do you have a “favorite” type of bottle?

Observations and Results

While results may vary slightly depending on an individual’s technique, you probably found that you had the most success with a bottle roughly 1/4 to 1/3 full of water. It was very difficult, almost impossible, to successfully flip an empty bottle or a completely full bottle.

The explanation for this phenomenon depends on angular momentum. Remember that angular momentum, which depends on moment of inertia and angular velocity, must be conserved when no outside torque acts on an object. When a water bottle is spinning through the air, no torque is exerted on it (neglecting air resistance). Also remember that the moment of inertia of a rigid object, like an empty water bottle, does not change as it spins. Therefore, the empty bottle’s angular velocity 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 the same.

All that changes when you use a partially filled bottle of water. Initially, the water’s mass is all 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 length of the bottle, 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 the More to Explore section for some excellent diagrams.

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