How Far Do You Sink in a Pool of Orbeez?

TL;DR

A person sinks about 85% into a pool filled with 25 million Orbeez because buoyancy depends on displacing an amount of material equal to the person's weight. The water balls are slightly denser than a person, and their spherical packing with surrounding water matters more than the friction between them.

Transcript

(whimsical music) - A while back I was eating lunch with some coworkers, and we were debating about just how far you would sink if you jumped into a pool of these water balls. For example, would it go up to your calf or to like your waist or all the way above your head? Everyone seemed to have a different theory. In fact, I asked about a hundred pe... Read More

Key Insights

• The experiment used 25 million Orbeez to determine how deeply a person would sink in a pool of water balls. Predictions gathered from about a hundred people varied from calf depth to complete submersion, showing that intuition produced no clear consensus.
• The observed result was that about 85% of the experimenter's body sank into the Orbeez. This was substantially deeper than his prediction of halfway, so he rejected his original hypothesis and investigated why friction had mattered less than expected.
• Buoyancy works by allowing an object to sink until it has displaced its own weight in the surrounding material. Once that amount has been displaced, the object reaches equilibrium and floats rather than continuing downward without limit.
• A 135-gram baseball sinks until it pushes aside 135 grams of water. Because its waterline settles about halfway up the ball, the demonstration indicates that the baseball is about half as dense as the water surrounding it.
• Melting floating ice does not change the water level in the demonstrated setup. Before melting, the ice already displaces water equal to its weight, so turning that ice into water leaves the level unchanged.
• Quicksand would make a person float after partial submersion because the person stops sinking after displacing their own weight. The transcript describes quicksand as about twice as dense as a person, producing a position roughly halfway down rather than total sinking.
• The Orbeez result depends on their density, their spherical packing efficiency, and the water surrounding them. Since the water balls are slightly denser than the experimenter, sinking to roughly 85% of his body is consistent with the buoyancy explanation.
• Unexpected experimental results can be more informative than correct predictions because they reveal which assumptions were wrong. In this case, the mistaken halfway estimate exposed an overestimation of friction between the balls and emphasized the stronger role of buoyancy.

Questions & Answers

Q: How far does a person sink in a pool of Orbeez?

A person in the demonstrated pool sank until about 85% of his body was submerged in the Orbeez. He had predicted that he would sink only halfway, but the actual result was much deeper. The explanation accounts for the water balls being slightly denser than him, their spherical packing efficiency, and the water surrounding the packed balls.

Q: Why does a person float instead of sinking completely in Orbeez?

A person stops sinking after displacing an amount of Orbeez equal to their own weight. This is the buoyancy principle presented in the experiment. Because the water balls are slightly denser than the person, equilibrium occurs with most of the body submerged, around 85% in the observed test, rather than with the person sinking completely beneath the surface.

Q: How does Archimedes' principle explain the Orbeez experiment?

Archimedes' principle explains that an object sinks until it has displaced its own weight in the surrounding material. In the Orbeez pool, the experimenter continued downward until the displaced water balls and surrounding water balanced his weight. Their density and packing allowed equilibrium only after roughly 85% of his body had entered the material.

Q: Why was the halfway-sinking prediction wrong?

The halfway prediction was wrong because it gave too much importance to friction between the individual water balls. The experiment showed that buoyancy, the slight density difference between the Orbeez and the person, and the packing efficiency of spheres surrounded by water better explained the result. These factors allowed about 85% of the body to sink.

Q: What does the baseball demonstration show about buoyancy?

The baseball demonstration shows that a floating object displaces an amount of water equal to its own weight. The baseball weighs about 135 grams, so it sinks until it has pushed aside 135 grams of water. At that point it reaches equilibrium. Its waterline sits about halfway up the ball, indicating that it is about half as dense as water.

Q: Does the water level change when floating ice melts?

The water level does not change when the floating ice melts in the demonstrated setup. While floating, the ice already displaces water equal to its own weight. After it melts, that same material becomes water, so the quantity added matches the displacement that previously supported the ice. The demonstration uses this result to introduce the principle governing flotation.

Q: Can a person sink completely in quicksand according to the explanation?

The explanation says a person would not sink completely in quicksand because sinking stops after the person has displaced their own weight. The transcript describes quicksand as about twice as dense as a person, so the person would bob roughly halfway down like a cork. This is presented as a correction to fictional scenes showing complete submersion.

Q: What factors determine sinking depth in a pool of water balls?

Sinking depth depends on reaching equilibrium after displacing material equal to the person's weight. For the Orbeez pool, the important factors are the water balls being slightly denser than the person, the packing efficiency created by their spherical shape, and the water surrounding them. Friction exists, but it played a smaller role than the experimenter originally expected.

Summary & Key Takeaways

• After coworkers offered widely different predictions about sinking in water balls, the experimenters spent six months stockpiling enough to fill a pool. The initial prediction was that a person would sink about halfway, but the test produced a much deeper result, with roughly 85% of the body submerged in the Orbeez.

• The result is explained using Archimedes' principle: an object sinks until it displaces its own weight in the surrounding material. A 135-gram baseball demonstrates the idea by sinking until it pushes aside 135 grams of water, then reaching equilibrium with its waterline about halfway up its body.

• The Orbeez are slightly denser than the person entering the pool, while their spherical packing and the water around them affect the pool's overall behavior. Friction between the balls contributed less than expected. The unexpected result showed why testing a hypothesis can be more informative than relying on intuition alone.