Buoyancy Brainteasers: Boat-in-Pool Puzzler

Resource for Grades 3-8

WGBH: Nova
Buoyancy Brainteasers: Boat-in-Pool Puzzler

Media Type:
Interactive

Size: 80.0 KB


Source: NOVA: "Voyage of Doom"

This resource can be found on the NOVA: “Voyage of Doom" Web site.

Resource Produced by:

WGBH Educational Foundation

Collection Developed by:

WGBH Educational Foundation

Collection Credits

Collection Funded by:

National Science Foundation

You're sitting in a boat in a pool of water. With you in the boat is a large rock. You toss the rock overboard. Will the pool's water level rise, fall, or stay the same? Put your understanding of the factors that influence buoyancy to the test in this interactive brainteaser from the NOVA Web site.

open Background Essay

The Greek mathematician Archimedes first noted that any object immersed or partly immersed in a fluid (i.e., a liquid or a gas) is buoyed upward by a force equal to the weight of the fluid displaced by the object. The crucial factor in determining an object's buoyancy is its density, the relationship between its weight, which is pulling it down, and its volume. If an object placed in water weighs more than an equal volume of water -- in other words, if the density of the object is greater than the density of water -- then the downward force of gravity will be greater than the upward buoyant force, and the object will sink. On the other hand, if the object weighs less than the fluid it would displace if submerged, it will sink only to the point where it displaces the amount of fluid equal to its weight. At that point, the upward force of the fluid equals the downward force of gravity, and the object floats.

In this featured brainteaser, when the 10-pound rock is in the boat, it is floating, so, in addition to what the boat would displace without the rock, it also displaces the amount of water with a weight equal to the rock. When the stone is hurled in the air -- so that it is out of the boat but not yet in the water -- the boat is relieved of the rock's weight and displaces less water. Consequently, the water level in the pool drops. When the rock enters the water, it displaces a volume of water equal to its size. Because the same volume of water weighs less (perhaps only 3 pounds) than the same volume of rock (10 pounds), the denser rock sinks to the bottom. The water level rises again, but not as high as it was when the rock was in the boat because it is now displacing less water. The net effect of throwing the rock in the pool, then, is a lowering of the water level.

open Discussion Questions

  • Before you select an answer, do you think the pool's water level will rise, lower, or stay the same when the rock is tossed into the pool? Why?
  • Explain why the pool's water level went down when the rock was thrown into the pool.
  • When an oil tanker is emptied of its oil, what do you think happens to the level of the tanker in the water? What do you think happens to the water level?

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