Density and Buoyancy: How Do Whales Sink So Deep?

Interactive, scaffolded model

Overview and Learning Objectives

Part A: The students observe several plastic soda bottles filled with various substances and discuss the varying density of those substances. After being given the numerical density of each substance, the students observe while some are placed in an aquarium tank filled with water. After the first observations, students are asked to predict whether the next couple of bottles will sink or float.

Part B: Students use the computer to measure the density of various combinations of atoms with varying mass and varying space between the atoms.

Students will be able to:

• describe how changing the mass or volume of a fixed number of atoms will affect the density of those atoms;
• predict if something of known density will float in another substance of known density.

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Classroom Practice

This activity is one of a larger module: Atoms in Motion (http://www.concord.org/~barbara/workbench_web/unit1/index.html)

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Central Concepts

Key Concept:

Density is the ratio of mass to volume, how much mass in each unit of volume.

Additional Related Concepts

Physics/Chemistry

• Buoyancy
• Density

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Macro Micro Link

The key factor in why a hot air balloon floats has to do with the density of the air inside as compared to the density of air outside the balloon. Less dense substances float in more dense substances. The bigger the difference in density and the larger the volume of the low density substance, the greater the buoyant (upward) forces will be. Heating the air causes the molecules to spread further apart, lowering the density of the gas inside the balloon.

Other macro connections:

• We can explain, or at least predict why certain things float or sink.
• Boats made of very dense metal float because the air sealed inside becomes part of their volume in the measurement of density.
• Whales can dive or surface without swimming by changing the density of a large organ in their head. When the density is high they sink, when low, they float, allowing them to conserve energy and not have to swim to change depths.

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Activity Credits

Created by CC Project: Molecular Workbench using Molecular Workbench + Pedagogica

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Requirements

• Adobe Flash Plugin

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