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Tufts OpenCourseware
Author: Fulcrum Institute Development Team

Snapshot: A Buoyancy Challenge


Mr H’s Goal: Students will understand that the density of a liquid determines the buoyancy of an object (whether it sinks or, if it floats, how high it will float).

The Snapshot: In Room 14, students are investigating floating and sinking in fresh and salty water. They have investigated objects made of different substances to find out which ones sink or float in fresh and salty water. They have found that an acrylic cube sinks in fresh water and floats in salty water. Mr H gives each pair of students four cups containing various concentrations (densities) of salt water. Each cup, he explains, contains a solution with a different amount of salt. In addition to the acrylic cube, he also gives them an acrylic rod and a graduated cylinder. Your challenge, he says, is to figure out how to arrange the solutions in order from least to most salty. As he moves from group to group, Mr H checks to see if students are using measurements to compare the behavior of the acrylic objects in the 4 solutions.

He notices one pair of students looking around the room rather than using the materials. He puts the acrylic cube and acrylic rod next to each other and asks If I put these two acrylic objects in water, do you think they will sink or float? They tell him that because they are both made of the same stuff, and since the cube sank, they think the rod will sink in water too. They try it. He then asks Does the acrylic cube sink or float the same way in each of the 4 salt solutions? The students pick up the cube and drop it into each of the 4 cups. Mr H listens as they talk about their observation that the cube looks like it’s floating a little differently each cup and begin to think about how they could tell exactly how high or low the cube floats in each solution. Mr H moves on to another group.