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Essential Science for Teachers: Physical Science
Rising and Sinking
Why does a hot air balloon rise into the sky? Why does ice rise in water, when a lump of solid wax will sink in a jar full of molten wax? In this session, participants generalize the model that has been developed about what rises and what sinks, using the idea of balance of forces.
Learning Goals
During this session you will have an opportunity to build understandings of the following concepts:
Hot air balloon.
- The volume of fluid displaced is equal to the volume of the immersed object.
- Archimedes’ principle states that the buoyant force on an object is equal to the weight of fluid displaced.
- Rising or sinking depends on the outcome of the battle between two forces: weight and buoyant force.
- Water is an “anomaly” because the shape and arrangement of its molecules make it less dense in its solid form than in its liquid form.
Video Overview
Why does a hot air balloon rise into the sky? Why does ice rise in water when a lump of solid wax will sink in a jar full of melted wax? In this session, we’ll generalize the model we’ve developed about what rises and what sinks, using the idea of balance of forces.
Video Outline
We begin this session by continuing to connect the ideas of buoyant force, pressure, and density. Hot air balloons provide a wonderful example of a fluid rising in a fluid, but why does the air have to be “hot”?
We revisit the Science Studio swimming pool where children are comparing the weight of objects in and out of the water. What principle might account for the consistent weight differences that they are measuring? “Archimedes’ principle” is the standard answer often given, but what does this really mean? Our hosts introduce a model called the “watery ghost” to help us understand this idea.
We then look more closely at the “special case” of floating and hear from yacht designer, Halsey Herreshoff, who shows us that boat builders take the balance of forces into account when determining the size and shape of their designs.
We visit the Young Achievers Science and Mathematics Pilot School in Boston, Massachusetts, where Monique Brinson’s third graders try their hands at designing reliable “sinkers” out of aluminum foil and clay. Then back in the Science Studio, a fourth grader compares the ability of two liquids of different densities to “hold up” different-sized pieces of a wax candle.
Joe Reilly’s first graders at the Lincoln School in Brookline, Massachusetts, get us thinking about objects “floating” in air with a parachute-making activity that segues nicely back to the Science Studio, where we ask a third grader their ideas about why some balloons rise and others sink.
The session ends with a look at how the shape of water molecules accounts for the geometry of snowflakes and for the “water anomaly” that allows ice to float in water (while other solids sink in their own liquid state).
