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Session 6. Rising and Sinking
Hot air balloon. |
Learning Goals
During this session you will have an opportunity to build
understandings of the following concepts:
- 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.
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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). View the video ==>
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