Question:
Hi-
I work with Special Education children-grade 7 and 8- and there is
a question that I cannot answer very clearly.... what are the
differences and similarities between density and buoyancy? The
text does not address this issue in a way that I can use to
explain it to them so that they actually "get it". yet, it asks
them to explain it. Go figure....
Replies:
DENSITY is the MASS (or for practical purposes at grades 7-8, the
WEIGHT) of an object divided by its VOLUME. The formula is:
DENSITY = WEIGHT / VOLUME.
BUOYANCY is a bit more complicated. BUOYANCY is the DECREASE in
the apparent MASS
(or for practical purposes at grade 7-8, the DECREASE in WEIGHT) of
an object when it is weighed while submerged in a fluid. The
DECREASE in the apparent WEIGHT of an object is the MASS (or WEIGHT)
of the amount of fluid that is DISPLACED (or REPLACED) by the object
being weighed.
Because the volume of air displaced by most objects is very
small compared to the volume of most objects being weighed there is
very little difference between the weight of an object measured in
air compared to its weight in a total vacuum, except for very
precise measurements. However, an object weighed in a fluid such as
water is significantly less than its weight in air (or vacuum). The
most common illustration of this is an ice cube. It floats -- with
about 1/12 of the volume above the surface of the water and 11/12
below the surface of the water. A "light" object, such as a cork,
floats with most of its volume above the surface of the fluid
(water). Because the amount of fluid that is displaced by an object
being weighed can depend upon its SHAPE, a crumpled piece of
aluminum foil sinks. But the same piece of aluminum foil in the
shape of a BOAT will float on the surface of water.
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