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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.

Vince Calder



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