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Name: Marie
Status: educator
Grade: 9-12
Location: MO
Date: November 2007

I have a student who would like to do a science fair project testing the effect of shape of an object on buoyancy. She wants to take equal blocks of clay and form them into several shapes and test buoyancy. My question is "How would she measure buoyancy?

If I were to do this science fair project I would place a hook on each object to be measured. Hang it from a spring balance and read the number in and out of the water. The difference would be caused by the buoyancy effect.

It might be interesting to do the experiment in fresh water and then in salt water. It could be tied to the difference in how low in the water (the draft) a ship floats in the ocean and in fresh water like the Great Lakes or the Mississippi River.

Larry Krengel

Buoyancy is basically measured as a difference. It is the difference in an object's weight when it is in different surroundings. So if your student has the clay blocks in water, she'd need to measure the weight of the object in water and out of water.

Remember when an object is floating on water, its "weight" is zero; if it is underwater and would rise if released, its weight is negative.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming

The simplest way would be to load each model with weights until it sank or was completely under water. She also needs to be sure that each "hull" weighs exactly the same amount.

Robert Avakian

DEFINITION: Buoyancy is the force (that is weight) of a fluid displaced by an object immersed in that fluid. The most common fluid used for such measurements is water, since 1 gm. of water occupies about 1 cc. at room temperature. You can find the "exact" value in any handbook or web site tabulation of the density of water as a function of temperature. If the weight of the volume of the fluid (water) is less than the weight of the object, it will "sink". In such cases the buoyancy can be measured by just weighing the object immersed in the fluid (water). The weight loss, compared to the weight of the object in air, which can be taken as negligible weight loss, is the buoyancy force.

For simple shapes where the volume can be calculated from the formula for volume for that shape, it is only necessary to compute the volume of the object without doing any weighings. Things get experimentally trickier if the shape of the object is such that the weight of the displaced fluid (water) equals the weight of the object when the object is only partially submerged. In such cases, the object will partially float. The system coming "to rest" when the weight of the displaced volume of fluid equals the weight of the part of the object that is immersed under the fluid (water). The most common example is of course an ice cube in water. Since the density of ice is about 90% that of liquid water, the ice cube will "sink" leaving about 10% of the ice cube above the water level.

The selection of clay (density much greater than water) as the construction material makes things simpler. Weigh the object suspended in air. Then weigh the object suspended in the fluid (water). That difference (weight loss) is the weight of the displaced fluid (water). Since the density of water is 1 gm per cubic centimeter, that is numerically the same as volume of displaced fluid (water).

With some care it should be possible to make both a solid sphere and a hollow sphere of the same size. You will then see the much greater buoyancy of the hollow sphere compared to the solid sphere, even though from the "outside" both objects appear to have the same size (that is, volume). You can also make a torus of the same amount of clay and be able to experimentally measure the volume of a torus compared to that of a sphere that weighs the same (in air). The key to keeping things simple is to be sure the difference in the shapes is always large enough for the object to completely "sink" rather than partially float. In such cases, even how you "float" the object will change the result since the amount of displaced fluid (water) will depend upon the orientation of the same object. Yet another variation would be to change the fluid from water to some other fluid. Then the change in the apparent weight in the new fluid compared to the weight loss suspended in water measures the density of the new fluid compared to water. A concentrated solution of some very soluble salt like Mg(SO4) (Epsom's salt) will measure the density of the solution. You could even dilute the salt solution with water and measure the density as a function of concentration.

Vince Calder

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