Optional Unit VI: Fluid Mechanics
D. Archimedes' Principle

Key Concepts

The apparent change in (or loss in) weight of an object immersed in a liquid is due to the buoyant force. (The buoyant force on an object in a gas is negligible in most applications.)

When an object is partially or fully submerged, the buoyant force, or apparent loss in weight, is equal to the weight of the fluid displaced. (Archimedes' principle )

The apparent weight equals the actual weight minus the buoyant force.

A floating object displaces its own weight of fluid. The buoyant force is equal to the weight of the object.

A floating object has an average density which is less than the density of the fluid in which it is floating.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: buoyant force, apparent weight.

2. State Archimedes' principle.

3. Explain what factors would need to be considered in determining whether or not an object will float.

4. Solve problems relating to Archimedes' principle.

Teaching Suggestions, Activities and Demonstrations

1. Perform an activity to measure the weight of an object in air and the weight of the same object in a liquid, measure the weight of the fluid displaced by the submerged object, and analyze the measurements obtained.

2. Determine the weight of a small metal object. Place the object into an overflow can. Collect the water displaced from the can in a graduated cylinder. (Using a wetting agent in the water helps to break up the surface tension and gives more accurate measurements of the displaced volume.)

   Record the weight of the object when submerged in water, by attaching it to a spring balance. Determine the weight of the liquid displaced and compare it to the apparent weight of the submerged object due to the buoyant force. The weight loss of the submerged object should be equal to the weight of water displaced.

   Repeat for an object which floats. Carefully examine how the apparent loss in weight of a floating object compares to the weight of the liquid it displaces.

3. Wine makers use a hydrometer to determine the starting specific gravity of the "must." More sugar is added, if necessary. The starting specific gravity determines the potential alcohol yield. (The type of yeast used, and the ability of the yeast to tolerate alcohol, may limit the potential yield.)

   Perform an activity to investigate the specific gravity of sugar solutions at different concentrations.

   Research wine making and the historical aspects of wine making. Alcoholic fermentation has a history that dates back to the seventh millennium B.C. It has important scientific, social, technical, historical, and economic aspects.

   In teams, make small batches of wine. Develop a set of criteria to be used in evaluating the wine's quality. Once the wine has been made and suitably aged, several teachers might volunteer to act as wine tasters to test the final product.

   Discretion is advised when considering this activity. Seek administrative support beforehand.