Category: Physics—Mechanics—Buoyancy
Project Idea by: Amy Grabina and Britanny Manchio
Question: How Does Density Affect the Buoyancy of Objects?
Buoyancy is the upward force of a fluid on an object placed in it. Archimedes (287–212 B.C.), a Greek mathematician, is given credit for explaining buoyancy. Floating is usually associated with water or air, but floating can describe any object that is suspended in any fluid, whether it is a liquid or a gas. The term buoyant is a measure of how well an object floats.
When an object such as a boat is placed in water, the submerged part of the object displaces (pushes aside) an amount of water whose weight is equal to the weight of the object. The heavier the object, the more water it displaces, and thus the lower it sits in the water.
An object floats or sinks in water because of the difference between the total downward force and the total upward force acting on the object. If no other forces are acting on the object, the downward force is equal to the weight (Fwt) of the object, and the upward force is equal to buoyancy (FB). Weight is the measure of the force of gravity acting on the object. If the weight of the object is less than the buoyancy on the object, then the object floats. If the weight of the object is greater than the buoyancy on the object, then the object sinks. In diagram A, the weight of the boat is equal to the buoyancy on it, which is indicated by equal-length arrows; thus, Fwt= FB, and the boat floats. In diagram B, the weight of the boat is greater than the buoyancy on it, which is indicated by unequal-length arrows; thus, Fw > FB, and the boat sinks.
Boats are made of different materials such as wood and steel. But boats are not solid pieces of material. Instead, boats have hollow spaces filled with air. Air is lighter than wood or metal. The more hollow spaces a boat has, the greater its volume. Since density is a measure of the mass of a given volume, a project question might be, "How does density affect the buoyancy of objects?"
Clues for Your Investigation
Build boats with the same mass but different volumes. Use the same kind and amount of material. Determine a way to measure the volume of each boat such as submerging the boats in water and measuring the amount of water displaced. With the mass and volume measurement, determine the density of each boat using this formula:
- density = mass ÷ volume.
Determine a way to compare buoyancy such as by measuring the height of the boat above the waterline.
Independent Variable: Boats with different volumes
Dependent Variable: Buoyancy measured by the height of the boat above the waterline
Controlled Variables: Mass of the boats, material used, testing container for holding water, method of measuring buoyancy
Control: Median density
Other Questions to Explore
- How does the placement of cargo in a boat affect its buoyancy?
- What effect do building materials have on buoyancy?
- what effect does salinity of ocean water have on buoyancy?
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