GRADES 4-8
TECHNOLOGY OF THE DEEP
EXPERIMENTS WITH BUOYANT FORCES

BACKGROUND INFORMATION: Fluid dynamics is the study of how gases and liquids behave. A fluid can be either a gas or a liquid. All substances have three states of matter: solid, liquid, and gas. When a substance is in a solid state, its molecules are all lined up and rigid. Its volume and shape are fixed. An ice cube (solid water) remains an ice cube no matter what size or shape a container you put it in. A liquid, on the other hand, has molecules that are more fluid and moveable. It has a fixed volume, but it can take the shape of its new container. You can pour a quantity of water from a glass to a cube, but if the cube has a smaller volume than the glass, the water will run over the top! A gas has molecules that are completely free to move about, contracting and expanding at will. A gas has no fixed volume or shape. If you take a deep breath of air (a gas), the air you take in expands to fill all the compartments of your lungs. If you blow that breath into a balloon, for instance, the volume in the balloon is much smaller than the volume in your lungs. We measure the number of molecules in a given volume of fluid and call it density. Since the volume of a liquid is constant, its density, the mass (measured by the number of molecules) in a fixed volume, is also constant. A liquid is considered to be incompressible; the density is constant and you can't squeeze more mass into the same volume. With a gas, you can continue to add molecules into a given volume, or you can squeeze the same number of molecules into a smaller volume. When you do this, the density changes and the gas is called a compressible fluid. Buoyancy is the final concept we want to discuss here. When an object is immersed in a fluid, what makes it float? Archimedes' principle states that the fluid exerts an upward force on the object equal to the weight of the fluid that is displaced by the object. This means that if you have an object whose mass is less than that of the fluid, the fluid forces will push that object up! This is why a boat floats or a hot air balloon floats through the air. The weight of the boat (its mass times gravity) is less than the weight of the water it displaces. In Activity 3, the students built boats and placed pennies in them. As long as the boats displaced a large volume of water, multiple pennies could be placed in them. If the foil boats didn't displace much water, then they wouldn't be able to hold many pennies. The wadded up piece of foil didn't displace very much water at all, and the weight of the wad was greater than the weight of the water it displaced, so it sunk to the bottom. Hot air balloons also work on the buoyancy principle. As the air heats up, its density decreases. Once the balloon is expanded to its full size, its volume is fixed. As the density continues to decrease, that means the mass is decreasing. Since weight is the mass times gravity, the weight of the hot air is less than the weight of the cooler air it is displacing, and the balloon rises into the air! Seawater is a mixture of water and salt. The salinity of a liquid is defined as the weight of the dissolved salt divided by the weight of the liquid. The salinity of seawater is typically about 0.035. Generally, this is written as 35 0/00 or 35 parts per thousand.