Exploring the Motion of Fluids

BACKGROUND:
A fluid, unlike a solid is a substance that can flow. Fluids can be of two types liquids or gases. The difference between a liquid and a gas is the positioning of the molecules. In gases the molecules are much farther apart than a liquid. This does not prevent either substance from flowing. Unlike solids where mass and the amount of force exerted on the mass controls the motion, fluids move in response to density and pressure.

The following are some of the fundamental principles that will be observed in lab. You may want to use examples to illustrate these principles. In lab, we will not ask students to figure out which principle applies to the different stations, because often there is a combination of principles at work. The lab will center around observing how fluids move. Stress to students that pressure within the fluid helps move it. The key objective is to introduce that fluids have motion just like solids, not to memorize the different principles.

Pascal's Principle. A change in the pressure applied to an enclosed container is transmitted without change throughout the fluid and acts in all directions.

Archimedes' Principle. A body immersed in a fluid will be buoyed up by a force equal to the weight of the fluid that it displaces.

Bernoulli's Principle. The pressure in a fluid decreases with increased velocity of the fluid. (The faster the liquid moves, the lesser is the pressure in that liquid.)

PROCEDURE:

Ask students to describe how they think fluids move. You may want to discuss what a fluid is. Students may not realize that anything that "flows" is a liquid. Most of their experience is with water, so the idea that toothpaste is a fluid is not familiar with them.


Demonstrate the following principles to students.

Pascal's Principle
An example would be when you squeeze one end of a tube of toothpaste. This principle is also the reason behind the Heimlich maneuver, in which a sharp pressure increase applied to the abdomen is transmitted to the throat, thereby ejecting any food particles that was lodged in the throat. The hydraulic pump is also an example.

Archimedes' Principle
An example would be a piece of wood floating on water. The wood will continue to rise out of the water until the force acting on it has decreased to a point equal to its weight. The piece of wood would be in equilibrium as it floats. Bubbles rising in the water would also be an example.

Bernoulli's Principle
A vivid example would be if the pressure of swiftly moving air inside the tornado is lower than that of the stagnant air in the house. The air inside the house blows the roof off. Liquid initially at rest in a container exerts a pressure on the sides. The pressure is reduced if there is a break at the bottom of the container. This type of motion is important to understand how lift occurs in airplanes. Bernoulli's Principle will be highlighted in a later activity.


Ask students again if they have any experience with fluid movement, and see if the list will grow. You may want students do a search on "fluids" on the Internet, to see if they can find more examples of fluids.


OBJECTIVES: Exploring the motion of fluids. Defining a fluid. VOCABULARY: density fluid pressure MATERIALS: glass of water spray can (i.e. air freshener) tube of toothpaste piece of wood and container of water a bottle with water that you can drill a hole in the bottom Internet	Students observe demonstrations of how fluids move.