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Properties of Fluids SCI 8: Fluids Unit Curriculum Outcomes Addressed: - Describe the movement of objects in terms of balanced and unbalanced forces (309-2) - Describe the science underlying hydraulic technologies (111-5) - Explain quantitatively the relationship between force, area, and pressure • We will study the following properties of fluids: 1. Density 2. Buoyancy 3. Displacement (Archimedes’ Principle) 4. Viscosity 5. Pressure (Pascal’s Principle) Key Terms • Pressure: A measure of the distribution of force over a given area P = _F_ A • Force: Strength or energy as an attribute of physical action or movement • Surface Area: Total space or area covered by the outside of an object • Pascal’s Principle: An external force exerted on a confined fluid is distributed evenly in all directions inside the surface area of the container Pressure - Pascal’s Principle • Pascal’s Principle states that “any change in pressure applied to a fluid in a confined space is transferred unchanged throughout that object”. • In other words, a fluid within a closed space exerts an equal amount of force (pressure) into all directions. • A good example of Pascal’s Principle can be observed in hydraulics in everyday life and in Cartesian divers. DEMONSTRATION Pascal’s Principle Example #1: Cartesian Diver Originally, the cartesian diver (pen cap with clay) is positively boyant (floating at the top). When the bottle is squeezed, the water exerts pressure in all directions, and the cap becomes negatively buoyant, sinking to the bottom. When the bottle is released, the pen cap becomes neutrally buoyant again, rising to the middle of the bottle. How the Cartesian Diver Works • There is air inside the diver (pen cap), making it positively buoyant so it floats at the top. • When the bottle is squeezed, pressure is exerted on the water. This pressure is exerted in all directions, and it increases the pressure on the air bubble inside the diver (pen cap). The air inside the diver (pen cap) compresses and reduces in volume, allowing more water to enter the diver (pen cap). The diver now becomes more dense and negatively buoyancy, so it sinks. • When the pressure on the bottle is released/let go, the air inside the diver (pen cap) expands again, making it less dense, and allowing it to become positively buoyant therefore rise and float at the top once again. Pascal’s Principle Example #2: Hydraulics Hydraulics are technologies, tools, or machines, that are operated by a liquid moving in a confined space under pressure Hydraulics: Force, Pressure, and Area • Force: Strength or energy that causes change • Pressure: Force per unit Area; The continuous physical force exerted on or against an object • Area: The amount of space occupied by something Pressure = Force Area P = _F_ A Force, Pressure, and Area cont’d • As area increases, pressure decreases, because the force is spread out over a bigger space/area • For example… In the first example, the pin is pushed into the wood. In the second example, the pin is pushed into the finger. The reason for this is the difference in pressure. Assuming the same force is applied, each case would have a different pressure acting on the thumb. In the first diagram the thumb pushes on a large area so the force is spread out and the pressure is low. In the second diagram the force is concentrated on a small area so the pressure is much higher. Hydraulics: Force, Pressure, and Area cont’d • Changing the amount of area (space) covered by something will change the force exerted • As area increases, pressure decreases, because the force is spread out over a bigger space/area • So, a bigger area will require less force to lift something on the other side of a hydraulic (since force is exerted in all directions equally in a fluid) Pressure = Force Area P = _F_ A Hydraulics: Force, Pressure, and Area cont’d F = Force • PHET SIMULATION of Pressure, Temperature, etc. http://phet.colorado.edu/en/simulations/categor y/physics/work-energy-and-power AND Spangler Science Bed of Nails/ Balloon Video