Ch. 14-2 Properties of Fluids PDF

Properties of Fluids Pascal Archimedes Bernoulli WHAT IS A FLUID? THE TERM FLUID IS USED TO REFER COLLECTIVELY TO BOTH LIQUIDS AND GASES – MATERIALS THAT CAN CHANGE THEIR SHAPES BECAUSE INDIVIDUAL...

Properties of Fluids Pascal Archimedes Bernoulli WHAT IS A FLUID? THE TERM FLUID IS USED TO REFER COLLECTIVELY TO BOTH LIQUIDS AND GASES – MATERIALS THAT CAN CHANGE THEIR SHAPES BECAUSE INDIVIDUAL PARTICLES (ATOMS, MOLECULES & IONS) ARE FREE TO CHANGE THEIR POSITIONS WHICH ALLOWS THEM TO FLOW.  PRESSURE Pressure is the force on an object that is spread over a surface area = force exerted per unit of area For a given force, if the surface area is smaller, the pressure will be greater. If you use a larger area, you are spreading out the force, and the pressure (or force per unit area) becomes smaller.  EXAMPLE WITH SHOES. An average shoe distributes the weight of the person over 20 square inches. Thus, a 100-pound person applies 100/20 = 5 pounds per square inch on the floor. Since a spike-heel is only 0.25 square inches, the 100-pound person would be applying 100/0.25 = 400 pounds per square inch on the floor at the heel! In some cases, that is sufficient to damage the floor. Consider a thumb tack on your hand: "ATMOSPHERIC PRESSURE is defined as the force per unit area exerted against a surface by the weight of the air above that surface. At sea level, air exerts a pressure of 14.7 pounds per square iinch! Since gases are compressible, in the mountains there is less air per unit volume above you so the atmospheric pressure is less. It will be harder to breathe because there is actually less O2 per volume compared to sea level. The greater the depth of a fluid, the greater the pressure. AIR PRESSURE VARIES WITH ELEVATION (ALTITUDE) ELEVATION OF ATLANTA = 1,050 FEET = 320 m Water is nearly 800 times denser than air and much heavier. As a scuba diver dives deeper and deeper, the pressure of all that water can affect the body. A diver who stays down too long, swims too deep, or comes up too fast can end up with a condition called "the bends." In this case, bubbles of gas in the blood can cause intense pain, even death. The Pascal Normal is the Atmospheric SI unit Air Pressure = 1for atm pressure. (atmosphere) = 101.3 kPa = 14.5 psi Note that the force must be in Newtons and the area in m2.  PRESSURE IN FLUIDS In a confined fluid - neglecting the effect of gravity on the fluid, the pressure is the same throughout the container, pressing equally on all the walls. When a liquid or gas is confined in a container or cylinder, you can create a pressure by applying an outside force with a solid piston. The pressure created in the cylinder equals the force applied divided by the area of the piston.  Pascal’s Principle: Pressure applied to a confined fluid is transmitted evenly throughout the fluid. P = F/A If the pressure is the same throughout, a small force over a small area at one end of the container of fluid can produce a large force if the area at the other end is large. This principle is used in most hydraulic systems such as pneumatic jacks and a car’s braking system. HYDRAULIC SYSTEMS Why more force where the area is larger? Think about the amount of fluid pushing up. HYDRAULIC SYSTEMS: Notice that the small force has to move over a greater distance to make up for the large force produced at the large piston. The Distance moved by the large piston is smaller The work done by each piston is the same! (W = F x d) PRESSURE-FORCE PROBLEMS: (Text p. 443 & 444) P = F/A Two cylinders contain pistons that are connected by fluid in a hydraulic system. A force of 7,500 N (F2) is exerted on one piston with an area of 0.05 m2 (A2). What is the force exerted on the other piston (F1) which has an area of 0.01 m2 (A1)? Pascal’s Principle: The pressure on Both cylinders is the same. P1 = P2: F1 = F2 F1 = F2 x A1 A1 A2 A2 F1 = 7500 N x 0.01 m2 = 1500 N 0.05 m2 PRACTICE PROBLEM: The small piston of a hydraulic lift has an area of 0.2 m2. A car weighing 12,000 N sits on a rack mounted on the large piston. The large piston has an area of 0.9 m2. How large force must be applied to the small piston to support the car? Pascal’s Principle: The pressure on Both cylinders is the same. P1 = P2: F1 = F 2 x A 1 = 12,000 N x 0.2 m2 = 2,667 N 0.9 m2 One for the road….. A 19,600 N car is lifted by a manually operated hydraulic lift. The area of the shaft of the lift is 80.0 cm2; the area of the piston that forces liquid into the system is 2.0 cm2. What force must be exerted on this piston to lift the car? 490 N  FLUID BEHAVIOR AND BUOYANCY When an object is immersed in a fluid, the pressure of the fluid exerts a force all around the object. But the pressure is greater at greater depth, so there is a net upward force on the object. The upward force exerted on an object immersed in a fluid is known as the BUOYANT FORCE The buoyant force is equal to the weight of the fluid that is displaced by the object. (remember that weight = m x g) Buoyancy: The ability of a fluid (liquid or gas) to exert an upward force on objects immersed in it. Positive Buoyancy: If the Remember that buoyant force is greater than the Weight = m x g objects weight, the object will float. Neutral Buoyancy: If the weight of the object & the buoyant force are equal, the object will “hover” in the water. Negative Buoyancy: If the buoyant force is less than the Weight of the object, the object will sink. The story of Archimedes’and the golden crown: BUOYANCY LINKS Click on the picture with the crown. Click the link below for an excellent buoyancy simulation activity: http://phet.colorado.edu/sims /density-and-buoyancy/ buoyancy_en.html Professor Dave & Buoyancy: https://www.youtube.com/watch? v=16HDJNoXQII Notes on Buoyancy – What is neutral Buoyancy? http://www.phillyseaperch.org /uploads/9/1/0/6/9106381/_buoyancy_for_hs.pdf Archimedes’ Principle:  SINK or FLOAT PROBLEMS Whether a substance sinks or floats depends on both density and buoyancy. 1. A slab of unknown material has a length of 14 cm, a width of 12 cm and a height of 7 cm. It has a mass of 1 kg. Float (Density = 1000 g/1176 cm3 Will this cube float or sink in water? = 0.9 g/cm3  less than water) 2. A toy boat with a mass of 850 g displaces 900 g of sea water a) What is the formula for weight? w (N) = m (kg) x g b) What is the actual weight of the boat in Newtons? W =.850 kg x 9.8 m/s2 = 8.3 N c) Based on Archimedes' principle, will the boat sink or float in water? Float – The buoyant force = weight of water displaced, It is greater than the weight of the boat.  AIR THAT IS NOT MOVING PUSHES IN ALL DIRECTIONS WITH EQUAL PRESSURE. Thus air is pushing down on the top of a table top and up on the underside of the table. Moving air however pushes with less pressure. If a fan pushes air across the top of a table, the downward pressure on the table is reduced. Bernoulli’s Principle As the velocity of a fluid increases, the pressure exerted by the fluid decreases. Try blowing paper Set up as in these Pictures. What happens? Fluids in motion - The BERNOULLI Effect – Applications: Place a ball in a funnel. Blow into the funnel. Hold it upside down or right side up. What happens to the ball? The air is faster above the ball and on the sides. The higher pressure below the ball keeps the ball in the funnel. Vacuum cleaner The BERNOULLI Effect – More Applications: Try this: Why does the roof of a house lift off in high winds? Is it pulled or pushed off? How does an airfoil really work? https://www.youtube.com/watch?v=aFO4PBolwFg  Viscosity: A fluid’s resistance to flow. Viscosity is due to how the molecules of a substance interact with one another – often referred to as the “internal friction” between the particles. (Friction is the resistance to movement) Flowing particles will transfer energy to the particles that are stationary. Flowing particles are pulling the other particles, causing them to flow as well. If the flowing particles do not effectively pull the other particles into motion - liquid has a high viscosity If the flowing particles pull the other particles in motion easily, then the liquid has low viscosity.  Low viscosity vs high viscosity liquids Low viscosity liquids: water, alcohol, milk High viscosity liquids: syrup, honey, molasses Viscosity Increase temperature = decrease viscosity Decrease temperature = increase viscosity Why? Increase in temperature = increase in KE Decrease in temperature = decrease in KE  READ ALL ABOUT IT! The great Boston molasses disaster: Think about the viscosity of molasses on a day in January and the pressure it can exert! Click on the newspaper to find out what happened!  CHECK OUT THESE WEBSITES: 1. Good explanation of properties of fluids http://www.bwsd.k12.wi.us/astitt/Physics/Fluids/fluids.htm 2. U tube video on Pascal’s Principal http://www.youtube.com/watch?v=ManGITM2Mbc 3. How objects float in fluids: http://www.school-for-champions.com/science/fluid_floating.htm 4. Pressure in fluids: http://www.school-for-champions.com/science/fluid_pressure.htm 5. Application of fluid principles: http://www.school-for-champions.com/science/fluid_applications.htm