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Forces in Fluids

Forces in Fluids. Pressure and Buoyant Force. What is a fluid?. A fluid must have particles that can easily flow past each other and take the shape of its container ( liquids and gases ) Tell your neighbor and list some specific examples: Water Air Oil.

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Forces in Fluids

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  1. Forces in Fluids Pressure and Buoyant Force

  2. What is a fluid? • A fluid must have particles that can easily flow past each other and take the shape of its container (liquids and gases) • Tell your neighbor and list some specific examples: • Water • Air • Oil

  3. What are characteristics of fluids? • All fluids exert pressure. • All fluids exert pressure equally in every direction

  4. More on pressure • Pressure refers to the amount of force exerted on a given area. Mathematically this is represented by the formula: Pressure = Force Area • The scientific unit for pressure is the pascal (Pa).

  5. More on pressure You’re the next goat! • 1 Pa is equal to 1 N/m2 • Another unit for pressure you commonly hear is psi = pounds per square inch • Most car tires have a pressure of around 30psior about 207 kPa (kiloPascals); 1000 Pa = 1 kPa • Which animals have the most pressure to their bite?

  6. Atmospheric Pressure • The atmosphereis the layer of nitrogen, oxygen, carbondioxide, and other gases that surround Earth. • The atmosphere goes up about 94 miles above us and the pressure of all those gas particles pushing down on us is called atmosphericpressure. • Atmospheric pressure is exerted on everything on Earth and is about 101 kPa. • This is like having the weight of a pineapple pressing on every square centimeter of your body!

  7. Atmospheric Pressure • The atmospheric pressure varies with elevation – higher altitudes = less atmospheric pressure • You aren’t crushed by atmospheric pressure because the fluid in your body (mostly water) pushes outward, against the atmosphere

  8. Water Pressure • Water and other liquids are more densethan air or other gases and exert more pressure. • The pressure exerted by water depends only on the depth. The deeper you go, the morepressure there will be.

  9. Another characteristic of fluids • Fluids always flow from highto lowpressure. • When you inhale and your lungs expand, the pressure inside your lungs decreases, allowing air to flowfrom the higher pressure area outside your body into your lungs.

  10. Pascal and his Principle • Blaise Pascal was a French scientist and mathematician who studied fluidsinmotion(hydrodynamics). • One of his findings was that a changein pressureat any point in an enclosedfluid will be transmitted equallyto allpartsof the fluid. This is called Pascal’s Principle.

  11. Pascal and his Principle • Hydraulicdevices use liquidsto transmit pressurefrom one point to another. • The brakes in a car use a hydraulic device to transmit the pressure from the brake pedal to the brake pads on each wheel.

  12. Buoyant Force • Pressure itself is nota force, it only describesthe amount of some type of force acting on a given area. • When we want to talk about the force that fluids exert, it is called buoyantforce. • Buoyantforce is an upwardforce.

  13. Buoyant Force • Water exerts pressure on allsides of an object. Pressure exerted horizontallyis equaland balanceeach other out. • Because pressure increaseswith depth, there will be more pressure on the bottom of the object. This creates the unbalanced, upwardforce known as buoyantforce.

  14. Archimedes and his Principle • A Greek mathematician named Archimedesis credited with discovering how to measure buoyant force – it’s a pretty crazy story • Archimedes’ principle says that the buoyant force of an object in a fluid is equal to the weightof the volumeof fluid that object displaces(takes the place of). • See if this helps!

  15. Sinking and Floating • Remember, weight is the gravitationalforceacting on the object. • Objects sink if their weightis greater than the weightof the fluid displaced. • If an object weighsthesame as the weight of the displaced fluid, the object will float withinthe water. • An object will float at the surfaceof the fluid if the buoyantforceis greater than the object’s weight. • Check thisout!

  16. Density • We can also describe whether an object sinks or floats based on its density. • Density = mass volume • Water has a density of about 1 g/cm3 (1 g/mL) • Objects with a density greater than 1 g/cm3 will sink. • The overall density of an object can be changed by changing the shape of the object to create more or less volume. This is what allows steel (8 times more dense than water) ships to float.

  17. Image Sources • Unless otherwise indicated, images are from Microsoft clip art • Slide 3: http://i0006.photobucket.com/albums/0006/findstuff22/GIRL-BLOWING-BUBBLES.jpg • Slide 8: http://l.thumbs.canstockphoto.com/canstock6971795.jpg • Slide 9: http://balloonsallover.com/wp-content/uploads/2010/07/helium-tanks.jpg • Slide 10: http://www-history.mcs.st-andrews.ac.uk/BigPictures/Pascal_6.jpeg • Slide 11: http://www.maritime.org/fleetsub/hydr/img/fig1-13.jpg • Slide 14: http://www.netstate.com/states/mottoes/images/ca_eureka.jpg

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