Forces in fluids

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  • 1. Forces in Fluids Pressure, Floating/Sinking, Pascal’s Principle & Bernoulli’s Principle
  • 2. Pressure (1) Pressure relates to anything exerting force on another object by pressing on it. Pressure = Force/Area Units will be in Newton per squared meter: N/m2
  • 3. Pressure (2) The amount of pressure you exert depends on the area over which you exert a force. Examples: – Snow shoes vs. regular shoes – Light pressure used with needles for shots – High heels in grass vs. regular shoes
  • 4. Fluid Pressure (1) Remember back to fluids… Fluids are a material that can easily flow and change shape. If you understand the pressure in fluids, you can predict whether an object will float or sink. **** Fluids are not just liquids!! They can also be gasses- look at the definition!!!
  • 5. Fluid Pressure (2) In a fluid, all of the forces exerted by the individual particles combine to make up the pressure exerted by the fluid. – The denser the fluid, the more pressure it will exert (because there are more particles to exert pressure).
  • 6. Fluid Pressure (3) Air pressure is also considered to be a type of fluid pressure. We are constantly surrounded by this pressure of the air around us. Although it changes at different elevations, our bodies are designed to withstand it without it crushing us. – Sea Level atmospheric pressure is a whopping 101,300 Pa!!!!! (Pascal units)
  • 7. Fluid Pressure (4) Pressure gets greater, the lower you are in a fluid. – You have the weight of all of the fluid on top of you and the lower you get, the more weight is on top of you. – It’s like playing “dog pile”. You never want to be on the bottom or you get crushed!
  • 8. Measuring Pressure We use Barometers to measure atmospheric pressure. People who make weather predictions use Barometers to help them understand weather patterns and make forecasts.
  • 9. Floating and Sinking (1) Remember Density? Mass per unit volume??? We can calculate density by dividing mass by volume! Density = Mass / volume
  • 10. Floating and Sinking (2) If you know the densities of objects you can predict whether they will sink or float when placed in a fluid of a given density. Remember if the density is larger than the fluid it’s placed in it will sink, if it’s smaller it will float. (let’s see what you’ve learned…)
  • 11. Floating and Sinking (3) If I had water (1g/ml) would the following float or sink? – Wood (.7 g/cm3 ) – Tar (1.02 g/ml) – Corn Oil (.925 g/ml) – Mercury (13.6 g/ml) – Corn Syrup (1.38 g/ml)
  • 12. Buoyancy (1) Buoyancy is the ability to float. Water and other fluids exert an upward force on any object submerged into it. Buoyancy acts in the opposite direction of gravity, so it makes an object feel lighter. – Ever notice how it’s much easier to lift or carry a friend in a pool than it is outside the water?
  • 13. Buoyancy (2) Weight of submerged objects is a downward force and the buoyancy of an object is an upward force. If an objects weight is greater than the buoyant force it will sink. If an objects weight is equal to the buoyant force it will not sink.
  • 14. Archimedes Principle (1) Archimedes Principle states that the buoyant force acting on a submerged object is equal to the weight of the volume of the fluid displaced by the object. Meaning, if your hand displaces 50 ml of water when I stick in a cup (stupid, I know) then the buoyant force acting on it would be equal to the weight of 50ml.
  • 15. Archimedes Principal (2) So how does this apply to ships? If the buoyant force of an object is equal to the weight of the displaced fluid, then a large fluid that displaces a lot of fluid should have a strong buoyant force. If you are confused, check out the picture on page 429 in your text book.
  • 16. Pascal’s Principle (1) Fluids exert pressure on any surface that it touches. Pascal’s Principle states that when a force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid. – An example of Pascal’s principal at work is hydraulic pistons.
  • 17. Pascal’s Principle (2) Hydraulic systems multiply forces by applying the force to a small surface area. The increase in pressure is then transmitted to another part of the confined fluid, which pushes on a larger surface area. – Hydraulic lifts – Hydraulic breaks
  • 18. Bernoulli’s Principle (1) Fluids (remember, not just liquids) flow from areas of high pressure to low pressure. This is an attempt to keep things balanced. The pressure of a moving fluid is different than the pressure of a fluid at rest (not moving).
  • 19. Bernoulli’s Principle (2) Bernoulli’s principle states that the faster a fluid moves, the less pressure the fluid exerts. – Therefore, as the speed increases, the pressure exerted by the fluid decreases.
  • 20. Bernoulli’s Principle (3) This principle is used to explain the flight of objects (kites, airplane, frisbee etc). If the air moves faster above the object, fluid pressure pushes the object upward. – The air moves faster due to the shape/design of the object (typically a curved shape with a slightly rounded top)