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Sub Unit 1.2 – Fluid Force

Sub Unit 1.2 – Fluid Force. Pressure in Fluid Systems. Objectives. Describe the four states of matter. Define density and pressure Explain why pressure in a fluid depends on depth in the fluid Explain why an object submerged in a fluid experiences a buoyant force

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Sub Unit 1.2 – Fluid Force

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  1. Sub Unit 1.2 – Fluid Force Pressure in Fluid Systems

  2. Objectives • Describe the four states of matter. • Define density and pressure • Explain why pressure in a fluid depends on depth in the fluid • Explain why an object submerged in a fluid experiences a buoyant force • Predict whether an object will sink or float in a given fluid. • Explain how a force can be multiplied in a hydraulic lift. • Explain where atmospheric pressure comes from. • Describe how a barometer measures atmospheric pressure • Explain the difference between absolute and gage pressure.

  3. States of Matter • Matter can exist in four states: • Solid (ice) • Liquid (water) • Gas (steam vapor) • Plasma (Extremely hot ionized atoms)

  4. Fluids • Fluids are materials that can flow, has no definite shape of its own, and conforms to the shape of its container. • Liquids • Gasses • Fluid systems use both liquids (hydraulic) and gasses (pneumatic) to operate mechanical devices.

  5. City Water System Hydraulic system under pressure

  6. Density and Pressure • Density is how much mass is contained in a given amount of space. • Amount of matter per unit of volume. mass Density = (rho) volume m r = v

  7. Units mass Density = • English volume SI (slugs) kg lb g or 3 3 ft 3 m cm

  8. Example • The mass of 1 cm3 of water has a mass of 1g; therefore the density is 1g/1cm3.

  9. Weight Density • Weight Density is the comparison of an object’s weight to it’s volume weight Weight Density = volume weight lb N r = w 3 V ft 3 m Units

  10. Weight Density of Water • Water has a weight density of 62.4 lb/ft3.

  11. Pressure • A force applied over a surface is pressure. force Pressure = area Units F lb N P = 2 2 A ft m English SI

  12. Pressure Units • N/m2 = 1 Pascal (Pa) • 1000 Pa = 1kilopascal (kPa) • lb/in2 = psi (pounds per square inch)

  13. Pressure and Depth • Pressure increase with depth because of the additional weight of the fluid above. Pressure = weight density x height P = r h x w

  14. Buoyancy and Archimedes’ Principle • Ptop = rw x h • Pbottom = rw x (h + d) • F = P x A • Ftop = Ptop x A = (rw x h) x A • Fbottom = Pbottom x A = [rw x (h+d)] x A • Fbuoyant = Fbottom – Ftop = rw Ad • Ad = Vbrick • Fbuoyant = rw x Vbrick = weight of water displaced

  15. Archimedes’ Principle • An object immersed in a fluid has an upward force exerted on it equal to the weight of the fluid displaced by the object. • Note: the buoyant force is based on the weight of the fluid displaced not on the weight of the object.

  16. Pascal’s Principle • A change in pressure at any point in a confined fluid is transmitted undiminished throughout the fluid. P = F / A or F = P x A A = p r2

  17. Atmospheric Pressure • The weight of the air above an area. • At sea level, a column of air extending up through the atmosphere, with a cross sectional area of 1m2, encloses about 10,000 kg of air. • This air weighs about 1 x 105 N • Therefore, atmospheric pressure is about 105 Pa or 100kPa at sea level. • Decreases with altitude • This is why your ears pop (equalization)

  18. Atmospheric Pressure • Barometer – instrument used for measuring atmospheric pressure. • At sea level the average atmospheric pressure is 101.3 kPa = 760mm of mercury = one atmosphere = 14.7 psi = 2117 lb/ft2

  19. Absolute and Gage Pressure • Absolute pressure is the total pressure measured above zero (perfect vacuum). • Gage pressure is the pressure measured above atmospheric pressure. • Absolute pressure = gage pressure + atmospheric pressure • Suppose a tire gage measures the pressure of a tire to be 30 psi; • Absolute pressure = 30 psi + 14.7 psi = 44.7 psi • The air inside the tire pushes out with a pressure of 44.7 psi. The atmosphere pushes in with a pressure of 14.7 psi. The difference is 30 psi – the gage pressure.

  20. Pressure is a Prime Mover • Pressure acts like a force to cause movement.

  21. Equilibrium

  22. Summary • Matter can exist in four states: solid, liquid, gas, and plasma. • Liquids and gases are called fluids. • The density of a substance is its mass per unit volume. • The density ofwater is 1g/cm3. • Weight density is weight per unit volume. • Pressure is force divided by the area over which the force acts. • We treat pressure as a scalar. • In SI units, pressure is measured in pascals, where 1 Pa= 1 N/m • Pressure increases with depth in a fluid.

  23. Summary • For a given fluid, the pressure does not depend on the size or shape of the container. • When an object is submerged in a fluid, an upward force is exerted on the object caused by the pressure difference between the top and the bottom of the object. This force is called a buoyant force. • The buoyant force exerted on a submerged object equals the weight of the fluid displaced by the object. • A pressure applied to a confined fluid is transmitted throughout the fluid. • Atmospheric pressure is caused by the weight of the air above a given area. • Atmospheric pressure can be measured with a barometer. • Absolute pressure is the sum of the gage pressure and atmospheric pressure.

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