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Buoyancy

Buoyancy. Barometer. Atmospheric pressure can be measured by balancing weight pressure against air pressure. A device that uses this is a barometer. This barometer uses the weight of a column of liquid mercury. The meter measures the height in inches (or mm). 1 atm = 29.92 inHg

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Buoyancy

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  1. Buoyancy

  2. Barometer • Atmospheric pressure can be measured by balancing weight pressure against air pressure. • A device that uses this is a barometer. This barometer uses the weight of a column of liquid mercury. The meter measures the height in inches (or mm). 1 atm = 29.92 inHg 1 atm = 760 mmHg

  3. Manometer • A manometer compares two different pressures. • Gas compared to air (tire gauge) • Liquid compared to air Gauge pressure is defined as the pressure minus atmospheric pressure.

  4. Pascal’s Law • A change in pressure in a fluid is transmitted to all parts of the fluid equally. Equal pressures, different forces

  5. Fluid Volume • Within a fluid, any volume is at equilibrium. • The pressure force on the sides of the volume balances the weight. Fg = -mg

  6. Buoyant Force • The force remains even without the original fluid. • It must balance the weight that would have been there. • This is the buoyant force. • Equals the fluid weight • Directed upward • Acts on the volume Fb = mg

  7. Sinking and Rising • An object in a fluid displaces a volume that had some mass. • If the object is heavier than the fluid it sinks. • If the object is lighter it rises. Fb = rVg Fb = rVg W= mg W= mg

  8. Archimedes’ Principle • An object suspended in a fluid has less apparent weight due to buoyancy. FT = mg - rVg Fb = rVg W= mg

  9. An iceberg has an average density of 86% of seawater. What fraction of the iceberg is underwater? The buoyant force is the weight of water displaced by the iceberg: Fb = rwaterVsubg. The weight is the total weight of the ice: Wi = riceViceg. Find the ratio of Vsub/Vice We know the ratio rice/rwater rwaterVsubg = riceViceg Vsub/Vice = rice/rwater = 0.86 Iceberg

  10. Center of Buoyancy • The force of buoyancy acts at the center of mass of the missing fluid. • The displacing object has a center of gravity at its center of mass. • The two forces may not act at the same point.

  11. Tippy Canoe • If a forces on an object act at different points there is a torque. • Buoyant force above the center of mass is stable. • The torque moves it back to the center • Buoyant force below the center of mass is unstable. • The torque moves it away from the center

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