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Fluids Archimedes’ Principle Name______ Fluids apply a force to objects which are immersed in them called a buoyant force. We know that pressure is greater at greater depths, so there is an upward force . P1A h P2A For a submerged object, we know that P2=P1 + ρgh, and that the difference between the force up and the force down is the buoyant force FB. So FB = P2A- P1A= ρghA where hA is the volume of fluid is the volume displaced by the object ρghA=mg so that means that FB=Wfluid displaced A solid object will float in a liquid if the density of the object is less than or equal to the density of the liquid. Example: See example 8 on page 310 Problems- Archmedes’ Principle 1. A duck is floating on a lake with 25% of its volume beneath the water. What is the average density of the duck? So mass of the water= (density of water) * ¼ the volume of the duck. ρduckVduckg= ρwater (1/4Vduck)g. Answer: 250 kg/m3 2. What is the radius of a hydrogen-filled balloon that would carry a load of 5750 N ( in addition to the weight of the hydrogen) when the density of air is 1.29 kg/m3. The buoyant force must be equal in magnitude to the weight of the balloon and its load. ρ of air is 1.29 kg/m3 and the ρ of hydrogen is 0.0899 kg/m3. Answer: 4.89 m 3. Only a small part of an iceberg protrudes above the water, while the bulk likes below the surface. The density of the ice is 917 kg/m3 and that of seawater is 1025 kg/m3. Find the percentage of the iceberg’s volume that lies below the surface. ΡiceViceg= ρwater Vwaterg. You need to find Vwater/Vice Answer: 89.5%.