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Lecture 02 - Buoyancy. Fluid dynamics

# Lecture 02 - Buoyancy. Fluid dynamics - Vacuum gun Sealed...

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1 Lecture 2 Buoyancy Buoyancy. Fluid dynamics. Vacuum gun Sealed tube, air pumped out Ping-pong ball What happens if we punch a little hole on one side? DEMO: Vacuum gun = 2 atm 2 atm 2 1 ~ 2 2 ~ ~ 300 km/s (speed of sound!!!) p FL L mv R p v L mR Length of tube L ~ 3 m Mass of ball m ~ 3 g Radius of tube R ~ 2 cm ACT: Side tube A sort of barometer is set up with a tube that has a side tube with a tight fitting stopper. What happens when the stopper is removed? vacuum A. Water spurts out of the side tube. stopper B. Air flows in through the side tube. C. Nothing, the system was in equilibrium and remains in equilibrium. DEMO: Side tube DoPaA1 Buoyancy and the Archimedes’ principle y bottom y top h A A box of base A and height h is submerged in a liquid of density ρ . Σ = top bottom F F F F top Net force by liquid : bottom top Ap Ap = ( ) ( ) atm bottom atm top A p gy A p gy ρ ρ = + + ρ = A hg Archimedes’s principle: The liquid exerts a net force upward called buoyant force whose magnitude is equal to the weight of the displaced liquid. ρ = direction up Vg F bottom

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Slide 3 DoPaA1 Hydraulic puzzle M-HS 8 Department of Physics and Astronomy, 8/24/2010
2 In-class example: Hollow sphere A hollow sphere of iron ( ρ Fe = 7800 kg/m 3 ) has a mass of 5 kg. What is the maximum diameter necessary for this sphere to be completely submerged in water? ( ρ water = 1000 kg/m 3 ) A. It will always be submerged. B. 0.11 m F B C. 0.21 m D. 0.42 m E. It will always only float. mg The sphere sinks if B F mg ρ π 3 water 4 3 R g mg πρ = 3 water 3 0.106 m 4 m R = = max Maximum diameter 2 0.21 m R Density rule A hollow sphere of iron ( ρ Fe = 7800 kg/m 3 ) has a mass of 5 kg. What is the

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