Lect09 - PHYS 172 Modern Mechanics Fall 2009 Lecture 9...

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Fall 2009 PHYS 172: Modern Mechanics Lecture 9 – Buoyancy, The Energy Principle Read 5.1 – 5.7
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CLICKER QUESTION #1 Reading Question (Sections 5.1 – 5.7) (This is a closed-book quiz, no consulting with neighbors, etc.) A. The force of Earth’s gravity causes work to be done on the Moon, giving it energy of motion. B. No work is done on the Moon because the net force on it is zero. C. The work done on the Moon causes it to be in a circular orbit. D. No work is done on the Moon. E. The Earth’s gravity gives the Moon kinetic energy. The Moon is in circular orbit about the Earth. Which statement is most correct ?
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Today’s Lecture • Buoyancy,Rest Energy, Kinetic Energy, Work
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Just a few words about: Pressure Balloon: Air molecules constantly hit surface and bounce off, exerting forces on surface. on small piece of surface of small piece of surface pressure area F P ! " Colliding ball exerts force on wall. on due to on due to on due to ball wall ball wall wall ball ball p F t F F ! = ! = " r r r r v r
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Just a few words about: Buoyancy There are more air molecules here than here Air is denser closer to the earth. (Why? We’ll see in Chapter 12!) Thus P bottom > P top OR Net force pushing up > Net force pushing down buoyancy
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6 Buoyancy: macroscopic view air m g r buoyant F r Archimedes principle: any body partially or completely submerged in a fluid or gas is buoyed up by a force equal to the
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