p140w07_ct_21 - Physics 140: Winter 2007 Lecture #20 March...

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Unformatted text preview: Physics 140: Winter 2007 Lecture #20 March 27, 2007 Dave Winn Racquetball Striking a Wall Copyright: Loren M. Winters Mt. Etna Andrew Davidhazy What happens when you apply a force to an object? • Solids: distortion allows them to push or pull back • There are other kinds of stress and strain: sideways shear, and bulk pressure which presses in from every side • Shear stress and strain: how easy to make it slip sideways? • Bulk stress and strain: how easy to squash it in L L Y Δ = A F L x S Δ = A F L F F Δ L Δ x L V V B Δ = A F Fluids • “Fluids”: materials which cannot support shear stresses • If you push on them they will slip sideways in any direction to escape your force • Solids: atoms are fixed in place • Fluids: atoms can move relative to one another, they can flow • Two kinds to consider: liquids and gases – Liquids: hard to compress and dense – Gases: easy to compress and tenuous Fluids and hydrostatic pressure • Fluids at rest: statics • Push on top of a sealed cylinder of fluid and you generate hydrostatic pressure • Force / unit area: measured in N/m 2 • 1 N / m 2 = 1 Pascal • Hydrostatic pressure acts in every direction, trying to escape any way it can • This pressure is transmitted everywhere through the fluid F P Fluids supporting themselves • Water in a glass: consider a slab • Forces on it balance: gh P A mg P P mg A P A P A P mg A P F top top bot top bot top bot y ρ + = + = + = = − − = Σ mg = ρ Ahg P bottom *A P top *A Pressure increases with depth: ( ) gd P d P top ρ + = d Pressure in a fluid • Pressure increases with depth d as: • What is pressure at the top? Atmospheric pressure P atm • Note that this pressure increase happens due to gravity!...
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This note was uploaded on 04/04/2008 for the course PHYSICS 140 taught by Professor Evrard during the Fall '07 term at University of Michigan.

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p140w07_ct_21 - Physics 140: Winter 2007 Lecture #20 March...

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