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PHYS16 – Lecture 29

PHYS16 – Lecture 29. Fluids: Pressure and Buoyant Force November 12, 2010. Review of Last Week…. Practice Question 1 . A black hole is an astronomical object that has a large mass concentrated at a single point. Given this information, what would be the escape velocity of a black hole?

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PHYS16 – Lecture 29

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  1. PHYS16 – Lecture 29 Fluids: Pressure and Buoyant Force November 12, 2010

  2. Review of Last Week…

  3. Practice Question 1 • A black hole is an astronomical object that has a large mass concentrated at a single point. Given this information, what would be the escape velocity of a black hole? • Undetermined • Zero • Infinity • Proportional to √(M)

  4. Practice Question 2 • A “day” on Mercury is very peculiar. It only takes Mercury from sunrise to noon (1/4 of a day) to sweep out half of it’s orbital area around the Sun. So how long must a day be? • ¼ year • ½ year • 1 year • 2 years • 4 years

  5. This Week • Pressure and Pascal’s Principle • Buoyant Force and Archimedes’ Principle • Equation of Continuity • Bernoulli’s Equation

  6. Pressure

  7. Liquid Pressure Varies with Depth • Find the pressure difference across a cube of water Would the pressure change be larger in salt water or fresh water? http://www.open2.net/open2static/source/file/root/0/30/19/124156/pressure_cube_b.jpg

  8. Example: Container Height • If I start filling the following container with water, where will the highest water level be? Manoharan Lectures, Harvard University, Fall 2010

  9. Example: Submarines • A submarine is submerged to a depth of 960 m. What air pressure is needed to blow water out of the ballast tanks? (density = 1024 kg/m3) http://www.yesmag.ca/how_work/submarine.html

  10. Air Pressure Varies with Height • As height increases pressure decreases and density decreases http://eo.ucar.edu/webweather/images/density.gif

  11. Example Question: Pistons • Given that the density of air is proportional to pressure, how does the air pressure inside a confined volume change if the volume decreases by half? http://www.tpub.com/fireman/25.htm Changes by 2…

  12. Buoyant Force

  13. Archimedes’ Principle • Buoyant force = the weight of the water displaced http://www.open2.net/open2static/source/file/root/0/30/19/124156/pressure_cube_b.jpg

  14. Example: Buoyant force on objects • Buoyant force on brick in water -Volume= 0.2 m X 0.08 m X 0.05 m -FB=8 N • Buoyant force on ball in water -Volume= (4/3)pi(0.025)^2 -FB=0.6 N • Buoyant force on Diet Coke vs. Coke -Volume= same in both cases -FB=same -Coke sinks more than Diet Coke bec. Coke more dense (Fgravity is more for Coke!)

  15. Main Points • Pressure = Force/Area • Pressure varies with height • Buoyant force is equivalent to the weight of the water displaced

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