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? A)Undetermined B)Zero C)Infinity D)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? A)¼ year B)½ year C)1 year D)2 years E)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 http://www.open2.net/open2static/source/file/root/0/30/19/124156/pressure_cube_b.jpg Would the pressure change be larger in salt water or fresh water?

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/m 3 ) 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? Changes by 2… http://www.tpub.com/fireman/25.htm

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 1)Buoyant force on brick in water -Volume= 0.2 m X 0.08 m X 0.05 m -F B =8 N 2)Buoyant force on ball in water -Volume= (4/3)pi(0.025)^2 -F B =0.6 N 3)Buoyant force on Diet Coke vs. Coke -Volume= same in both cases -F B =same -Coke sinks more than Diet Coke bec. Coke more dense (F gravity 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