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# Notes! 71234

SFSU
GPA 4.0
Physics 101
J. Fielder

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COURSE
Physics 101
PROF.
J. Fielder
TYPE
Class Notes
PAGES
4
WORDS
KARMA
25 ?

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This 4 page Class Notes was uploaded by Adrianna Minichiello on Friday March 13, 2015. The Class Notes belongs to 71234 at San Francisco State University taught by J. Fielder in Spring2015. Since its upload, it has received 24 views. For similar materials see Physics 101 in Science at San Francisco State University.

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Date Created: 03/13/15
Physics Lecture Notes Week of March 9 2015 Pressure Buoyancy and Floatation March 9 2015 0 Solid materials are not all brittle They have a certain degree to which they can bend Elasticity 0 We can harness this elasticity to exert forces 0 Hooke s Law Force AX O The more you stretch or compress a spring the more force it can exert when released 0 This works up until the elastic limit of the material Pressure 0 Pressure is force per unit area 0 P FArea 0 SI Unit Nm2 0 1 Nm2 1 Pascal Pa 0 Other common pressure units 0 Pounds per Square Inch PSI tires etc 0 mm Hg blood pressure 0 inches Hg weather barometer 0 Pressure changes with contact area 0 The same force applied over a smaller area results in greater pressure think of poking a balloon with your finger and then with a needle 0 Pressure in LiquidsFluids O A uid is any substance that can ow liquids and gases 0 Liquid pressure weight density X depth 0 Total pressure liquid pressure atmospheric pressure 0 Atmospheric pressure 1013 kPa More Buoyancy Archimedes and Pascal s Principle March 11 2015 0 Water seeks its own level 0 Pressure is the same at the bottom of each container because they are connected 0 Pressure depends on DEPTH not volume 0 Pressure depends on depth weight of what s above determines pressure Buoyancy 0 Pressure depends on depth 0 Consider a large rock The bottom of the rock is at a lower depth so it feels more pressure than the top of the rock 0 This net upward force is called buoyancy Displacement of liquid 0 Objects displace a volume of liquid equal to their volume 0 This is useful for determining irregular or odd volumes Archimedes Principle 0 The buoyant force on an immersed object is equal to the weigh tof the uid it displaces I Immersed partially or completely submerged I Buoyant force does not depend on depth Helpful to know 0 Properties of water I 1kg of liquid water takes up 1 L of volume I 1kg of water weights 10 N I Water has a mass density of 1gcm3 or 1000 kgm3 0 What would it take for the buoyant force to be greater than the weight of the object displacing some volume of water Example What is the water pressure at the bottom of a 20m deep lake Will it oat 0 Any object immersed in a liquid will feel a buoyant force but that doesn t mean it will oat I 1 An object more dense than the uid in which it is immersed will sink I 2 An object less dense than the uid in which it is immersed will oat I 3 Same density Neither will sink or oat Remember Archimedes Principle An immersed object feels a buoyant force equal to the weight of the uid it displaces Objects with density less than the uid will oat Floating depends on average density 0 Imagine a sold block of iron and a sheet of iron bent into a boatlike shape 0 We have increased the effective volume of the sheet of iron 0 This lowers its average density allowing it to oat A oating object displaces a weight of uid equal to its own weight Pascal s Principle 0 A change in pressure at any point in an enclosed uid is transmitted undiminished to all points in the uid Example Pascal s Principle 0 Remember Pressure forcearea 0 Pressure on both piston is the same 0 Imagine the are of the small piston is 1 m2 and the area of the large piston of 5m2 0 What force acts on the large piston I Side A Fmg Side B P 100 Pa P FA 5 X 100 F F 500 N 10 kg X 10 ms2 F 100 N P FA 100 N 1 m2 P 100 Pa Gas Pressure the atmosphere Friday March 13 2015 Gases 0 It s easy to forget about atmospheric pressure but it is pushing all the time 0 100kPa 0 Main difference between gasses and solidsliquids Gases are compressible 0 In an ideal gas we can ignore interactions between molecules The molecules speed depends only on temperature 0 Most common example of gas is air which is a mixture of nitrogen and oxygen gases 0 Density of water 1000 kgm3 0 Density of air 12 kgm3 0 Water is 1000 times denser than air 0 Weight of air is small but not ignorable 0 Atmospheric Pressure 0 Density of air in the atmosphere decreases with increasing altitude 0 Most of atmosphere in the 10 km of altitude 0 Drinking out of straw and Vacuum Pressure 0 There s no such thing as vacuum forcepressure 0 With lungs you reduce pressure in your mouth and the straw 0 The higher pressure on the outside pushes the liquid up the straw 0 Force due to the pressure difference must match or exceed the weight 0 Boyle s Law Pressure vs Volume 0 Density of a gas increases as it is compressed 0 As the density of a gas increase the pressure in the gas also increases I P1V1 P2V2 FB weight of air displaced mfg p mV I m pV density 9 water 1000 kgm3 9 air 12 kgm3 Example Two cylinders P1 200 Pa V2 12 V1 P1V1 P2V2 PaV1 V1 P2 Pa Archimedes Principle 0 An object surrounded by air is buoyed up by a force equal to the weight of the air displaced 0 1 m3 of air has a mass of about 12 kg whereas 1 m3 of water has a mass of 1000 kg 0 Buoyant force due to air is usually very small Which object feels the largest buoyant force due to the atmosphere 0 An elephant Which object feels a buoyant force due to the atmosphere that is closest in magnitude to its weight 0 A heliumfilled balloon Bernoulli s Principle 0 Where the speed of a uid increases internal pressure in the uid decreases 0 Due to continuous ow of a uid what goes in must come out 0 Fluid ows faster through a narrower pipe

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