University:
The University Of Western Ontario
Course:
2223B | Organic Chemistry of Biological Molecules
Academic year:

2023
Views:

313

Pages:

2
Author:

Cole M.

Fluids at Rest The Buoyant Force does not get smaller as you sink: On earth, surface pressure is about 101 kPa (or 1 atm). 1 Pa = 1 N/m2 and 1 atm = 101.325 kPa Unlike with air, pressure rapidly increases with water (since it is much more dense than air) Pressure increases as you dive deeper into water because there is more and more water sitting on top of you (weighing - down on you → on another planet the pressure could be different) Buoyant force is the force that allows something to stay afloat The buoyant force is constant no matter the surrounding pressure (since water is incompressible) Archimedes principle shows the equation that gives you the size of the buoyant force 𝐹! = ⍴"#$%& 𝑉'(!)#&*+% 𝑔 ⍴"#$%& is around 1 g/cm3 or 1000 kg/m3 Also, the buoyant force on an object equals the total weight of water that it pushes out of the way Pressure & Pascal’s Principle: A fluid is anything that takes the shape of its container (gas or liquid) The difference between a liquid and a gas is that a gas is compressible while a liquid is incompressible (technically compressible but by only such a small degree that we don’t even consider it) Pressure is defined as a force per given area Pascal’s Principle states that any external pressure that is applied to a fluid, that pressure will be distributed throughout the fluid equally (P = F/A) Pressure at a Depth in a Fluid: Density = mass/volume (kg/m3) 𝑃 = ⍴ 𝑔ℎ where h is the depth in the liquid and rho is the density of the liquid (gives in units of Pa) Finding Height of Fluid in a Barometer: Specific gravity is the ratio of how dense that substance is to water (mercury is 13.6 meaning its 13.6 times denser) Archimedes Principle & Buoyant Force: Remember, pressure is acting in every single direction when submerged, it’s just that some vectors cancel out with one another, where only the upwards/downwards ones end up mattering The pressure at a lower part of the submerged object will be higher than the pressure on top 𝐹+%$ = ⍴ 𝑔𝑉 where V is the volume of the liquid displaced (also the volume of the object) V and ⍴ can both me replaced by mass, leaving you with mass of the liquid displaced time gravity to give you the net force, stating that the weight of liquid displaced is the net force acting on the object The fact that there is more pressures on the bottom of an object than the top is why things float and is known as buoyant force To find how much of an object is submerged, use the formula 𝑉𝑠𝑢𝑏𝑚𝑒𝑟𝑔𝑒𝑑 𝑉𝑜𝑏𝑗𝑒𝑐𝑡 = ⍴𝑜𝑏𝑗𝑒𝑐𝑡 ⍴𝑙𝑖𝑞𝑢𝑖𝑑 Specific Gravity: Specific gravity of an object is simply - ⍴𝑜𝑏𝑗𝑒𝑐𝑡 ⍴𝑤𝑎𝑡𝑒𝑟 (a ratio between the density of the object to water → can even be used as a ratio to other types of liquids such as oil) and can be used in order to find the fraction of submerged object in th liquid (as shown in the equation two above) No units because they end up canceling each other out For something that floats, the specific gravity tells you the fraction of that object that is submerged Anything over 1, then the object will not float (it’s density is greater than that of water’s) Can use a ratio of: 𝐹𝐵 𝐹𝑔 = ⍴ ⍴ 𝑙𝑖𝑞𝑢𝑖𝑑 𝑜𝑏𝑗𝑒𝑐𝑡 (??) Extra Notes: Absolute pressure is simply the atmospheric pressure plus the gauge pressure 𝑃 = 𝑃/ + ⍴𝑔ℎ where PO is atmospheric pressure