2Topics Pressure Pressure in a Liquid Buoyancy Archimedes’ Principle FlotationPascal’s PrincipleSurface TensionCapillarity
3Pressure Units: N/m2 -- named the Pascal (Pa) very small unit; kPa lb/in2
4Pressure in a Liquidaverage water pressure acting against dam depends on average depth of waternot on volume of water held backlarge shallow lake exerts only one half the average pressure that the small deep pond exerts.
5Pressure in a LiquidLiquid pressure is same for any given depth below surfaceIndependent of shape of the containerLiquid rises to same height in each
6Pressure in a Liquidforces of a liquid pressing against a surface add up to a net force that is perpendicular to the surface
7Pressure in a LiquidWater pressure acts perpendicular to the sides of a container, and increases with increasing depth
8Buoyancybuoyant force is a consequence of pressure increasing with depthgreater pressure against the bottom of a submerged object produces an upward buoyant force
9Water DisplacementA submerged object displaces a volume of water equal to volume of object
10Archimedes’ Principle An immersed body is buoyed up by a force equal to the weight of the fluid it displaces
11Check YourselfDoes Archimedes' principle tell us that if an immersed object displaces liquid weighing 10 N, the buoyant force on the object is 10 N?
12Check YourselfDoes Archimedes' principle tell us that if an immersed object displaces liquid weighing 10 N, the buoyant force on the object is 10 N?Yes. Looking at it another way, the immersed object pushes 10 N of fluid aside. The displaced fluid reacts by pushing back on the immersed object with 10 N.
13Check YourselfA 1-liter container completely filled with lead has a mass of 11.3 kg and is submerged in water. What is the buoyant force acting on it?
14Check YourselfA 1-liter container completely filled with lead has a mass of 11.3 kg and is submerged in water. What is the buoyant force acting on it?The buoyant force is equal to the weight of the liter of water displaced. One liter of water has a mass of 1 kg, and a weight of 9.8 N. So the buoyant force on it is 9.8 N.
15Sinking and FloatingIf an object is denser than the fluid in which it is immersed, it will sink.If an object is less dense than the fluid in which it is immersed, it will float.If an object has a density equal to the density of the fluid in which it is immersed, it will neither sink nor float.
16FlotationThe weight of a floating object equals the weight of the water displaced by the submerged part.
17Boats & FloatingA floating object displaces a weight of fluid equal to its own weight.When an iron boat displaces a weight of water equal to its own weight, it floats. This is sometimes called the principle of flotation:
18Check YourselfWhy is it easier for you to float in salt water than in fresh water?
19Check YourselfWhy is it easier for you to float in salt water than in fresh water?It's easier because salt water is more dense. In order to displace your weight, a lesser amount of your body is immersed—you don't “sink” as far. You'd float even higher in mercury (density 13.6 g/cm3), and you'd sink completely in alcohol (density 0.8 g/cm3).
20Floating IcebergsThe tip of a floating iceberg above the ocean surface is approximately 10 percent of the whole iceberg.That's because ice is 0.9 times the density of water, so 90 percent of it submerges in the water.
21Floating Iceberg Newfoundland. divert path away from rig by towing them with ships!They estimated the weight at 300,000,000 tons.
22Pascal’s PrincipleA change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.
23Pascal’s Principleincreased air pressure produced by an air compressor transmitted to the surface of oil in underground reservoiroil in turn transmits the pressure to a piston, which lifts the automobile.
24Cohesion & AdhesionMolecules experience strong intermolecular attractive forcescohesive forces -- between like moleculeswater dropletssurface tensionadhesive forces – between unlike moleculescapillary action
25Cohesive ForcesPolar nature of water molecules allows intermolecular bonding
26Cohesive ForcesMolecules on surface have stronger intermolecular bonds with nearest neighbors on surface than molecules below surface
27Surface Tension Surface tension of water 72 dynes/cm at 25oC decreases with increased temperature
28Capillary Action Result of adhesion and surface tension Adhesion of water to the walls of the tube cause an upward force on the liquidSurface tension holds surface intact so that whole liquid surface is dragged upward