Ch. 19 Liquids Notes

Name_________________________________Block______Date______March 2007___Ch. 19 Liquids NotesMrs. PeckObjectives:1. Describe what determines the pressure of a liquid at any point. 19.12. Explain the cause of a buoyant force on an immersed or submerged object. 19.23. Relate the buoyant force on an immersed or submerged object to the weight of the fluid itdisplaces. 19.34. Describe what determines whether an object will sink or float in a fluid. 19.45. Given the weight of a floating object, determine the weight of fluid it displaces. 19.56. Describe how Pascal’s principle can be applied to increase the force of a fluid on a surface. 19.6Liquid- the state or phase in which matter takes the shape of its container (molecules slide past ea. oth)any material that flows and takes on the shape of its containerthe atoms or molecules have enough kinetic energy (movement) to overcomethe attractions between them. The particles slide past another until the liquidtakes on the shape of its container.can change shape yet not change volume (shape determined by container)fluid-anything that flows; in particular; any liquid or gas3 properties:1. Surface Tension-force on the particles at the surface of a liquid that causes theliquid to form spherical droplets2. Viscosity- the liquid’s resistance to flowthe more attraction between the particles in a liquid---the more viscous3. cohesion- intermolecular attraction that holds molecules togetherhydrophobic- a substance that does NOT “like” water, repels watereg: lipids: fat............solid found in animalsoil............liquid found in plantswax.........solid found in both plants & animalshydrophilic- a substance that “likes” waterreadily diffuses or dissolves in waterdiffusion- when 2 fluids are introduced to same space, because of KE of molecules, theywill push & jostle each other until they are uniformly mixed.motion of molecules continually tries to mix themsome molecules are denser and sink into another fluid-at surface b/w 2 fluids diffusion occurs

Liquid Pressure 19.1 2fluids exert pressure equally in all directions (onto walls of its container)Pressure = Force ÷ Area SI unit Pascal (Pa) = N/ m 2 Symbol P-fluid pressure depends on weight of fluid and area on contact-weight of fluid depends on fluid’s densitydenser liquids exert more pressurefluid pressure increases as depth increases fig. 19.2liquids with same density: pressure greater at bottom of deeper liquidliquid pressure at any point in the liquid depends only on its density and its depthnot on the shape of the containerdenser liquids exert more pressureliquid molecules are virtually incompressible; therefore the density of a liquiddoes not change with depthfluids flow from areas of high pressure to areas of low pressure until pressureis equalpressure liquid = weight density X depth wt density = wt/ vol wt=mg

19.1 Liquid Pressure 3fig. 19.3 water pressure is greater at the bottom of the deeper lake, not the lakewith the most water.why are dams built thicker at bottom?fig. 19.4 pressure of liq. is same at any given depth below surface regardless of shape ofcontainerat any point in a liq., the forces that produce pressure are exerted equally in all directionsfig. 19.5 when liquid is pressing against a surface (obj. or container), there is a net force directedperpendicular to the surface.if there is a hole in the container’s surface then liquidwill initially flow out perpendicular to the surface(gravity eventually causes path of flow to curve)at greater depths....net force & Pressure is greater....velocity of escaping liq. is greater

19.2 Buoyancy 4Buoyancy- the apparent loss of weight of an object immersed or submerged in a fluidBuoyant Force- the net upward force exerted by a fluid on a submerged or immersed objectFb-the upward force that fluids exert on all matter opposite direction of gravitythe force exerted horizontally on one side of an object is = to the force exertedon the opposite side (at the same depth). The horizontal forces cancel each other out,therefore, there is no sideways movementbecause pressure increases with depth, the force on the bottom of an object(acting upward) is greater than the force on the top (acting downward)Fb-the difference in the upward and downward forces is the buoyant forceas an object increases it’s depth, the difference between the upward and downwardforces on the object is the same (buoyant force) at any given depth.buoyant force for an object does not change with depthsee fig. 19.6forces acting horizontallycancel each other out. Theyare equal in magnitude &act in opposite directions-when the weight of a submerged objectis greater than the buoyant force, the object sinks-when the weight of a submerged objectis = to buoyant force, the object stays at that level-when the weight of a submerged objectis less than buoyant force, the object floats

19.2 Buoyancy 5displaced-term applied to the fluid that is moved out of the way when an object is placed in fluid.fig. 19.7A submerged object always displaces a volume of liq. equal to its own volumeused to determine volume of an irregular shaped object (completely submerged)vol. object = vol. water displacedArchimedes’ Principle 19.3Archimedes’ Principle-the relationship between buoyancy and displace fluid:an immersed object is buoyed up by a force = to weight of volume of fluid it displacesimmersed- either completely or partially submerged-true for liquids or gases wh/ are both fluidsweight of object has nothing to do with the buoyant forceit is the weight of the displaced fluid that determines buoyant force-Newton’s 3rd law: if you put something in water that pushes 100 N of water outof the way (displaces 100N), then the water pushes back with a force of 100N-buoyant force is = & opp. to the weight of water you displacewhy does Fb not act sideways on a submerged object?what happens to the Fb as an object moves deeper in a fluid?fig 19.9 if you completely submerge a 1 liter bottle, it will displace 1L and be buoyed upby the weight of the volume of water displaced.1 ml = 1 g 1L = ______kgwhat is volume of water displaced?____________________what is mass of volume of water displaced?_______________________what is the weight of volume of water displaced?_____________________________what is the Fb acting on the bottom of the 1 liter bottle?________________________

19.3 Archimede’s Principle 6upward force due to fluid pressure on bottom of a submerged object, minus thedownward force due to liquid pressure on the top of object equals the weight of fluiddisplaced.fig. 19.11 Adepth makes no difference on buoyant force....even though there is more pressure with greater depththe differences in the pressures on the bottom and topof an object are the same at any depthBCyellow ?’s pg 279if you hold someone in water, they “appear” to have less weight. Why?apparent weight: equal to the actual weight minus buoyant force fig 19.10Fa = Fg - FbExamples:1. if submerge 2.5 kg block, it displaces 3L of water: then Fb and Fa is?Fg of block =Fb = weight of water displaced( 1 L = 1 kg for water only)Fa = Fg - Fbblock seems to weigh ______________ in water

Examples: 7Fa = Fg - Fb2. If a 300g block is submerged & displaces 200 ml of water, then what is the object’s Fb and Fa?3.A 2 liter container filled with mercury has a mass of 27.2 kg and weighs 272 N. When it issubmerged in water, what is the buoyant force acting on it? What is it’s apparent weight?4. A 5 kg mass is hung from a newton scale and is submerged in a large beaker of water. Whensubmerged the newton scale reads 15N. What is the buoyant force?

19. 3 Archimede’s Principle 8Archimede’s Principle: an object completely or partially immersed in a fluid is pushed up by a forcethat is equal t the weight of the displaced fluid.buoyant force= density x g x volumeD= m therefore m x V = m times g = Fg fluidVVFb = DgVuse this equation for submerged or floating objectsfor floating objects: object only partially submerged therefore, use vol. of submerged portionthe Fb floating objects = weight of vol. of fluid displace and = weight of object itselfFb = DgVExamples1. An object will float when placed in water so that most of the object is suspended below thesurface, and only a small fraction sticks up above the water line. The dimensions of the object are.02m x .02m x .02m. Its density is 950 kg/ m 3 . What is the buoyant force acting on the object?2. An ice cube, whose density is 917 kg/ m 3 , has the dimensions of 3cm x 2cm x 2cm and isfloating in a glass of water. What is the buoyant force acting on the ice cube?PB pg 129 #15 and pg 133 A-8

19.4 Does it Sink, or Does it Float? 9smaller obj. displaces less water ...smaller buoyant force acts on itlarger obj. displaces more water...larger buoyant force acts on it*submerged objects volume not its weight determines buoyant forcesinking or floating dep. on: both buoyant force (up) & its weight (down)how great the Fb is compared w/ obj.’s weightDensity of waterDwater =1g/ 1ml= 1g/ 1cm 3 Dwater = 1000g / 1000ml Dwater= 1kg/ 1literDwater= 1000 kg/ m 3 = 1kg/ 1liter3 Rules: fig. 19.121. an obj. more dense than the fluid in wh/ it is immersed sinks2. an obj. less dense than the fluid in wh/ it is immersed floats3. an obj. w/ density equal to the density of the fluid in wh/ it is immersed neither sinks nor floats*to float more easily you must reduce your density.....D=m.....increase your V.....decrease your DVto float in water: object’s density < 1kg/ 1liter or 1000kg/ m 3to float in salt water: object’s density < 1.03 kg/ 1 liter or 1030kg/ m 319.5 Flotationdensity of object compared to density of fluid determines whether an object will sink or floatif change the shape of an object to occupy a greater volume....it will displace a greater volume ofliquid. If weight of displaced volume of water is greater than weight of object...object will float.fig19.14principle of floatation- a floating object displaces a weight of fluid equal to its own weightwhen weight of displace water = weight of the object...it will sink no farther because the Fb equalsthe weight of the object.every ship must be designed to displace a weight of water equal to its own weight.as a floating cargo ship takes on more cargo.....the ship sinks deeper in the water....asit sinks.....it displaces more water, ....so the buoyant force increasesyellow ?’s pg 283 #1 and 2

19.6 Pascal’s Principle 10Pascal’s Principle: the change in pressure on one part of a confined fluid is = to the change inpressure on any other part of the confined fluida change in pressure at any point in an enclosed fluid will be transmitted equally to allparts of the fluidhydraulic device- a device that uses liquids to transmit pressure from one point to anotheruse liquids because they are virtually incompressible, (can’t be squeezed into a smaller space)∆P = F1 = F2 a small force exerted over a small area will result inA1 A2 a large force exerted over a large areaFFused in brakes in cars: foot applies a low force on a small area of brake fluid which exerts a largeforce over a large area onto the brake pad...which brings car to a stop(same amount of Pressure in both cases F )AHydraulics work because:1. one piston is larger than the other piston2. pascal’s principle is operating3. liquids transmit pressure undiminished4. liquids transmit pressure in all directions equally5. liquids are incompressibleuse hydraulic fluid (usually type of oil) instead of water