Name Block Date March 2007 Ch. 19 Liquids Notes Mrs. Peck Objectives: 1. Describe what determines the pressure of a liquid at any point. 19.1 2. Explain the cause of a buoyant force on an immersed or submerged object. 19.2 3. Relate the buoyant force on an immersed or submerged object to the weight of the fluid it displaces. 19.3 4. Describe what determines whether an object will sink or float in a fluid. 19.4 5. Given the weight of a floating object, determine the weight of fluid it displaces. 19.5 6. Describe how Pascal s principle can be applied to increase the force of a fluid on a surface. 19.6 Liquid- 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 container the atoms or molecules have enough kinetic energy (movement) to overcome the attractions between them. The particles slide past another until the liquid takes 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 gas 3 properties: 1. Surface Tension-force on the particles at the surface of a liquid that causes the liquid to form spherical droplets 2. Viscosity- the liquid s resistance to flow the more attraction between the particles in a liquid---the more viscous 3. cohesion- intermolecular attraction that holds molecules together hydrophobic- a substance that does NOT like water, repels water eg: lipids: fat...solid found in animals oil...liquid found in plants wax...solid found in both plants & animals hydrophilic- a substance that likes water readily diffuses or dissolves in water diffusion- when 2 fluids are introduced to same space, because of KE of molecules, they will push & jostle each other until they are uniformly mixed. motion of molecules continually tries to mix them some molecules are denser and sink into another fluid -at surface b/w 2 fluids diffusion occurs
Liquid Pressure 19.1 2 fluids 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 density denser liquids exert more pressure fluid pressure increases as depth increases fig. 19.2 liquids with same density: pressure greater at bottom of deeper liquid liquid pressure at any point in the liquid depends only on its density and its depth not on the shape of the container denser liquids exert more pressure liquid molecules are virtually incompressible; therefore the density of a liquid does not change with depth fluids flow from areas of high pressure to areas of low pressure until pressure is equal pressure liquid = weight density X depth wt density = wt/ vol wt=mg
19.1 Liquid Pressure 3 fig. 19.3 water pressure is greater at the bottom of the deeper lake, not the lake with 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 of container at any point in a liq., the forces that produce pressure are exerted equally in all directions fig. 19.5 when liquid is pressing against a surface (obj. or container), there is a net force directed perpendicular to the surface. if there is a hole in the container s surface then liquid will 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 4 Buoyancy- the apparent loss of weight of an object immersed or submerged in a fluid Buoyant Force- the net upward force exerted by a fluid on a submerged or immersed object Fb-the upward force that fluids exert on all matter opposite direction of gravity the force exerted horizontally on one side of an object is = to the force exerted on the opposite side (at the same depth). The horizontal forces cancel each other out, therefore, there is no sideways movement because 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 force as an object increases it s depth, the difference between the upward and downward forces on the object is the same (buoyant force) at any given depth. buoyant force for an object does not change with depth see fig. 19.6 forces acting horizontally cancel each other out. They are equal in magnitude & act in opposite directions -when the weight of a submerged object is greater than the buoyant force, the object sinks -when the weight of a submerged object is = to buoyant force, the object stays at that level -when the weight of a submerged object is less than buoyant force, the object floats
19.2 Buoyancy 5 displaced-term applied to the fluid that is moved out of the way when an object is placed in fluid. fig. 19.7 A submerged object always displaces a volume of liq. equal to its own volume used to determine volume of an irregular shaped object (completely submerged) vol. object = vol. water displaced Archimedes Principle 19.3 Archimedes Principle-the relationship between buoyancy and displace fluid: an immersed object is buoyed up by a force = to weight of volume of fluid it displaces immersed- either completely or partially submerged -true for liquids or gases wh/ are both fluids weight of object has nothing to do with the buoyant force it 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 out of the way (displaces 100N), then the water pushes back with a force of 100N -buoyant force is = & opp. to the weight of water you displace why 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 up by the weight of the volume of water displaced. 1 ml = 1 g 1L = kg what 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 6 upward force due to fluid pressure on bottom of a submerged object, minus the downward force due to liquid pressure on the top of object equals the weight of fluid displaced. fig. 19.11 A depth makes no difference on buoyant force... even though there is more pressure with greater depth the differences in the pressures on the bottom and top of an object are the same at any depth B C yellow? s pg 279 if you hold someone in water, they appear to have less weight. Why? apparent weight: equal to the actual weight minus buoyant force fig 19.10 Fa = Fg - Fb Examples: 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 - Fb block seems to weigh in water
Examples: 7 Fa = Fg - Fb 2. 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 is submerged 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. When submerged the newton scale reads 15N. What is the buoyant force?
19. 3 Archimede s Principle 8 Archimede s Principle: an object completely or partially immersed in a fluid is pushed up by a force that is equal t the weight of the displaced fluid. buoyant force= density x g x volume D= m therefore m x V = m times g = Fg fluid V V Fb = DgV use this equation for submerged or floating objects for floating objects: object only partially submerged therefore, use vol. of submerged portion the Fb floating objects = weight of vol. of fluid displace and = weight of object itself Fb = DgV Examples 1. An object will float when placed in water so that most of the object is suspended below the surface, 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 is floating 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? 9 smaller obj. displaces less water...smaller buoyant force acts on it larger obj. displaces more water...larger buoyant force acts on it *submerged objects volume not its weight determines buoyant force sinking or floating dep. on: both buoyant force (up) & its weight (down) how great the Fb is compared w/ obj. s weight Density of water Dwater =1g/ 1ml= 1g/ 1cm 3 Dwater = 1000g / 1000ml Dwater= 1kg/ 1liter Dwater= 1000 kg/ m 3 = 1kg/ 1liter 3 Rules: fig. 19.12 1. an obj. more dense than the fluid in wh/ it is immersed sinks 2. an obj. less dense than the fluid in wh/ it is immersed floats 3. 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 D V to float in water: object s density < 1kg/ 1liter or 1000kg/ m 3 to float in salt water: object s density < 1.03 kg/ 1 liter or 1030kg/ m 3 19.5 Flotation density of object compared to density of fluid determines whether an object will sink or float if change the shape of an object to occupy a greater volume...it will displace a greater volume of liquid. If weight of displaced volume of water is greater than weight of object...object will float. fig19.14 principle of floatation- a floating object displaces a weight of fluid equal to its own weight when weight of displace water = weight of the object...it will sink no farther because the Fb equals the 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...as it sinks...it displaces more water,...so the buoyant force increases yellow? s pg 283 #1 and 2
19.6 Pascal s Principle 10 Pascal s Principle: the change in pressure on one part of a confined fluid is = to the change in pressure on any other part of the confined fluid a change in pressure at any point in an enclosed fluid will be transmitted equally to all parts of the fluid hydraulic device- a device that uses liquids to transmit pressure from one point to another use 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 in A1 A2 a large force exerted over a large area F F used in brakes in cars: foot applies a low force on a small area of brake fluid which exerts a large force over a large area onto the brake pad...which brings car to a stop (same amount of Pressure in both cases F ) A Hydraulics work because: 1. one piston is larger than the other piston 2. pascal s principle is operating 3. liquids transmit pressure undiminished 4. liquids transmit pressure in all directions equally 5. liquids are incompressible use hydraulic fluid (usually type of oil) instead of water