Pressure F Pressure is force per unit area: P = A The direction of the force exerted on an object by a fluid is Pressure toward the object and perpendicular to its surface. At a microscopic level, the force is associated with the atoms and molecules in the fluid bouncing elastically from the surfaces of the object. The SI unit for pressure is the pascal. 1 Pa = 1 N/m2 1 2 The arrows below illustrate the directions of the force of Atmospheric pressure pressure acting on a shark at various point of its body. They show that the force is always pointing towards and perpendicular to the surface of the shark. 1 atm = 1.01 x 105 Pa = 14.7 psi = 760 torr (psi = pounds / square inch) At atmospheric pressure, every square meter has a force of 100,000 N exerting on it, coming from air molecules bouncing off it! Why don’t we, and other things, collapse because of this pressure? We, human, have an internal pressure of 1 atmosphere. General solid objects do not collapse because they possess an elastic modulus (i.e., rigidity) enabling them to deform only 3 little when compressed by the atm. pressure. 4 Conceptual example 1: Blood pressure Conceptual example 2. Rank by pressure Question A typical reading for blood pressure is 120 over 80. What do the two numbers represent? What units are A container, closed on the right side but open to the atmosphere on the left, is almost completely filled with they in? water, as shown. Three points are marked in the container. Rank these according to the pressure at the points, from Ans. highest pressure to lowest. 120 mm Hg (millimeters of mercury) is a typical systolic pressure, the pressure when the heart contracts. 1. A = B > C 80 mm Hg is a typical diastolic pressure, the blood 2. B > A > C 3. B > A = C pressure when the heart relaxes after a contraction. PC 4. C > B > A 760 mm Hg is the typical atmospheric pressure. The blood 5. C > A = B pressure readings are not absolute – they tell us how 6. some other order much above atmospheric pressure the blood pressure is. PB PB 5 See explanation on the next page. 6 1 Pressure in a static fluid Pressure in a Static Fluid A static fluid is a fluid at rest. In a static fluid: To understand the equation, • Pressure increases with depth. P2 = P1 + ρgh, consider a • Two points at the same vertical position experience the column of fluid with area A Area, A same pressure, no matter what the shape of the container. and height h as shown. The weight of the fluid column is P If point 2 is a vertical distance h below point 1, and the ρghA. This weight will cause 1 h P pressure at point 1 is P1, the pressure at point 2 is: an extra pressure of 2 ρghA/A = ρgh at the bottom PP21=+ρ gh of the fluid column and Point 2 does not have to be directly below explains the equation. point 1 - what matters is the vertical distance. 7 8 Conceptual example 2b. Rank by pressure Measuring pressure The relationship between pressure and depth is exploited in PP21=+ρ gh manometers (or barometers) that measure pressure. What is the pressure at A standard barometer is a tube P1 (usually with one end sealed. The sealed ~ 0) this level? end is close to zero pressure, while the other end is open to the h atmosphere. The pressure P difference between the two ends of 2 the tube can maintain a column of 1. Cannot be determined fluid in the tube, with the height of 2. PA – ρfluidg(0.1m) the column being proportional to 3. PA + ρfluidg(0.1m) the pressure difference. 4. Atmospheric pressure 5. 2 and 4 9 6. 3 and 4 10 Example 3. Water pressure The origin of the buoyant force At the surface of a body of water, the pressure you The net upward buoyant force is experience is atmospheric pressure. Estimate how deep PP= + ρ gh you have to dive to experience a pressure of 2 21 the vector sum of the various atmospheres. Given that density of water is 1000 kg/m3 forces from the fluid pressure. and g = 10 m/s2 (or N/kg). Because the fluid pressure Solution: increases with depth, the upward force on the bottom surface is larger than the downward force on PP=+ρ gh 21 the upper surface of the immersed object. 200000 Pa=+ 100000 Pa (1000 kg/m3 ) ×× (10 N/kg) h FPAghAgVnet=Δ×=ρ fluid ×=ρ fluid h works out to 10 m. Every 10 m down in water increases This is for a fully immersed object. the pressure by 1 atmosphere. For a floating object, h is the height below the water level, so we get: 11 FgVnet= ρ fluid disp 12 2 Conceptual example 4. When the object goes Conceptual example 4. When the object goes deeper (cont’d) deeper … Explanation: PP21= + ρ gh Question: If we displace an object deeper into a fluid, what happens to the buoyant force acting on the object? You may assume that the fluid density is the same at all depths. The buoyant force … 1. increases 2. decreases 3. stays the same If the fluid density does not change with depth, all the forces increase by the same amount, leaving the buoyant force unchanged! 13 14 Flowing fluids – fluid dynamics We’ll start with an idealized fluid that: 1. Has streamline flow (no turbulence) 2. Is incompressible (constant density) 3. Has no viscosity (flows without resistance) Fluid Dynamics 4. Is irrotational (no swirling eddies) 15 16 The continuity equation An application of continuity We generally apply two equations to flowing fluids. One comes from the idea that the rate at which mass flowing past a point The concept of continuity (a constant mass flow rate) is is constant, otherwise fluid builds up in regions of low flow rate. applied in our own circulatory systems. When blood flows from ΔΔmVAxρ ρ Δ a large artery to small capillaries, the rate at which blood mass flow rate == = =ρAv leaves the artery equals the sum of the rates at which blood ΔΔtt Δ t flows through the various capillaries attached to the artery. The mass flow rate of a fluid must be continuous. Otherwise fluid accumulates at some points. So, we have: ρ111AvAv= ρ 2 22 Density of fluid ⇒ A11vAv= 2 2 (continuity equation) doesn’t change 17 18 The fluid flows faster in narrow sections of the tube. 3 Applying energy conservation to fluids Bernoulli’s equation Our second equation for flowing fluids comes from energy conservation. P1A1 P2A2 U1 + K1 + Wnc = U2 + K2 11 11 mgy++=+ mv22 W mgy mv ρρgyvP+ 22+= ρρ gyvP + + 1122nc 2 2 111222 22 WFxFxPAxPAxnc =Δ−Δ11 22 = 111 Δ− 222 Δ 11 mgy++Δ=++Δ mv22 P A x mgy mv P A x 111112222222 Dividing by volume: 11 ρρgyvP++=++22 ρρ gyvP 111222 22(Bernoulli’s equation)19 20 Simple Interpretation of Bernoulli’s equation Fluid in a pipe Wnc by the forces of pressure in moving a unit volume of the – question 1 fluid from point 1 to point 2. The pipe is narrow where point A is located, widens out 1 1 some where points B and C are located, and widens again ρgy + ρv2 + P + (−P ) = ρgy + ρv2 where point D is located. The level of points C and D is 1 2 1 1 2 2 2 2 lower than that of points A and B. If the fluid is at rest, rank the points based on their pressure. PE per unit KE per unit PE per unit KE per unit 1. Equal for all four 5. C=D>A=B volume of volume of volume of volume of 2. A>B=C>D 6. A>B>C>D the fluid at the fluid at the fluid at the fluid at 3. D>B=C>A 7. D>C>B>A point 1 point 1 point 2 point 2 4. A=B>C=D 8. It's ambiguous - there are two possible answers 21 22 vA > vB ⇒ PA < PB By similar argument, PC < PD Fluid in a pipe Fluid in a pipe – question 2 – question 3 The pipe is narrow where point A is located, widens out The pipe is narrow where point A is located, widens out some where points B and C are located, and widens again some where points B and C are located, and widens again where point D is located. The level of points C and D is where point D is located. The level of points C and D is lower than that of points A and B. If the fluid is flowing lower than that of points A and B. If the fluid is flowing from left to right, rank the points based on the fluid speed. from left to right, rank the points based on the pressure. 1. Equal for all four 5. C=D>A=B 1. Equal for all four 5. C=D>A=B 2. A>B=C>D 6. A>B>C>D 2. A>B=C>D 6. A>B>C>D 3. D>B=C>A 7. D>C>B>A 3. D>B=C>A 7. D>C>B>A 4. A=B>C=D 8. It's ambiguous - there are 4. A=B>C=D 8. It's ambiguous - there are two possible answers two possible answers Use the concept of continuity to figure out this problem.