Presentation on theme: "Clicker Question Room Frequency BA FB mplasticg"— Presentation transcript:
1 Clicker Question Room Frequency BA FB mplasticg A solid piece of plastic of volume V, and density ρplastic is floating partially submerged in a cup of water. (The density of water is ρwater.) What is the buoyant force on the plastic?A) ZeroB) ρplastic VC) ρwater VD) ρwater V gE) ρplastic V gFBmplasticgThe plastic is in equilibrium so FB = mplasticg = ρplastic V g !
2 Announcements CAPA assignment #13 is due on Friday at 10 pm. This week in Section: Assignment #6Start reading Chapter 11 on Vibrations and WavesI will have regular office hours 1:45 – 3:45 in the Physics Helproom today
3 Fluids in Motion: Fluid Dynamics Many, many different types of motion depending on particular properties of fluid: waves, rivers, geysers, tornados, hurricanes, ocean currents, trade winds, whirlpools, eddies, tsunamis, earthquakes, and on and on!We’ll focus on the simplest motion: flow
4 Fluids in Motion: Flow Two main types of flow: Laminar and Turbulent We’ll focus on the simplest flow: laminar flow
5 Analysis of Flow Use conservation laws! Analyzing flow at the force level is mathematically complex Use conservation laws!1) Conservation of Mass: the Continuity Equation2) Conservation of Energy: Bernoulli’s Equation
6 Continuity EquationConsider the flow of a fluid through a pipe in which the cross sectional area changes from A1 to A2The mass of fluid going in has to equal the mass of fluid coming out: conservation of mass!The speed of the fluids must be different!
7 Continuity EquationTo analyze this mass conservation, we calculate the mass flow rate:Flow rate in =must equalFlow rate out =
8 Mass Flow RatesFlow rate in =Flow rate out =Continuity Equation:
9 Continuity for Incompressible Fluids If the fluid is incompressible: ρ1 = ρ2 soDoes this make sense?
10 Clicker Question Room Frequency BA “Incompressible” blood flows out of the heart via the aorta at a speed vaorta. The radius of the aorta raorta = 1.2 cm. What is the speed of the blood in a connecting artery whose radius is 0.6 cm?vaorta2 vaorta(2)1/2 vaorta4 vaorta8 vaorta
11 Bernoulli’s Equation: Conservation of Energy Earlier in the course we learned:Applied to fluid flow, we consider energy of pieces of fluid of mass Δm
12 Bernoulli’s Equation: Incompressible Fluids Now usingand the continuity equation you getBernoulli’s Equation:
13 Applications of Bernoulli’s Equation Bernoulli’s Equation is behind many common phenomena!Curve ballsAerodynamic LiftSailing into the windTransient Ischemic Attacks (“mini-strokes”)Light objects getting sucked out your car windowShower curtains bowing inFlat roofs flying off houses in Boulder!Ping pong ball demo
14 Flat Roof ExampleWind flows over a flat roof with area A = 240 m2 at a speed of voutside = 35 m/s (125 km/h = 80 mi/h). What net force does the wind apply to the roof?Inside Outsidehhv = 35 m/sh
15 Clicker Question Room Frequency BA For an airplane wing (an air-foil) the upward lift force is derivable from Bernoulli’s equation. How does the air speed over the wing compare to the air speed under the wing? It is……FasterSlowerSameUnknownF = lift=(Pbot-Ptop)(Wing Area)fasterslowerOn the top side, the air has to travel farther to meet at the back edge of the wing!
16 Oscillations!Throughout nature things are bound together by forces which allow things to oscillate back and forth.It is important to get a deeper understanding of these phenomena!We’ll focus on the most common and the most simple oscillation: Simple Harmonic Motion (SHM)Requirements for SHM:There is a restoring force proportional to the displacement from equilibriumThe range of the motion (amplitude) is independent of the frequencyThe position, velocity, and acceleration are all sinusoidal (harmonic) in time
18 A Simple Harmonic Oscillator: Spring and Mass! Note:restoring force is proportional to displacementforce is not constant, so acceleration isn’t either: a = -(k/m)x“amplitude” A is the maximum displacement xmax, occurs with v = 0mass oscillates between x = A & x = -Amaximum speed vmax occurs when displacement x = 0a “cycle” is the full extent of motion as shownthe time to complete one cycle is the “period” Tfrequency is the number of cycles per second: f = 1/T (units Hz)