Physics 101, Practice Test 2 This is a closed book exam. You have one hour to complete it. The test has 30 problems. Submit your answers on the “Scantron Answer Sheet”. Use a black or blue ink pen or a #2 pencil. Fill a circle completely. Fill in you name and your UT ID. DO NOT use your social security number. Formulas: Fluids in general P = F/A pressure = force/area ρparticle = N/V, ρ = M/V particle density = number of particles/volume, density = mass/volume Ideal gas: PV = N*k*T Pressure * volume = number of particles * Boltzmann constant * absolute temp. B=w Buoyant force = weight of the displace fluid Incompressible fluids, laminar flow A1*v1 = A2*v2 Equation of continuity: (Area 1)*(velocity 1) = (Area 2)*(velocity 2) Bernoulli's equation: P + ρgh + ½ρv2 = pressure + density*g*height + ½*density*velocity2 = constant Volume flow rate = pi*(pressure difference)*(pipe diameter)4/[128*(pipe length)*viscosity) Turbulent flow Reynolds number = density * characteristic length * flow speed /viscosity Reynolds number > 2300 --> turbulent flow Heat c = ΔQ/(m*ΔT) specific heat = amount of energy/(mass * temperature change) Lf = ΔQ/m latent heat of melting or latent heat of fusion Lf = energy needed to melt/mass Lv = ΔQ/m latent heat of vaporization Lf = energy needed to vaporize/mass Stefan Boltzmann Law: Radiated power = emissivity * σ * T4 * Area Wien Law λmax = 3*106/T (λ in units of nanometer = 10-9 m and T is in Kelvin) Thermodynamics increase in internal energy = heat put into the system + work done on the system (1st ΔU = ΔQ + ΔW. law) Q1/T1 = Q2/T2. The best any heat engine (device) can do when moving heat. (2nd law) ΔS = ΔQ/T change in entropy ΔS = heat added or removed/ absolute temperature Water (H2O) Specific heat of ice 0.49 kcal/(kgoC) Latent heat of fusion 80 kcal/kg Specific heat of water 1 kcal/(kgoC) Latent heat of vaporization 540 kcal/kg Specific heat of steam 0.48 kcal/(kgoC) Problem 1: Your apartment window opens 20 meters (65 feet) above a lemonade stand. Your friend lowers a long plastic tube out the window until its end enters the tank of delicious lemonade far below. She then begins to suck on the other end of the tube in hopes of getting a free drink. To her dismay, she never tastes a drop because (A) atmospheric pressure cannot support a column of lemonade 20 meters tall. (B) lemonade is too thick to pass through a tube that long. (C) the tube has an average density that is larger than that of lemonade, so the buoyant force cannot support its weight. (D) the tube’s acceleration is downward, so it prevents the lemonade’s velocity from being upward. Problem 2: You are seated at a table in a Paris Café, contemplating life and watching the bubbles rise upward in your glass of carbonated water. You can think of many physical reasons why those carbon dioxide bubbles should rise upward through the water. Which of the follow observations is not true and therefore does not help explain why the bubbles rise in your Perrier? (A) A bubble displaces more than its weight of water. (B) The pressure below a bubble is greater than the pressure above it. (C) The pressure inside a bubble is much less than the pressure in the water around it. (D) A bubble’s average density is less than that of water. Problem 3: Why are failures in dams more likely to occur closer to the bottom of the dam? (A) Water is considerably more dense at the bottom. (B) Water pressure is higher at the bottom. (C) The temperature is higher at the bottom. (D) Water weighs more at the bottom. Problem 4: For an object that is floating on a fluid, (A) the weight of fluid displaced is less than its weight. (B) the weight of the fluid displaced is greater than the object’s weight. (C) the weight of the fluid displaced equals the object’s weight. (D) the weight of the object becomes less than in air. Problem 5: Suppose you are ill and have to go to the hospital for shot. When the nurse points the fluid – filled syringe skyward to clear the air out of it, he presses the plunger down, but no fluid comes out because the needle is plugged. When the plunger is pressed down and no fluid is moving, (A) the pressure of the fluid in the needed is much less than in the larger syringe. (B) the pressure of the fluid in the needle is much more than in the larger syringe. (C) the pressure of the fluid in the needle and much larger syringe are the same. (D) the pressure of the fluid everywhere is zero. Problem 6: The diameter of a pipe is doubled while the pressure difference across the pipe remains the same. The volume flow rate of the pipe increases by a factor of (A) 2. (B) 4. (C) 8. (D) 16. Problem 7: You construct a water-powered go-cart by attaching a large tank of water to a cart. The water can be forced out a nozzle by connecting a tank of high-pressure air to the water tank. In preparation for your first ride, you position this contraption facing away from a brick wall. You climb on and start spraying the water against the wall. To your delight you accelerate away from the wall and begin to move. When you have moved far enough away from the wall that the stream of water can no longer hit it, you (A) continue to accelerate as the water sprays out the back of the cart. (B) continue to move, but stop accelerating since the water can no longer push against the wall. (C) feel lighter than normal since the water hitting the ground will produce an upward force. (D) continue to accelerate, but less since it takes more force to accelerate a moving object than a stationary one. Problem 8: You are filling a jar of honey from the spigot at the bottom of a large barrel at the grocery store. The honey flows extremely slowly, so the store manager has the barrel refilled. Now the honey flows much more rapidly from the spigot because (A) the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases. (B) the density of the honey decreases as the height of the honey in the barrel increases. (C) the viscosity of the honey decreases as the height of honey in the barrel increases. (D) the viscosity of the honey increases as the height of honey in the barrel increases. Problem 9: As the Reynolds number increases, (A) flow is becoming more laminar. (B) flow is going from being viscous dominated to inertia dominated and therefore is becoming turbulent. (C) flow is slowing down. (D) flow is speeding up. Problem 10: You have just set up a weather station for your daughter to observe wind speeds at home. You had to move it up from the ground because when it was too close to the ground the wind speed readings are all too small. This is because (A) The air near the ground is too dense to move well. (B) The ground is hot so air movement there is always upward, never sideways. (C) The fluid in the boundary layer moves slower than the fluid farther away from the boundary. (D) Someone kept stepping on the wind speed gauge. Problem 11 One of the many things that make an artery blockage dangerous is (A) The amount of fluid passing through the blockage is proportional to the square of the diameter of the opening. (B) The amount of fluid passing through the blockage is proportional to the fourth power of the diameter of the opening. (C) The amount of fluid passing through the blockage is proportional to the area of the opening. (D) The amount of fluid passing through the blockage is proportional to the diameter of the opening. Problem 12: The four things that affect the amount of water flowing through a hose are (A) air viscosity, hose length, inlet and outlet pressure difference, and hose diameter. (B) water viscosity, hose composition, inlet and outlet pressure difference, and hose diameter. (C) water viscosity, hose length, inlet pressure and hose diameter. (D) water viscosity, hose length, inlet and outlet pressure difference, and hose diameter. Problem 13: At what place in or near the jet engine is gas moving the fastest relative to the flying airplane? (A) In the air flowing into the engine’s inlet duct. (B) In the plume of exhaust gas flowing out of the engine’s outlet duct. (C) In the engine’s turbine section. (D) In the engine’s compressor section Problem 14: When a fish hovers over the bottom of a lake, what forces act on it, and what is the net force? (A) Lift only, for a net force up. (B) Lift only, for a net force zero. (C) Lift and weight, for a net force up. (D) Lift and weight, for a net force zero. Problem 15: Water pours gently from an open hose because it (A) receives little energy from the water company. (B) lost energy going through the final opening. (C) lost energy as it entered the hose. (D) lost energy throughout its trip to the hose end. Problem 16: Suppose you have gotten a new job where you have to design balls that fly through the air with as little resistance as possible. You know that in front of the ball there is laminar flow, so in back of the ball there should be (A) turbulent flow, because there will be lower density. (B) turbulent flow, because there will be less drag. (C) laminar flow, so the leading and trailing pressure environments are similar. (D) no air flow at all. Problem 17: How much heat must be added to 0.5 kg of ice at 0 oC to completely convert it into steam at 100 oC? (A) 320 kcal (B) 360 kcal (C) 310 kcal (D) 720 kcal Problem 18: A shiny, white transparent surface prevents heat transfer because (A) it has emissivity of zero and reflects all light that strikes it. (B) it has emissivity of 1 and reflects all light that strikes it. (C) it has emissivity of zero and absorbs all light that strikes it. (D) it has emissivity of 1 and absorbs all light that strikes it. Problem 19: You are watching a science fiction movie where people on a space ship are traveling around the universe observing stars with different temperatures. Which of the following stars has the lowest surface temperature? (A) A bluish star. (B) A yellowish star. (C) The sun. (D) A reddish star. Problem 20: You are hired as an engineer at a thermometer factory. Suppose you built a water thermometer by replacing the mercury in a mercury thermometer with water. You want to measure temperatures above freezing with it. How would it behave as the temperature drops from 8o C to 1o C? (A) The reading will decrease and then begin increasing. (B) The reading will steadily decrease. (C) The reading will steadily increase (D) The reading will increase and then begin decreasing. Problem 21: You are talking with your fiancée and looking up into a beautiful night sky and she comments “You know, it is sad that we couldn’t tell if there were any black objects out there no matter how hot they were because a black object would never emit light at any temperature.” You would comment: (A) Nothing. Even though you know it is a misconception and that black objects absorb and emit radiation perfectly, you don’t want to ruin the evening. (B) Nothing, because she is correct. (C) Nothing, because black objects absorb light but don’t emit any. (D) Nothing, because black objects emit light but don’t absorb any. Problem 22: The gulf stream is one of nature’s ways of transferring energy through (A) conduction. (B) convection. (C) transmission. (D) radiation. Problem 23: Suppose you are teaching a science course and have prepared a demonstration to teach how a thermometer works. When you plunge the thermometer into hot water, one of your students points out that the mercury in the thermometer actually went down a little bit before it shot up. It behaved in such a way because (A) the water pressure squeezed in on the mercury. (B) the mercury contracted at first because of the temperature shock. (C) the glass expanded before the mercury had a chance to, creating space. (D) the thermometer lost heat to the surrounding water. Problem 24: A pedestrian bridge crosses a street. This bridge is entirely supported by columns from below. A gap at each end of the bridge separates the bridge’s surface from the sidewalks leading to the bridge. The width of each gap changes with time. This width is smallest (A) on hot days. (B) at noon. (C) at midnight. (D) on cold days. Problem 25: You are a mechanic in a garage and are working on a carburetor that is made of metal, has a glass inspection window and plastic gears for the choke. If all three types of material are at the same temperature and each type of material is in contact with the other two, then (A) heat will flow from the plastic to the glass, and from the glass to the metal. (B) heat will flow from the metal to the glass, and from the glass to the plastic. (C) heat will flow from the glass to both the plastic and the metal. (D) no heat will flow between any parts. Problem 26: “Two objects in thermal equilibrium with a third object are in thermal equilibrium with each other.” Which Law of Thermodynamics is this? (A) Zeroth (B) First (C) Second (G) Third Problem 27: Suppose that you place a timer and a tiny pile of gunpowder in an absolutely rigid box. You then seal the box completely and insulate it perfectly so that no heat can flow in or out of the box. Soon the timer ignites the gunpowder and it burns to form smoke. Since the box is sealed, the smoke remains inside it. If you look into the box a few minutes later, you will find that the smoke has not converted back into the gunpowder because (A) there is not enough energy in the box to reform the gunpowder. (B) there are not enough atoms in the box to reform the gunpowder. (C) such an event, unburning the smoke to form gunpowder, is extraordinarily unlikely. (D) the basic laws of motion, otherwise known as Newton's three laws, forbid the unburning of smoke to form gunpowder. Problem 28: In a car engine, the compression stroke involves the piston compressing the air/gas mixture so rapidly that virtually no heat loss can occur. In the compression stroke, the air/gas mixture (A) heats because of friction between the gas and the cylinder. (B) cools because the gas is radiating its energy away. (C) heats because work is being done on the air/gas mixture. (D) cools because the air/gas mixture is doing work on the piston. Problem 29: A thermoelectric cooler is a type of heat pump that uses electric power to move heat against its natural direction of flow. In other words, it takes energy from a system and transfers it to another system which is at a higher temperature. In such a heat pump (A) the entropy of the system must be conserved. (B) Newton's laws of motion prevent heat from moving from a cool region to a warm region. (C) the cooler requires power input because the flow of heat from a cool region to a warm region alone would decrease the entropy (disorder) of the system. (D) the cooler requires power input because the flow of heat from a cool region to a warm region alone would violate the conservation of energy. Problem 30: The first law of thermodynamics is a re – statement of the law of (A) conservation of momentum. (B) thermal equilibrium. (C) conservation of energy. (D) averages.