When is a system a closed system, and when is it a control

Problem 41P If the airplane of Problem 1–42 weighs 1450 lbf, esti­mate the lift force produced by the airplane’s wings (in lbf and newtons) when flying at 55.0 m/s.

Problem 33P The gas tank of a car is filled with a nozzle that dis­charges gasoline at a constant flow rate. Based on unit considerations of quantities, obtain a relation for the filling time in terms of the volume V of the tank (in L) and the discharge rate of gasoline ( , in L/s).

A 4-kW resistance heater in a water heater runs for 2 hours to raise the water temperature to the desired level. Determine the amount of electric energy used in both kWh and kJ.

Problem 34P A pool of volume V (in m3) is to be filled with water using a hose of diameter D (in m). If the average discharge g velocity is V (in m/s) and the filling time is t (in s), obtain a relation for the volume of the pool based on unit considerations of quantities involved.

Problem 36P A forklift raises a 90.5 kg crate 1.80 m. (a) Showing all your work and using unity conversion ratios, calculate the work done by the forklift on the crane, in units of kJ. (b) If it takes 12.3 seconds to lift the crate, calculate the useful power supplied to the crate in kilowatts.

Problem 35P Based on unit considerations alone, show that the power needed to accelerate a car of mass m (in kg) from rest to velocity V (in m/s) in time interval t (in s) is proportional to mass and the square of the velocity of the car and inversely proportional to the time interval.

Problem 39P Water at 20°C from a garden hose fills a 2.0 L con­tainer in 2.85 s, Using unity conversion ratios and showing all your work, calculate the volume, flow rate in liters per minute (Lpm) and the mass flow rate in kg/s.

Problem 38P A student buys a 5000 Btu window air conditioner for his apartment bedroom. He monitors it for one hour on a hot day and determines that it operates approximately 60 percent of the time (duty cycle = 60 percent) to keep the room at nearly constant temperature. (a) Showing all your work and using unity conversion ratios, calculate the rate of heat transfer into the bedroom through the walls, windows, etc. in units of Btu/h and in units of kW. (b) If the energy efficiency ratio (EER) of the air conditioner is 9.0 and electricity costs 7.5 cents per kilowatt-hr, calculate how much it costs (in cents) for him to run the air conditioner for one hour.

Problem 40P An airplane flies horizontally at 55.0 m/s. Its propeller delivers 1500 N of thrust (forward force) to overcome aero­dynamic drag (backward force). Using dimensional reasoning and unity conversion ratios, calculate the useful power deliv­ered by the propeller in units of kW and horsepower.

Problem 1P Define internal, external, and open-channel flows.

Problem 2P Define incompressible flow and incompressible fluid must the flow of a compressible fluid necessarily be treated as compressible?

Problem 3P Consider the flow of air over the wings of an aircraft. Is this flow internal or external? How about the flow of gases through a jet engine?

Problem 42P What is the difference between precision and accu­racy? Can a measurement be very precise but inaccurate? Explain.

Problem 43P What is the difference between the analytical and experimental approach to engineering problems? Discuss the advantages and disadvantages of each approach?

Problem 44P What is the importance of modeling in engineering? How are the mathematical models for engineering processes prepared?

Problem 4P How is the Mach number of a flow defined? What does a Mach number of 2 indicate?

Problem 5P Consider the flow of air at a Mach number of 0.12. Should this flow be approximated as being incompressible?

Problem 6P What is the no-slip condition? What causes it?

Problem 45P When modeling an engineering process, how is the right choice made between a simple but crude and a complex but accurate model? Is the complex model necessarily a better choice since it is more accurate?

Problem 46P How do the differential equations in the study of a physical problem arise?

Problem 47P What is the value of the engineering software packages in (a) engineering education and (b) engineering practice?

Problem 7P What is forced flow? How does it differ from natural flow? Is flow caused by wind forced or natural flow?

Problem 8P What is a boundary layer? What causes a boundary layer to develop?

Problem 9P What is the difference between the classical and the statistical approaches?

Problem 48P Determine a positive real root of this equation using EES: 3.5x3 ? 10x0.5 ? 3x = ?4

Problem 49P Solve this system of two equations with two unknowns using EES: x3 ? y2= 10.5 3xy + y = 4.6

Problem 50P Solve this system of three equations with three unknowns using EES: 2x-y+z =5 3x2+2y = z+2 xy+2z =8

Problem 10P What is a steady-flow process?

Problem 11P Define stress, normal stress, shear stress, and pressure.

Problem 12P What are system, surroundings, and boundary?

Problem 52P Consider the flow of air through a wind turbine whose blades sweep an area of diameter D (in m). The average air velocity through the swept area is V (in m/s). On the bases of the units of the quantities involved, show that the mass flow rate of air (in kg/s) through the swept area is proportional to air density, the wind velocity, and the square of the diameter of the swept area.

Problem 51P Solve this system of three equations with three unknowns using EES: x2y ? z = 1.5 x ? 3y0.5 + z = ?2 x + y ?z = 4.2

Problem 53P The weight of bodies may change somewhat from one location to another as a result of the variation of the gravita­tional acceleration g with elevation. Accounting for this varia­tion using the relation in Prob. 1–27, determine the weight of an 110-kg person at sea level (z = 0), in Denver (z = 1610 m), and on the top of Mount Everest (z = 8848 m). Problem: At 450 latitude, the gravitational acceleration as a function of elevation z above sea level is given by g = a - bz, whew a = 9.807 m/s2 and b = 3.32 x 10-6s-2. Determine the height above sea level where the weight of an object will decrease by 1 percent.

Problem 13P When is a system a closed system, and when is it a control volume?

Problem 14P You are trying to understand how a reciprocating air compressor (a piston-cylinder device) works. What system would you use? What type of system is this?

Problem 15P When analyzing the acceleration of gases as they flow through a nozzle, what would you choose as your sys­tem? What type of system is this?

Problem 54P A man goes to a traditional market to buy a steak for dinner. He finds a 12-oz steak (1 lbm = 16 oz) for $3.15. He then goes to the adjacent international market and finds a 300-g steak of identical quality for $2.95. Which steak is the better buy?

Problem 55P The reactive force developed by a jet engine to push an airplane forward is called thrust, and the thrust developed by the engine of a Boeing 777 is about 85,000 1bf. Express this thrust in N and kgf.

Problem 56P The drag force exerted on a car by air depends on a dimensionless drag coefficient, the density of air, the car velocity, and the frontal area of the car. That is, FD = function (CDrag, Afront, ?, V). Based on unit considerations alone, obtain a relation for the drag force.

Problem 20P In a news article, it is stated that a recently devel­oped geared turbofan engine produces 15,000 pounds of thrust to propel the aircraft forward. Is “pound” mentioned here lbm or lbf? Explain.

Problem 21P What is the difference between pound-mass and pound-force?

Problem 22P What is the difference between kg-mass and kg- force?

Problem 23P What is the net force acting on a car cruising at a constant velocity of 70 km/h (a) on a level road and (b) on an uphill road?

Problem 24P A 4-kg plastic tank that has a volume of 0.2 m3 is filled with liquid water. Assuming the density of water is 1000 kg/m3, determine the weight of the combined system.

Problem 25P Determine the mass and the weight of the air contained in a room whose dimensions are 6 m × 6 m × 8 m. Assume the density of the air is 1.16 kg/m3.

Problem 27P The acceleration of high-speed aircraft is sometimes expressed in g’s (in multiples of the standard acceleration of gravity). Determine the net upward force, in N, that a 90-kg man would experience in an aircraft whose acceleration is 6 g’s.

Problem 28P A 5-kg rock is thrown upward with a force of 150 N at a location where the local gravitational acceleration is 9.79 m/s2. Determine the acceleration of the rock, in m/s2.

Problem 29P Solve Prob. 1–18 using EES (or other) software. Print out the entire solution, including the numerical results with proper units.

Problem 30P The value of the gravitational acceleration g decreases with elevation from 9.807 m/s2 at sea level to 9.767 m/s2 at an altitude of 13,000 m, where large passenger planes cruise. Determine the percent reduction in the weight of an airplane cruising at 13,000 m relative to its weight at sea level.

Problem 31P While solving a problem, a person ends up with the equation E = 25 kJ + 7 kJ/kg at some stage. Here E is the total energy and has the unit of kilojoules. Determine how to correct the error and discuss what may have caused it.

Problem 26P At 45° latitude, the gravitational acceleration as a function of elevation z above sea level is given by g = a - bz, where a =9.807 m/s2 and b = 3.32 × 10-6 s-2. Determine the height above sea level where the weight of an object will decrease by 1 percent.