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HW2_Solution

# HW2_Solution - Problem 2.20 Known The drag force on a truck...

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Problem 2.20 Known: The drag force on a truck is known as a function of velocity and other parameters. Find: Determine the power required by the truck to overcome drag. Schematic & Given Data: Power required to overcome drag Drag force Velocity Drag coefficient Projected frontal area Air density d d d W F V C A ρ = = = = = = ± Model and Assumptions: The truck is the system. Solution: “The rate of energy transfer by work is called power and is denoted by W ± . The rate of energy transfer by work is equal to the product of the force and the velocity at the point of application of the force.” WF V =⋅ ± (Eq.2.13 in a textbook) Applying Eq. 2.13, the power required to overcome drag is dd V ± (1) The drag force, d F , is given by 2 1 2 FC A V = (2) Substituting Eq. (2) to Eq. (1), 23 11 22 d WC A V V C A V ρρ ⎛⎞ == ⎜⎟ ⎝⎠ ± (3)

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Finally, () 3 3 2 32 3 1 kg km 10 m 1h 1N 1kJ 0.65 10m 1.1 110 2 m h 1km 3600s 1kg m s 10 N m kJ 1kW 102 s1 k J / s 102kW d W ⎛⎞ = ⎜⎟ ⋅⋅ ⎝⎠ = = ± Problem 2.33 Known: Carbon monoxide gas within a piston-cylinder assembly undergoes three processes in series. Find: Sketch the processes in series in a P-V diagram and evaluate the work for each. Schematic & Given Data: Process 1-2: Expansion from P 1 =5bar, V 1 =0.2m 3 to V 2 =1m 3 , during which the pressure-volume relationship is PV C = (constant). Process 2-3: Constant-volume heating from state 2 to state 3, where P 3 =5bar. Process 3-1: Constant-pressure compression to the initial state Work done (+:by the system, while -:on the system ) Pressure System volume Constant W P V C = = = = Model and Assumptions: 1. The gas is the closed system.
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HW2_Solution - Problem 2.20 Known The drag force on a truck...

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