Quiz1-Key-Rubric - Fall 2010 7B Quiz 1 Fluids (last)...

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Unformatted text preview: Fall 2010 7B Quiz 1 Fluids (last) RubricCode: (first) DL Sec# Student ID:________________ Name:________________ , ________________ |______| A cube is submerged in a fluid as shown where the top is at a depth of yA=1m and the bottom is at a depth of yB=1.5m. The fluid has a density of f=1000kg/m3 , the cube has a density of c=1200Kg/m3 and each side has a length of a=0.5m. Answer the following questions: Consider the pressure on the left, right, front and back walls of the cube. Note the total horizontal forces balance to zero. yA yB a) Find the change in pressure between the top and the bottom faces (Be sure to show the energy diagram, and use Bernoulli's principle also known as the energy density equation). PE V P Fromtoptobo7omheightdecreases(PE/V decreases)andduetodepth,gaugepressure increases PE+P=0 P=fg(yAyB)=1000*9.8*(0.5)=4900Pa Withpressurehigheratthebo7om c) Find the difference between the force on the top face and the force on the bottom face (This is called the Buoyant force Archimedes' Principle). FromP=F/AweknowthatF=A*P=(.5)2*4900=1225N Noteitisthedifferenceinpressurethatgivesahigherforceatbo7om. d) Calculate the force of gravity on the block which is given by F=mg. Will it sink or float to the top? TocalcF=mgweneedm.Fromc=mc/Vcwesolveform=V=1200*(.5)3=150kg F=mg=(150kg)(9.8)=1470NThusthecubemustsink,wealsoknowthisfromc>f V F 5 I = t = Av , [ AA vA = AB vB ] , P = A , [ Pgauge = gd] , [ Ptotal = Patm + Pgauge ] , [Patm =110 Pa ] , [PV = nRT ] PE KE 1 2 PE KE m 1 g = 9.8 m , = , [PE = mgh ] , KE = mv 2 , = gh , = v , + + P = 0 2 s V V V 2 2 V V [ Rubric Part a) 33% Q R S T U X 10 9 8 6 2 0 As shown in key, with diagram, Energy-density equation, and P=4900N Minor Math error Used density of cube instead of density of fluid for pressure due to fluid Considered P due to cube as well as water, conceptual or major math errors. Multiple math and conceptual errors, unintelligible Little or no progress Part b) 33% Q R S T U X 10 9 8 4 2 0 As shown in key Buoyant force is 1225N Math error, less than thorough explanation Calculated force on top and bottom but not the difference, major math error. Stated equal pressure on top and bottom, no force. Calculated change in pressure again, major math and conceptual errors. Little or no progress Part c) 33% Q R S T X 10 9 7 2 0 As shown, Fgrav=1470N, block sinks because Fgrav>Fbuoyancy Math error but correct answer that denser block sinks in less dense fluid Correct or minor error in calculation, but no conclusions about whether block floats or sinks / major calculation error or block floats. Wrong answer, no work, incorrect and incomplete math, tried to calculate g constant. Little or no progress ...
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This note was uploaded on 09/08/2010 for the course PHY 7B PHY7B taught by Professor Cebra during the Winter '10 term at UC Davis.

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