HW_Ch11_Sol - PHY126 HW Set 2 (Chap.11) Problem 11.42...

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PHY126 HW Set 2 (Chap.11) Problem 11.42 Description: A physics student is standing on an initially motionless, frictionless turntable with rotational inertia 0.31 . She's holding a wheel of rotational inertia 0.22 spinning at 130 about a vertical axis, as in the figure. When she turns the wheel upside down, student and turntable begin rotating at 70 . Part A What is the student's mass, considering her to be a cylinder 30 in diameter? Express your answer using two significant figures. Part B How much work did she do in turning the wheel upside down? Neglect the distance between the axes of the turntable and wheel. Express your answer using two significant figures. Solutions (a) Since the turn table is frictionless, there are no external torques about its axis, and the z component of angular momentum is conserved. Initially this is all spin of the wheel, When the wheel is inverted, the student and turrable acquire anugular momentum, such that or rpm). 130 )( m kg 22 . 0 ( 2 0 = L z L 0 0 L L L z = . 2 0 L L z = If is the total moment of inertia about the z-axis, or Now tot I rpm), 130 )( m kg 2(0.22 rpm) 70 ( 2 = = tot z I L . m kg 817 . 0 2 = tot I tot I
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includes the inertia of the turntable, 0.31 kg m 2 , the student (a cylinder0, (1/2)M(0.5 m) 2 , and CM of the wheel, mh 2 9which can be neglected if the axes of the wheel and turntable coincide, and the spin axis is frictionless). Then (1/2)M(0.15 m) 2 =(0.817-0.31) kg m 2 , or M=45.1 kg. (b) The work done by the student’s muscle equals the change in kinetic energy,
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This note was uploaded on 05/28/2011 for the course PHY 126 taught by Professor Staff during the Summer '08 term at SUNY Stony Brook.

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HW_Ch11_Sol - PHY126 HW Set 2 (Chap.11) Problem 11.42...

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