# h25 - hernandez(ejh742 homework 25 Turner(58120 This...

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hernandez (ejh742) – homework 25 – Turner – (58120) 1 This print-out should have 13 questions. Multiple-choice questions may continue on the next column or page – fnd all choices beFore answering. 001 (part 1 oF 2) 10.0 points A skater spins with an angular speed oF 16 . 8 rad / s with his arms outstretched. He lowers his arms, decreasing his moment oF in- ertia From 41 kg · m 2 to 43 kg · m 2 . a) Calculate his initial rotational kinetic energy. Correct answer: 5785 . 92 J. Explanation: Basic Concepts: L = KE = 1 2 2 Given: I i = 41 kg · m 2 I f = 43 kg · m 2 ω i = 16 . 8 rad / s Solution: KE i = 1 2 I i ω 2 i = 1 2 (41 kg · m 2 )(16 . 8 rad / s) 2 = 5785 . 92 J 002 (part 2 oF 2) 10.0 points b) Calculate his fnal rotational kinetic en- ergy. Correct answer: 5516 . 81 J. Explanation: Solution: Angular momentum oF the system is con- served, so I f ω f = I i ω i ω f = I i ω i I f KE f = 1 2 I f p I i ω i I f P 2 = 1 2 I 2 i ω 2 i I f = 1 2 (41 kg · m 2 ) 2 (16 . 8 rad / s) 2 43 kg · m 2 = 5516 . 81 J 003 (part 1 oF 3) 10.0 points Assume: The small objects are point parti- cles. The system oF seven small 4 kg objects is rotating at an angular speed oF 2 rad / s. The objects are connected by light, ±exible spokes that can be lengthened or shortened. x y 4 kg 4 kg 4 kg 4 kg 4 kg 4 kg 4 kg 2 r a d / s 9 m What is the initial angular momentum oF the object? Note: An e²ect similar to this ocurred in the early stages oF the Formation oF our Galaxy. As the massive cloud oF dust and gas that was the source oF the stars and planets contracted, an initially small angular speed increased with time. Correct answer: 4536 kg m 2 / s. Explanation: Basic Concepts: The angular momentum is L = I ω = ( M r 2 ) ω . With no external torques, angular momentum is concerved.

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hernandez (ejh742) – homework 25 – Turner – (58120) 2 Let : m = 4 kg , r 1 = 9 m , r 2 = 7 m , and ω 1 = 2 rad
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h25 - hernandez(ejh742 homework 25 Turner(58120 This...

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