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HW04-solutions

# HW04-solutions - sela(ds39632 HW04 janow(11196 This...

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sela (ds39632) – HW04 – janow – (11196) 1 This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0points A person of mass 96 . 3 kg escapes from a burn- ing building by jumping from a window situ- ated 24 . 7 m above a catching net. The acceleration of gravity is 9 . 8 m / s 2 . If air resistance exerts a force of 89 . 2 N on him as he falls, determine his speed just before he hits the net. Correct answer: 20 . 9371 m / s. Explanation: The forces acting on him are the gravita- tional force m g acting downward, and the air resistance, F a , acting upward. The accelera- tion is downward and the net force is F net = m a = m g F a a = m g F a m The person is in free fall, so his final speed is defined by v f = 2 a h = radicalbigg 2 ( m g F a ) h m = radicalBigg 2 [(96 . 3 kg)(9 . 8 m / s 2 ) 89 . 2 N] (24 . 7 m) 96 . 3 kg = 20 . 9371 m / s . 002 10.0points A 4000 kg helicopter accelerates upward at 3 . 19 m / s 2 . The acceleration of gravity is 9 . 8 m / s 2 . What lift force is exerted by the air on the propellers? Correct answer: 51960 N. Explanation: The propellers exert a force F upward and gravity acts downward. The net force (di- rected upward) on the helicopter is F = m a = summationdisplay F up summationdisplay F down = F m g Thus F = m a + m g = m ( a + g ) Note: Here a and g are absolute values of the helicopter’s acceleration and the acceleration of gravity, respectively. 003(part1of3)10.0points A 3 . 1 kg object is subjected to two forces, vector F 1 = (1 . 4 N) ˆ ı + ( 1 . 2 N) ˆ and vector F 2 = (4 . 4 N) ˆ ı + ( 11 . 5 N) ˆ . The object is at rest at the origin at time t = 0. What is the magnitude of the object’s ac- celeration? Correct answer: 4 . 50378 m / s 2 . Explanation: Let : vector F 1 = (1 . 4 N) ˆ ı + ( 1 . 2 N) ˆ  , vector F 2 = (4 . 4 N) ˆ ı + ( 11 . 5 N) ˆ  , and vectorv o = 0 m / s .

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HW04-solutions - sela(ds39632 HW04 janow(11196 This...

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