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HW 4-solutions

# HW 4-solutions - kuruvila(lk5992 HW 4 opyrchal(11113 This...

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kuruvila (lk5992) – HW 4 – opyrchal – (11113) 1 This print-out should have 12 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points A person of mass 88 . 1 kg escapes from a burn- ing building by jumping from a window situ- ated 23 . 9 m above a catching net. The acceleration of gravity is 9 . 8 m / s 2 . If air resistance exerts a force of 110 . 5 N on him as he falls, determine his speed just before he hits the net. Correct answer: 20 . 2111 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 [(88 . 1 kg)(9 . 8 m / s 2 ) - 110 . 5 N] (23 . 9 m) 88 . 1 kg = 20 . 2111 m / s . 002 (part 1 of 3) 10.0 points A boat moves through the water with two forces acting on it. One is a 2112 N forward push by the motor on the propeller, and the other is an 1877 N resistive force due to the water around the bow. What is the acceleration of the 1125 kg boat? Correct answer: 0 . 208889 m / s 2 . Explanation: Given : F 1 = 2112 N , F 2 = 1877 N , and m = 1125 kg . 2112 N 1877 N The forces are unbalanced, so the net force is F net = m a = F 1 - F 2 and a = F 1 - F 2 m = 2112 N - 1877 N 1125 kg = 0 . 208889 m / s 2 forward. 003 (part 2 of 3) 10.0 points If it starts from rest, how far will it move in 15 . 5 s? Correct answer: 25 . 0928 m. Explanation: Given : Δ t = 15 . 5 s Under acceleration, Δ x = v i t + 1 2 a t ) 2 = 1 2 a t ) 2 since v i = 0 m/s. Thus Δ x = 1 2 a t ) 2 = 1 2 ( 0 . 208889 m / s 2 ) (15 . 5 s) 2 = 25 . 0928 m .

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HW 4-solutions - kuruvila(lk5992 HW 4 opyrchal(11113 This...

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