# hw13 - patel(kp9583 HW13 Mackie(20211 This print-out should...

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patel (kp9583) – HW13 – Mackie – (20211) 1 This print-out should have 16 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 10.0 points The small piston of a hydraulic lift has a cross-sectional area of 8 . 2 cm 2 and the large piston has an area of 11 cm 2 , as in the figure below. F 8 . 2 cm 2 area 11 cm 2 What force F must be applied to the small piston to maintain the load of 44 kN at a constant elevation? Correct answer: 32800 N. Explanation: Let : A 1 = 8 . 2 cm 2 , A 2 = 11 cm 2 , and W = 44 kN . According to Pascal’s law, the pressure ex- erted on A 1 must be equal to the one exerted on A 2 . The pressure P 1 = F A 1 must be equal to the pressure P 2 = W A 2 due to the load. F A 1 = W A 2 , so F = A 1 A 2 W = (8 . 2 cm 2 ) (11 cm 2 ) (44000 N) = 32800 N . 002 10.0 points A hydraulic brake system is shown. The area of the piston in the master cylinder is 6 . 35 cm 2 , and the area of the piston in the brake cylinder is 1 . 68 cm 2 . The coefficient of kinetic friction between the brake shoe and the wheel drum is 0 . 488. Master cylinder Brake cylinder Brake shoe μ k = 0 . 488 Wheel drum Pedal How large is the frictional force between the brake shoe and the wheel drum when a force of 42 N is exerted on the pedal? Correct answer: 5 . 42256 N. Explanation: Let : A mc = 6 . 35 cm 2 , A br = 1 . 68 cm 2 , μ k = 0 . 488 , and F p = F mc = 42 N . F k = μ k F br P br = P mc F br A br = F mc A mc = F p A mc . F br is the normal force exerted on the brake shoe, so F k = μ k parenleftbigg F p A mc A br parenrightbigg = μ k F p parenleftbigg A br A mc parenrightbigg = (0 . 488)(42 N) parenleftbigg 1 . 68 cm 2 6 . 35 cm 2 parenrightbigg = 5 . 42256 N . keywords:

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patel (kp9583) – HW13 – Mackie – (20211) 2 003 10.0 points A balloon filled with helium plus air at atmo- spheric pressure is designed to support a mass of 115 kg (payload + empty balloon). Air density is 1 . 29 kg / m 3 , helium plus air density in the balloon is 0 . 486 kg / m 3 , and helium density is 0 . 179 kg / m 3 .
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