45. Water pressurized to 3.5 × 105 Pa is flowing at 5.0 m/s in a horizontal pipe which contracts to 1/3 its former area. What are the pressure and flow speed of the water after the contraction?
(A) 2.5 × 105 Pa, 15 m/s
(B) 3.0 × 105 Pa, 10 m/s
(C) 3.0 × 105 Pa, 15 m/s
(D) 4.5 × 105 Pa, 1.5 m/s
(E) 5.5 × 105 Pa, 1.5 m/s

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統計： A(37), B(0), C(3), D(2), E(0) #2751660

詳解 (共 1 筆)

#5002086

首先根據流量相等, A = 3A' , V'=3V,可以得知v=15m/s 剩下(A)(C)可選
再來偷吃步一點的算法是5->15m/s 在1g下是剛好為10m的加速距離,
可換算為1atm(~=105Pa)的差值,即較高速的成因理解為是把壓力消耗為加速的過程,這麼一來 3.5-1 = 2.5*105 (A)

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