二力构件是指()
A.只受两个力作用的构件
B.只受两个互相平行的力作用的构件
C.只受两个互相垂直的力作用的构件
D.只受两个力作用而处于平衡的构件
参考答案:D
-What are you looking ______?
-My watch. I can't find it now. Have you seen it anywhere? [ ]
A. up
B. for
C. out
D. back
已知某空调系统进风管道截面为圆形截面,其直径为200mm,测得风速为7m/S.求进风量