已知物体在 F1、F2、F3三共点力作用下处于平衡,若F1=20 N,F2=28 N,那么F3的大小可能是( )
A.46 N
B.50N
C.60 N
D.6 N
答案:A
Not until he failed the exam _______he had left his books untouched .
A.did he realize
B.had he realized
C.he realized
D.he had realized
如图所示为小华同学探究“让线圈转起来”的实验装置图,线圈能够转动的原因是_________________;要改变线圈转动方向,可采用_________________或________________的方法。