虚功原理
参考答案：
约束力不作功的力学系统实现平衡的必要且充分条件是对结构上允许的任意位移（虚位移）施力所作功之和为零。
The students were all ___after they heard the ____story.
A．inspired; inspiring
B．inspired; inspired
C． inspiring; inspired
D．inspiring; inspiring
