题干

我国研制的“嫦娥三号”月球探测器于2013年12月1日发射成功,并成功在月球表面实现软着陆.探测器首先被送到距离月球表面高度为H的近月轨道做匀速圆周运动,之后在轨道上的A点实施变轨,使探测器绕月球做椭圆运动,当运动到B点时继续运动,探测器携带的传感器测得自由落体运动时间为t,已知月球半径为R,万有引力常量为G.则下列正确的是(   )

A:“嫦娥三号”的发射速度必须大于第一宇宙速度

B:探测器在近月轨道和椭圆轨道上的周期相等

C:“嫦娥三号”在A点变轨时,需减速才能从近月圆轨道进入椭圆轨道

D:月球的平均密度为 3h2πGRt2

上一题 下一题 0.0难度 选择题 更新时间:2018-07-27 12:37:43

答案(点此获取答案解析)

A,C,D

同类题1

根据短文内容,选择最佳答案。

    There were three big fish in the lake. One of them, Demo, was wise; another one, Jimmy, was half-wise; Martin, the third one, was stupid.

    Some fishermen came to the bank of the lake with their nets. The three fiss saw them.

    Demo decided at once to leave, to make the long and difficult trip to the ocean. He thought, "I won't discuss with these two about this. They love this place so much that they call it home and will stay on. How silly they are! They still haven't realized we are in great danger now. Their ignorance will cost them their lives."

    Seeing the wise fish had left, Jimmy thought, "My guide has gone and now I've lost my chance to escape." He felt sorry for it and then thought, "What can I do to save myself from these men and their nets? Perhaps I should pretend(假装)to be already dead, giving myself totally to the water."  So he did that. He went up and down with the water, helpless, within arm's reach of the fishermen.

    "Look at this! The best and biggest fish is dead." One of the men lifted him by the tail, and threw him up on the ground. He rolled(滚动)over and over and moved secretly toward the water, and then, back in.

    The third fish was aimlessly jumping about, trying to escape with his speed and cleverness. The net, of course, finally closed around him, and as he lay in the hot frying-pan bed, he thought, "If I get out of this, I'll never live again in the limits of the lake. Next time, the ocean! I'll make it my home."