题目

As space science develops, man has learned more and more about space. Space is not only amazing but also dangerous. While working in space, spacemen are facing danger as well as success. Scientists have found out that__________________to spacemen in space. When spacemen are working in space, they are in danger of radiation from the sun and other stars, which is bad for their health. The damage(危害) done by radiation won't be discovered until their children or even grandchildren are born. Some special medicine may work a little, but no really helpful medicine has been. Space junk is 如图6-3-2所示，一细线的一端固定于倾角为θ=30°的光滑楔形块A的顶端处，细线的另一端拴质量为m的小球. 图6-3-2（1）当滑块至少以多大的加速度a向左运动时，小球对滑块压力为零？（2）当滑块以a=2g的加速度向左运动时，小球对线的拉力为多大？