题目

Those who suffer from headache will find they get _______from this medicine . A. relief   B. safety   C. defense  D. shelter 答案：A 质量为m的滑块从半径为R的半球形碗的边缘滑向碗底，过碗底时的速度为v，若滑块与碗间的动摩擦因数为μ，则滑过碗底时滑块受到的摩擦力大小为 [　　] A．μmg B． C．μm() D．μm()