漫画阅读。阅读对话,判断正误。 (1) Jack often gets up at 8 o'clock on the weekend. (2) Jack doesn't have class on Saturday. (3) Jack usually goes shopping on the weekend. (4) Jack is happy because he can go hiking today. 答案:错误
正确
错误
错误
如图所示,桌面离地高度为h,质量为m的小球,从离桌面H高处由静止下落.若以桌面为参考平面,则小球落地时的重力势能及整个过程中小球重力做功分别为
[ ]
A.
mgh,mg(H-h)
B.
mgh,mg(H+h)
C.
-mgh,mg(H-h)
D.
-mgh,mg(H+h)