物理问题求解

投坛高人不但赚钱厉害，出题解题也是不减当年勇。小弟帮儿子功课，有两道题想不出来为什么答案是这样，请各位高人帮帮。

 

没有质量，怎么算energy? 这个到底是哪一部分的题目？

多谢各位高人解惑！

所有跟帖： 

• 是F=ma 2010 的题目 -sneezy888- ♂ (0 bytes) () 01/16/2013 postreply 10:28:38

• I don't see the potential given anywhere in your post. -borisg- ♀ (0 bytes) () 01/16/2013 postreply 10:31:38

• sorry missed a condition in this chart... -sneezy888- ♂ (33 bytes) () 01/16/2013 postreply 10:40:37

• 19.: particle can stay at 15 (I), move from 10 to 15 (III), can -fouquets- ♂ (0 bytes) () 01/16/2013 postreply 10:46:56

• can not stay at -5 (II), has to go down -fouquets- ♂ (0 bytes) () 01/16/2013 postreply 10:48:49

• that is right... LOL 钻了牛角尖 -sneezy888- ♂ (0 bytes) () 01/16/2013 postreply 10:49:22

• 多谢了！ -sneezy888- ♂ (0 bytes) () 01/16/2013 postreply 10:50:12