I'm familiar with 一人じゃない, but I'm not sure what 一人じゃ means. Is it a contracted version?
I found the word in this context below:
一人じゃ運べそうもないから、手伝ってくれる?
(Since it seems like I'm unable to carry it by myself, could you help me?)
1人じゃない = 1人ではありません。 Consider that the negative form of “です" is “ではありません",which can also be “じゃありません" So I think “じゃ” is used as a simple form of"では".
Since it seems it cannot be carried by yourself, would you like me to help you?<- The subject of 一人じゃ運べそうもない is the speaker (=I), and the subject of 手伝ってくれる is the hearer (=you), so it'd be like "Since it seems I cannot carry it by myself, would you help me?" – chocolate Jul 27 '16 at 07:24