题目

如图所示，用一根长度为L的丝线，系一质量为m的带电小球，当小球在匀强电场中处于静止时，悬线与竖直方向的夹角为θ，现用力将小球拉到悬点O的正下方，并处于静止，则所加的外力对小球做的功为多少？ 答案：解析：将小球拉到悬点O的正下方的过程中有外力、重力和电场力对小球做功，其中电场力做负功，则有：W外＋mgL(1-cosθ)-qELsinθ=0解得W外=qEsinθ-mgL(1-cosθ)①当悬线与竖直方向夹角为θ时小球处于平衡状态，则有tanθ=,即qE=mgtanθ②由①②得W=.答案：--- This history book is of great value. --- Exactly. ____________ can be enjoyed from it ______ you have a deep understanding of it, however.A.Nothing; unless B.Few; till C.Something; until D.Little; since