如图所示，质量为m_B=24kg的木板B放在水平地面上，质量为m_A=22kg的木箱A放在木板B上．一根轻绳一端拴在木箱上，另一端拴在天花板上，轻绳与水平方向的夹角为θ=37°．已知木箱A与木板B之间的动摩擦因数μ₁=0.5．现用水平向右、大小为200N的力F将木板B从木箱A下面匀速抽出（sin37°=0.6，cos37°=0.8，重力加速度g取10m/s²），求

《中国国家地理》杂志社特别策划了“三纵一横”四条寻访中国的路线，即东线（A线）、中线（B线）、西线（C线）和北纬35°线（D线）。图13示意《中国国家地理》寻访中国路线，读图完成下列各题。

    Alice and John are in the music club at their school. Alice plays the violin and John plays the drums. There are two groups in the school music club: the training (训练) group and the concert(音乐会) group. Everyone starts in the training group!

     "When I first started to play the violin it sounded terrible!" Alice said. "My family asked me to practice outside."

    After a year's practice, Alice and John play much better now. They are both in the concert group and they are learning more difficult music. "We're learning the music from the film Pirates of the Caribbean. It sounds fantastic!" John said. "Playing in the club with other kids is a lot of fun."

    Playing an instrument (乐器) is not only fun. It also helps children learn to concentrate (集中注意力). "I have to concentrate when I'm practicing," Alice said. "And now it's easier for me to concentrate on my schoolwork."

    Playing an instrument can also help children become more confident (自信的). As John said, "I was scared when I played in front of a lot of people for the first time. But now I really enjoy it."