A:30∘
B:60∘
C:120∘
D:150∘
俗话说“一方水土养一方人”。下列自然景观和人文活动与我国北方地区相符的是( )
—How about joining us in the summer camp?—I wish I could, ___________ I have to practice for the piano test.
测量滑轮组机械效率的装置如图所示.用5N拉力F将重为8N的物体匀速提升0.5m,用时4s,拉力做得总功W总=____ J,有用功W有用=____ J,滑动组的机械效率η=____ ,拉力的功率P=____ W.
—I'd like to, but I have a meeting to ______.