题干

如图所示,质量M=1kg的木板静置于倾角θ=37°、足够长的固定光滑斜面底端. 质量m=1kg的小物块(可视为质点)以初速度v0=4m/s从木板的下端冲上木板,同时在 木板上端施加一个沿斜面向上的F=3.2N的恒力.若小物块恰好不从木板的上端滑 下,求木板的长度l为多少?已知小物块与木板之间的动摩檫因数μ=0.8,重力加速度 g=10m/s2,sin37°=0.6,cos37°=0.8.

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答案(点此获取答案解析)

解:由题意,小物块向上匀减速运动,木板向上匀加速运动,当小物块运动到木板的上端时,恰好和木板共速.

设 小物块的加速度为a,由牛顿第二定律得,mgsinθ+μmgcosθ=ma

设木板的加速度为a′,由牛顿第二定律得,F+μmgcosθ﹣Mgsinθ=Ma′

设二者共速的速度为v,经历的时间为t,由运动学公式得,

v=v0﹣at

v=a′t

小物块的位移为s,木板的位移为s′,由运动学公式得,

同类题5

根据短文理解,从A、B、C、D四个选项中选择最佳答案填空。

    The International Climate Champions (ICC) project began in 2007. It gives young people of school age a chance to speak publicly on climate change and to call on people to take action to reduce its 1 .

    Each country involved selects three teenagers to be Climate Champions, who take part in local and international activities.Climate Champion Irene Sanna lived on the Italian island of Sardinia. Irene is interested in solar energy, and  2 believes that Sardinia should use the waves around its coast to produce electricity. 'We must make our plans to save our coast, which still has no pollution. We must protect the animals, birds and fish in danger from global warming. And we must recycle.'

    Chinese student Ding Yinghan is the Beijing Climate Champion. Ding feels it is not  3 to say that just one country - his own - is causing climate change. He says the air pollution that leads to global warming comes from many parts of the world, including poorer countries that are now growing more quickly. He believes the only way to  4 the situation getting even worse is for rich and poor countriesto work together.

    Sophia Angelis, a junior student in California, is a US Champion. She's against young people's generation lack of interest in politics and feels they need to discuss the problems that really matter to their generation. Sophia strongly believes that climate change is an important issue for her generation. For her, 5 in the way teenagers behave are an important way of influencing choices made by parents.

In 2008, the Climate Champion attended the International Conference of Environment Ministers in the Japanese city of Kobe.  6 , 30 countries are involved in the ICC, and more countries are expected to join soon.