问题 计算题

(13分)如图所示,一质量m=0.4kg的小物块,以v0=2m/s的初速度,在与斜面成某一夹角的拉力F作用下,沿斜面向上做匀加速运动,经t=2s的时间物块由A点运动到B点,A、B之间的距离L=10m。已知斜面倾角θ=30o,物块与斜面之间的动摩擦因数。(重力加速度g取10 m/s2     

(1)求物块加速度的大小及到达B点时速度的大小。

(2)拉力F与斜面的夹角多大时,拉力F最小?拉力F的最小值是多少?

答案

(1)3m/s2    8m/s   (2)300     

题目分析:(1)设物块加速度的大小为a,到达B点时速度的大小为v,由运动学公式得:

       (2分)

        (2分)

联立以上两式,代入数据解得:      (2分)

(2)设物块所受支持力为FN,所受摩擦力为Ff,拉力与斜面之间的夹角为α。受力分析如图所示,由牛顿第二定律得:

       (1分)

         (1分)

       (1分)

联立解得:         (1分)

由数学知识得:         (1分)

由上式可知对应的F最小值的夹角α=300              (1分)

代入数据得F的最小值为:            (1分)

完形填空
Chinese tennis is entering a golden time. In recent years there has been an explosion of __36__ female tennis players bursting onto the global scene. And this couldn’t have come at a _37_ time with the 2008 Beijing Olympics .
These young girls have ­_38__ strong will and talent. _39  the way is Li Na, who may have the _40_name in women’s tennis, but she has a big reputation. Li Na is China’s top ranked player and the first to have _41_into the world’s top 20.
After _42_ nearly six months of the season with a serious injury, the 25-year-old_ 43_ an impressive comeback by winning the Australian Women’s Hardcourt Tennis Championships.
In the world of professional tennis, Li’s refreshing sense of humor proved to be a hit both on and _44_ the court.
“My husband said he had a special gift for me _45_I won,” Li Na said after lifting the prize cup. “That’s _46__ I tried so hard today. I wanted a special gift.”
The Wuhan girl __47_ tennis at the age of eight after being discovered by a coach who had   _48_ her playing badminton. After years of hard training, Li Na turned __49_ in 1999 and has become one of the most successful Chinese tennis players.
With her solid groundstroke and aggressive mindset, she _50  the first Chinese tennis player to win a WTA Tour _51_. Then two years later, she was the first Chinese to make   __52_ into the last 16 of Wimbledon, which led to her being _53_ number 16 in the world last January.
Li is now looking forward to winning a _54_at the Olympics. “The Olympics are very important for us, ” she said. “I am more motivated after my recovery _55_ I feel my desire for the Olympics is bigger than ever before.”
小题1:.
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20080801

 
20080505
 
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小题14:.
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小题19:.
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小题20:.
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判断题