一个质量为60kg的体操运动员做“单臂大回环”训练，用一只手抓住单杠，伸展身体，以单杠为轴做圆周运动，如图所示．要完成此动作，运动员的臂力至少为（忽略空气阻力，g取10m/s2）（ ）
A．约600N
B．约2400N
C．约3000N
D．约3600N
把人看成质量为m=60kg的质点，距杠距离为L绕杠做圆周运动，令人在最高点时的速度为v0，当人运动最低点时的速度根据动能定理可得：
mg2l=
mv2-1 2
mv02得人的速度v=1 2
可知，人在最低点时手臂的拉力和重力的合力提供向心力4gl+v02
则F-mg=mv2 R
即F=mg+m
=5mg+mv2 R v02 l
因为求F的最小值，故当v0=0时Fmin=5mg=3000N．
故选C．
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| Dear Li Hua, Thank you 1 your last letter. Here are some photos 2 me and my twin sister Tina. As you can see, in some ways we look 3 , and in some ways 4 look different. We both have blue eyes and brown 5 , although my hair is shorter 6 hers. We both 7 running and swimming, although Tina is more athletic than me. She is 8 outgoing, and I'm quieter. I think I'm smarter than her. My 9 subjects are physics and chemistry, and her favorite subject is P.E. However, we both enjoy 10 to parties and making friends there. By the way, we haven't seen each 11 for more than a month, and I miss you very much. Can you write 12 me or come to see me soon? Love, Tara | |||
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