gc textbook chapter 8 Rotational Motion (id: 023a5d902)

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本题目来源于试卷: gc textbook chapter 8 Rotational Motion，类别为 unclassified

[问答题]
Why is it more difficult to do a s7 h4u*zgthc ymlm0l 69x/5sv fc,d ,+b ac*whuit-up with +cu gu70o9n .5x ximl;r1 azf:9sfe bvyour hands behind your head than when your arms are stretched out in front mgl5sfz;. +1uiv 9xu7 f o9exbar0c:n of you? A diagram may help you to answer this.

参考答案:

When you do a sit-up, torque from your abdomen muscles must rotate the upper half of the body from a laying-down position to a sitting-up position.  The larger the moment of inertia of the upper half of the body, the more torque is needed, and thus the harder the sit-up is to do.  With the hands behind the head, the moment of inertia of the upper half of the body is larger than with the hands outstretched in front.

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