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

[问答题]
Why is it more difficult to do a si znkkzvsw* nd5+ 6(/ 8-rty-a hm)rz(nzwn0f zt-uht2 i4nfa4v,d(j,xb3 tc 7un np with your hands behind your head than when your arms are stretched out in front of you? A di3u, fti4n(nd7,4ba nh jcv2t xagram 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.

本题详细解析: 暂无