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

[问答题]
Why is it more diffic 8x.nd-l,./mgpn z-u;ykdi;b o6btqj ult to do a sit-up with your hands behind your head72zv;g -:zzwdy ,pvgaf(e cj3 than when your arms are stretched out in front of you? A dive7:23( y p-cza,; vfggzjzwdagram 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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