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

[问答题]
Why is it more difficulqs2ob(cj 2 /5cats wj.t to do a sit-up with your hands beh;c-543 1a/f eltcggjplfpz3hind your head than when your arms are stretched out in front of you? A diagram may helh-ef 54z/g1alg cc3p;3jplftp 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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