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

[问答题]
Why is it more difficult to do a sit,ub 3:b ogr-ztz/9o 9ru3zro) oht k6i-up with your hands be cr+2,e(ytbft+)ap ocg7w*fi+vz yz. ae80 d mhind your head than when your arms are stretched out in front of you? A diagram may help you to answmvozb+(ipag+.ae e0d r*z y+2fcw,tc8 t7 )fyer 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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