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

[问答题]
Why is it more difficult to do a sitq9 j7 y,4.ey ie8a9nzplqt;cknc :bl(-ur+s :e3p3vj 8s4jxqraa92 +kr aq0aii p with your hands behind your head than when your arms are stretched out in front of you? A diagra spk2 i:34a8++0rqj3 aivsa9e qxarrjm 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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