
Concept explainers
To Explain: It is easier to do sit-up with arms stretched out and tougher with hands behind your back.

Explanation of Solution
Introduction:
With hands held behind head, one increases rotational inertia.
The resistance to an angular rotation of the upper body is increased and the torque required to complete the sit-up is greater when arms are behind the head. In the lowered arm-front position, the centre of mass of the rotating parts of the body is closer to the axis, which makes it easier to do complete the sit-up as the rotational inertia has been reduced.
Conclusion:
It is difficult to do sit-ups with hands behind the head.
Chapter 8 Solutions
Physics: Principles with Applications
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