Chapter 8, Problem 6Q

To determine

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.