Understand the Difficulty of Torque Situps: Expert Response

[hshphyss]

Can anyone help me with this question?

Why is it more difficult to do a sit-up with your hands behind your head than with your arms stretched out in front of you?

 

[Jordan_]

Can anyone help me with this question?
Why is it more difficult to do a sit-up with your hands behind your head than with your arms stretched out in front of you?

I would guess it is because with your arms over your head, they are further from your bodies center of mass, meaning it would take more energy to sit up.

 

[Astronuc]

As Jordan_ indicated, with the arms extended forward, the moment of intertia is changed (decreased) as compared to putting hands behind one's head. Putting the hands behind the head moves the mass away from the axis of rotation, the hips.