Chapter 8, Problem 4Q

Why is it more difficult to do a sit-up with your hands behind your head than when your arms are stretched out in front of you? A diagram may help you to answer this.

To determine

To analyze:

The reason why it is difficult to perform a sit-up when your hands are at the back of your head than when the arms are stretched out in front of you.

Answer to Problem 4Q

Solution:

To do a sit-up, you must rotate your upper body about an axis through your hips. The moment of inertia will be high when you compare with the arm behind your head and the arms stretched out. So, the torque required to rotate your upper body will be large.

Explanation of Solution

We have to work against the body’s moment of inertia when doing a sit-up. When our arms are stretched out in front of us, they are closer to the body’s center of gravity, reducing the body’s moment of inertia. When we move the hands behind our head, the moment of inertia increases.