Question

Short answer

It is difficult to do sit-ups when your knees are bent because less opposing torque is produced by the lower half of the body.

Step-by-step solution

Understanding torque exerted on the body

In this problem, one of the essential points is that during the sit-up, the abdomen muscle will exert a torque to rotate the body upward, away from the ground.

Explaining the difficulty in doing sit-ups when your knees are bent than when your legs are stretched out

While doing a sit-up, the upper half of the body feels the force of gravity which pulls the body down toward the ground.

So, the lower half of the body applies some torque that counters the torque produced by the force of gravity on the upper half body, making sit-ups slightly easier.

If the legs are bent during sit-ups, the lever arm for the lower part of the body is short. So, the opposing torque will reduce, and this will make sit-ups difficult.