Where does the center of gravity lie, when a boy is doing sit-ups? Does weight vector pass through the base or move away from the base? Explain.

The center of gravity of an object is the point you can suspend the object from without there being any rotation because of the force of gravity .when a boy is doing sit-ups the center of gravity move up and down, thus it is not fixed it keeps changing with the upward and downward movement of the boy. The figure below illustrates it.

The weight vector passes through the base but the move slightly with respect to the previous position.