Final answer:
Stability around the longitudinal axis of a body is dependent on the center of gravity and the base of support. Lowering the center of gravity or widening the base can increase stability. Skeletal structure, including the axial skeleton and ligaments like the anterior longitudinal ligament, also contributes to stability in rotational movements.
Step-by-step explanation:
Stability about the longitudinal axis of a body, such as a human or an object like a pencil, is largely determined by the center of gravity and the base of support. The center of gravity is the point where the weight of the body or object is evenly distributed in all directions. When the center of gravity is high above the base of support, as in an adult standing with feet close together, the stability is decreased because small displacements can result in toppling. Moving the center of gravity closer to the base, by spreading the feet apart or bending over to lower the body's center of mass, increases stability.
In terms of a round pencil lying on its side, it is in neutral equilibrium relative to displacements perpendicular to its length, meaning it is neither stable nor unstable, because it can roll without a particular trend towards a new position. However, when it comes to displacements parallel to its length, stability depends on if it is being displaced along a flat surface or if it is on an incline where gravity would pull it downwards.
Rotational movement is another form of stability where the bone rotates about its longitudinal axis. The anterior longitudinal ligament and skeletal structures like the axial skeleton provide essential support and stability for rotational motions.