All objects with mass have a center of gravity. In human movement, such as gymnastics, balance is achieved around this point. The position of the center of gravity within the body, relative to the apparatus, will determine which moves can be successfully completed.
The Force of Gravity
Gravity is the primary external force affecting a gymnast’s movement. It is the force of attraction of the planet Earth which pulls the mass of the human body toward it. The internal force of energy supplied to muscles allows us to overcome the force of gravity and get out of bed in the morning.
The Gymnast’s First Law of Motion
This is based on Isaac Newton’s First Law of Motion referring to rigid bodies with mass: The First Law of Motion states: "A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force." This can be defined as inertia.
A rigid body is a solid object which resists being deformed into a different shape. As a gymnast is a flexible, living body, many other parameters also apply, including motion directed by an internal force.
The Rigid Body
A hard, smooth sphere, like a billiard ball, has its center of gravity at the mid-point of the sphere. A cube with sides of equal length also has the center of gravity at the mid-point if it is balanced on one corner. If a cube is balanced on one of its sides, then the center of gravity is a plane through the center of the cube and parallel to the side it is resting on. This assumes that the composition of the material making up its mass is consistent through the object.
A hard cylindrical rod also has the center of gravity at the mid point. However, the distance from the ends of the rod to the center and the radius in cross section may be quite different. Stand a long, thin rod upright on one end and it is more likely to fall over than a short, fat cylinder, when an external force is applied.
Center of Gravity and Gymnastic Skills
All gymnastic skills are actions around the center of gravity of an individual gymnast’s body. Yet there is another sense of balance which is critical for gymnastic skills: the balance between external and internal forces. When these are applied efficiently, then particular skills can be more easily achieved.
The bottom line is this: the greater the mass, the greater is the force needed to get the body moving.
The exception is when it is falling. Gravity is a force which acts on all matter and is a function of both mass and distance.
Balance and the forces affecting it will be discussed in more detail in following articles.