Centre of Gravity

Centre of Gravity

The centre of gravity (CG) is the point at which the whole weight — or mass — of a body may be taken to be concentrated. For a person standing in the anatomical position, the CG marks the intersection of the three primary planes and axes of the body. This is a cornerstone idea in the NCERT Physical Education Class 12 chapter on Biomechanics and Sports.

The human body's flexibility and fluidity make the CG hard to locate precisely: although the mass centre can be fixed for any one momentarily held stance, any real movement is accompanied by a shift in its position. In other words, in most sporting skills the mass centre is constantly moving.

Locating the centre of mass of a rigid object is far easier, and easier still if the object is of uniform density and regular (symmetrical) shape — in that case the CG lies at the exact geometric centre, and an object suspended from this point rests in rotational equilibrium.

The ability to balance — whether still or moving — is central to success in almost every sport, and an athlete's stability depends directly on the position of the CG. Examples:

• In the snatch and jerk, wrestlers widen the legs and lower the body to maintain stability, which brings the CG down.
• While running, the runner's CG sits low in the pelvis and slightly in front of the body, because the upper body leans forward — a low, forward CG is an advantage for acceleration.

Importance and Application of the Centre of Gravity in Sports

Control of the CG helps the athlete to:

(a) move the body; (b) stop a moving object; (c) accelerate; (d) throw objects; (e) lift objects; and (f) pull objects.

Examples of the Centre of Gravity in Action

• In basketball and volleyball, high-defence players spread their legs to lower the CG towards the base and take up a better position against the attacker. …