Correct balance—at rest and in motion—is a vital factor in track and field events, and must often be achieved under difficult circumstances.
Balance and stability at rest. The first consideration for balance (or equilibrium, as it is called in mechanics) is that the resultant of all the forces acting on the object shall be zero.
Balanced objects, however, possess varying degrees of firmness or steadiness in their positions; their stability varies. We can see at once that it is easier to upset the brick towards the right in position (a) than in (b); to do this the Centre of Gravity, marked X, must pass over and beyond the far edges, the angle through which it must tilt to lose balance in (a) being considerably less than in (b); less work is performed, too, in raising the Centre of Gravity. And the required horizontal force, acting through the Centre of Gravity, is also less. But a heavier brick would be more stable in both positions; it would take more work and more force to topple it. It follows, therefore, that an object’s stability depends upon:
The area of its base
The height of its Centre of Gravity
The horizontal distance between the Centre of Gravity and the pivoting edge
Its weight Figs. 41 and 48 exemplify the application of these principles to athletics. To be steady on his marks the sprinter adopts a position where his base is long and fairly broad , with the Centre of Gravity above it. But for a quick, forceful getaway he crouches with his Centre of Gravity as high and as far forward as circumstances permit; his stability is not great.
On the other hand, an initial position for a standing put is much more stable in a forward direction. The base (which includes not only the feet, but also the intervening area) is wider and the Centre of Gravity (common to both man and shot) comparatively far away from the eventual pivot, the front foot. But a plumb-line dropped from his Centre of Gravity falls within the base, as with the sprinter.
The athlete is also balanced, but balanced precariously on one foot. To improve stability and avoid fouling he flexes the supporting leg and lowers his trunk, thus lowering his Centre of Gravity. Body mass outside the base does not upset balance unless it alters the position of the athlete’s Centre of Gravity.
The standing starting position is certainly more stable than the thrower’s but in comparison to a crouched stance the base is too narrow and short, and the Centre of Gravity too high—which accounts for the ‘rolling’ starts seen so often in distance running. In all these positions a heavier athlete, equal in all other respects, will have greater stability.
Although a balanced position is more difficult to attain the higher the Centre of Gravity, the correction of balance will be easier—because the speed of the Centre of Gravity will develop less quickly the greater its distance above the base. Thus, a knife is more easily balanced point downward, by moving the finger, than point upward. But for a stationary balanced position the point of the knife should be uppermost.
Balance in motion. Let us consider, for the present, balance in motion to mean that, as a whole, an object or athlete is moving without rotation.
Take the example of a stick is balanced upright in the palm of a hand. When the hand moves forward force is applied to the bottom of the stick, causing it to rotate backward. On a second attempt, however, the stick is tilted forward as the hand accelerates. The force of the stick’s weight, acting through the Centre of Gravity (now outside the base), tends to rotate it forward, and if the stick leans neither too much nor too little for the force applied by the hand it remains balanced. Thus, clockwise and counter-clockwise rotations cancel out —the second condition for equilibrium.
When force ceases to be exerted and the motion of stick and hand is therefore uniform, the stick must be upright again to stay in balance. Or, if its speed is such by now that air resistance is sufficiently strong, it must lean forward just enough to offset the force of the air.
When the hand applies a braking force, the tendency will be for the stick to fall forward. Now, to stay balanced, it must lean backward , and the greater the retarding force the greater this backward lean must be. Finally, to remain balanced when the hand is no longer moving, the stick must be upright again.
This simple example illustrates an important aspect of the interplay of forces in the maintenance of balance in motion. To take an example from athletics, where a runner’s final position in driving his body forward. The reaction to the force of his drive against the ground (here assumed to pass through his Centre of Gravity) is divided into its forward (Fl) and upward (F2) components, which tend, respectively, to rotate the athlete backward and forward. To run with balance (ignoring air resistance) the force turning him forward at this instant of the stride cycle must exactly counteract the force rotating backward. For balance, any alteration of one of these forces requires an adjustment in the other.
Again, at the start of a sprint the athlete’s effective horizontal driving force is much greater than in our previous illustration, for it is easier to push backward with full force against the ground; the tendency to rotate backward is therefore greater. Now, for balanced running the turning effect of the force of the upward component of drive is increased by working at a greater horizontal distance from the Centre of Gravity.