Biomechanical principles: Factors affecting stability.

The idea

Stability describes how resistant a body is to being displaced from its balanced position, and it depends on four interacting mechanical factors: the height of the centre of mass, the area of the base of support, the position of the line of gravity relative to that base, and the body mass. A lower centre of mass reduces the turning effect of any disturbing force, so a crouched skier is harder to topple than an upright one. A larger base of support widens the range of positions in which the line of gravity still falls inside the base, so a wrestler spreads the feet. If the line of gravity passes outside the base, a restoring moment cannot be generated and the body rotates. Greater body mass increases inertia, so a heavier performer resists a given disturbing impulse more than a lighter one.

What earns the marks

• Stability is the resistance of a body to being displaced from its balanced position.
• Lowering the height of the centre of mass reduces the moment produced by a disturbing force about the edge of the base, increasing stability.
• Increasing the area of the base of support widens the range of positions in which the line of gravity remains within the base, increasing stability.
• Stability is greatest when the line of gravity falls within the base of support; if it passes outside the base, the body rotates and balance is lost.
• Greater body mass increases inertia, so a larger disturbing impulse is needed to produce the same change in motion, increasing resistance to displacement.
• The four factors interact: for example, a low centre of mass combined with a wide base gives more stability than either factor alone.

Common mistakes and how to put them right

• Thinking that a higher centre of mass increases stability.

  Put it right: Raising the centre of mass increases the moment arm of a disturbing force about the base edge, so stability decreases.

• Believing that body mass alone guarantees balance.

  Put it right: Mass increases inertia and resistance to a disturbing impulse, but if the line of gravity falls outside the base of support the performer still topples.

• Assuming a wide base always means maximum stability.

  Put it right: The base must be wide in the direction of the expected disturbing force, and the line of gravity must remain inside it.

• Confusing centre of mass with centre of gravity or treating them as unrelated.

  Put it right: In a uniform gravitational field they coincide, and the line of gravity acts vertically through the centre of mass.

Exam tips

• Read the stem carefully and identify which single factor is being changed before selecting an option; distractors often describe a different factor.
• For each option, ask whether it lowers the centre of mass, widens the base, keeps the line of gravity inside the base, or increases mass, and reject options that do the opposite.
• Use sporting examples such as a crouched start in sprinting or a wide stance in rugby to test each option against a concrete situation.