AQA A-level PE · Paper 2 · Biomechanical movement · Stability

Low & wide

Four factors decide how stable an athlete is: how low the centre of mass is, where the line of gravity falls in the base of support, how big the base is and how much mass they have. Change the stance and watch.

Centre of mass0cm high
Line of gravity0cm to nearest edge
Shove from in front0N to tip backwards
Shove from behind0N to tip forwards
Verdict– 

Model: shove needed to topple = weight × horizontal distance from the line of gravity to the edge of the base ÷ height of the centre of mass. You won't need this equation in the exam, but it shows why all four factors matter. Simplified: the body is treated as rigid, the shove acts at the height of the centre of mass and the feet don't slide.

centre of mass line of gravity base of support athlete faces right →
Mode:
Try:
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The exam idea, drawn live

Four factors. One verdict.

Each row is one of the four AQA factors for the athlete's current stance. Green helps stability, red reduces it. The strip shows where the line of gravity falls inside the base of support: the further it is from an edge, the bigger the shove needed to tip the athlete over that edge.

Height of centre of mass
Line of gravity in the base
Area of base of support
Mass of performer
Base of support (side-on)
← back edgecentrefront edge →
Shove from in front (tips backwards)
Shove from behind (tips forwards)
Centre of mass outside the body

The Fosbury flop

The centre of mass can move, and it can even lie outside the body. Arch the high jumper's back over the bar: the body is draped over the bar in a curve, so the centre of mass sits below the arch, outside the body. In a well-timed flop it can pass just below the bar while each body part goes over in turn.

The path of the centre of mass is fixed at take-off (a parabola), so the dashed path never moves. Arching the back doesn't raise the centre of mass; it changes where the body is around it, so the body can clear a bar that is higher than the centre of mass. Viewed from the end of the bar. Simplified: the body is treated as a uniform curved rod 1.8 m long and 16 cm thick.

Game

More stable or less stable?

Sort each sporting position. Some athletes want to be unstable on purpose. Think about the four factors before you choose.

Card 1 / 8Score 0
Sporting position
01 · AQA definitions

Learn these word for word

Centre of mass
The point at which a body is balanced in all directions. It is the point from which weight appears to act. It moves when the body changes shape and can lie outside the body.
Line of gravity
An imaginary line extending vertically downwards from the centre of mass.
Base of support
The area enclosed by the points of contact with the surface. For example, the area between and including the feet, or the hands and feet.
Stability
The ability of a body to resist being toppled or moved out of a balanced position. A stable athlete is hard to knock over; an unstable athlete can be moved easily.

Balanced or not? A body stays balanced while its line of gravity falls inside its base of support. Once the line of gravity moves outside the base, the body overbalances and topples. Push the Lean slider in the sandbox until it happens.

02 · The four factors

What makes an athlete stable

FactorMore stable when…Sporting example
Height of the centre of massThe centre of mass is lower.A rugby player lowers their centre of mass by bending their knees before a tackle.
Position of the line of gravityThe line of gravity is central within the base of support (further from the edge it would have to cross).A wrestler keeps the line of gravity central so they can resist a push from any direction.
Area of the base of supportThe base of support is larger: feet further apart or more points of contact.A headstand (three points of contact: head and two hands) is more stable than a handstand (two hands).
Mass of the performerThe performer has greater mass (more inertia, so a bigger force is needed to move them).Props in rugby and sumo wrestlers benefit from a large mass.

Unstable on purpose. A sprinter on "set" and a swimmer on the starting block move their line of gravity close to the front edge of their base of support. A tiny movement then takes the line of gravity outside the base, so they overbalance forwards and move off quickly.

03 · Applying it

How athletes use it

  • To resist force: lower the centre of mass, widen the base of support and keep the line of gravity central (rugby scrum, wrestling, judo, defensive stances).
  • To lean into a force: a scrummager moves the line of gravity forwards, away from the back edge of the base, so a much bigger shove is needed to drive them backwards.
  • To move quickly: raise the centre of mass or move the line of gravity to the edge of the base (sprint start, swim start, a goalkeeper shifting weight before a dive).
  • To clear a height: change body shape so the centre of mass passes close to or below the bar (Fosbury flop).
04 · Common slips

Where marks go missing

  • Write "lower the centre of mass", not "get lower" or "lower the body".
  • Say where the line of gravity is in relation to the base of support (central, near the edge, outside).
  • Don't confuse base of support with "feet": it is the whole area enclosed by the points of contact.
  • Greater mass increases stability. Don't write "weight" on its own when the question asks about the factor.
  • Being unstable is sometimes the aim. If the question is about a start, explain why instability helps.
05 · Exam practice

Try, then check

Question 1 · Define1 mark

Define the term centre of mass.

Show mark scheme
  • The point at which a body is balanced in all directions / the point from which weight appears to act
Question 2 · Identify4 marks

Identify four factors that affect the stability of a performer.

Show mark scheme
  • Height of the centre of mass (lower is more stable)
  • Position of the line of gravity in relation to the base of support (central is more stable)
  • Area of the base of support (larger is more stable)
  • Mass of the performer (greater mass is more stable)
Question 3 · Explain3 marks

Explain why a sprinter wants to be unstable in the "set" position.

Show mark scheme
  • The line of gravity is moved towards / close to the front edge of the base of support (over the hands)
  • When the hands are lifted at the gun, the line of gravity falls outside the base of support
  • So the sprinter overbalances forwards, giving a faster start / quicker movement off the blocks
Question 4 · Apply4 marks

A rugby player is about to be tackled. Explain how they can increase their stability.

Show mark scheme
  • Lower the centre of mass, e.g. by bending the knees / hips
  • Widen the base of support, e.g. by moving the feet further apart
  • Keep the line of gravity central within the base of support / lean into the tackle so the line of gravity moves away from the edge it would be pushed over
  • A greater mass makes them more stable / harder to move
Question 5 · Explain3 marks

Explain how a high jumper using the Fosbury flop can clear a bar that is higher than their centre of mass.

Show mark scheme
  • The jumper arches their back over the bar
  • This changes body shape so the centre of mass lies outside the body / below the arch
  • The centre of mass can pass below / close to the bar while each body part passes over it in turn, so less height (energy) is needed for the same bar
Question 6 · Explain6 marks

Using examples from sport, explain how performers change their stability to improve performance.

Show mark scheme
  • Stable when resisting force, e.g. scrum, tackle, wrestling, defensive stance
  • Lower the centre of mass to increase stability
  • Widen the base of support / more points of contact, e.g. a headstand rather than a handstand
  • Keep the line of gravity central within the base of support
  • Greater mass gives greater stability / more inertia, e.g. props, sumo
  • Unstable when moving quickly, e.g. sprint start / swim start: line of gravity moved to the edge of the base
  • Changing shape to move the centre of mass, e.g. the Fosbury flop
  • Max 6 marks. Examples must be linked to the correct factor to be credited.