Biomechanical principles — AQA A-Level Physical Education

Biomechanical principles are fundamental to understanding how the human body moves and how forces interact during physical activity. This topic covers Newton's laws of motion, types of levers, force summation, and the factors affecting stability and centre of mass. It is essential for analysing and improving technique in any sport.

In the AQA A-Level PE specification, biomechanics is part of the 'Applied Physiology' unit and is often examined through both short-answer and extended-writing questions. Mastering these principles allows students to explain performance, suggest improvements, and apply theoretical knowledge to practical sporting contexts.

• →Newton's three laws of motion: inertia, acceleration (F = ma), and action-reaction, and their application to sporting movements.
• →Levers: first, second, and third-class levers in the body, with examples such as the triceps (first-class), calf raise (second-class), and biceps curl (third-class).
• →Force summation: the sequential summation of forces from large muscle groups to smaller ones to produce maximum force or speed.
• →Centre of mass and stability: how the position of the centre of mass, area of base of support, and line of gravity affect balance.
• →Projectile motion: factors affecting the flight path of objects, including angle, speed, and height of release.

• State Newton’s first law: a body remains at rest or moves with constant velocity unless acted upon by a resultant force.
• Apply Newton’s first law to sporting examples, such as a stationary ball remaining still until kicked.
• State Newton’s second law: the resultant force is proportional to the rate of change of momentum, often expressed as F = m × a.
• Apply Newton’s second law to calculate force, mass or acceleration in a sporting context.
• State Newton’s third law: for every action there is an equal and opposite reaction.
• Apply Newton’s third law to sporting examples, such as a sprinter pushing back on starting blocks to move forwards.
• First law (inertia): a body remains at rest or moves at constant velocity unless acted on by a resultant external force.
• Second law (acceleration): resultant force equals mass multiplied by acceleration, F = m × a, so acceleration is proportional to resultant force and inversely proportional to mass.
• Third law (action/reaction): for every action force there is an equal and opposite reaction force acting on a different body.
• Force is a push or pull measured in newtons (N) that can change a body's state of motion or shape.
• Applying the laws to sport: a lighter performer accelerates more for the same force (second law), and a swimmer's backward push on water produces a forward reaction (third law).
• A scalar is a quantity with magnitude only and no direction, whereas a vector has both magnitude and direction.
• Distance is the total length of the path travelled, measured in metres (m).
• Speed is the rate of change of distance, given by speed = distance ÷ time, measured in metres per second (m/s).
• Mass is the quantity of matter in a body, measured in kilograms (kg), and time is measured in seconds (s).
• Energy is a scalar measured in joules (J), and recognising which quantities are scalars helps select the correct equation and unit.
• Distance is a scalar quantity: it is the total length of the path travelled and has magnitude only.
• Distance is measured in metres (m).
• Speed is a scalar quantity: it is the rate of change of distance with respect to time.
• Speed is calculated using speed = distance ÷ time.
• The unit of speed is metres per second (m s⁻¹).
• Average speed uses total distance divided by total time; instantaneous speed is the speed at a given moment.
• Distance differs from displacement, which is a vector measured in a straight line from start to finish.
• Speed differs from velocity, which is a vector quantity.
• The centre of mass is the point at which the whole mass of a body or object is considered to be concentrated.
• It is the point about which a body balances in all directions.
• In a uniform symmetrical object, the centre of mass is at its geometric centre.
• In a standing human, the centre of mass lies around the lower abdomen and shifts as body parts move.
• Stability depends on the position of the centre of mass relative to the base of support.
• Stability is greater when the centre of mass is lower and the base of support is wider.
• A body topples when the line of gravity passes outside the base of support.
• 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.
• The height of the centre of mass is the vertical distance of the centre of mass above the base of support.
• A lower centre of mass increases stability because the body must be tilted through a greater angle before the line of gravity falls outside the base of support.
• The area of the base of support is the region enclosed by the contact points with the ground; a larger area increases the range of positions in which the line of gravity remains inside the base.
• The line of gravity acts vertically downwards through the centre of mass; balance is maintained while it falls within the base of support.
• Body mass determines inertia; a greater mass means a given disturbing impulse produces a smaller change in motion, so the performer is harder to displace.
• The four factors combine: for example, a gymnast on a beam uses a low centre of mass, a narrow but carefully aligned base, a controlled line of gravity and body mass to maintain balance.

• 💡In MCQs, match the key phrase in the question to the law: 'constant velocity' points to the first law, 'F = m × a' to the second, and 'equal and opposite' to the third.
• 💡When calculating with F = m × a, ensure mass is in kilograms and acceleration in m s⁻² to give force in newtons.
• 💡For action–reaction questions, identify the two different objects involved to avoid the misconception that forces cancel.
• 💡Read the stem carefully to identify which law is being tested before looking at the options, since inertia, acceleration and action/reaction are easily confused.
• 💡Check the units in each option: force is in newtons (N), mass in kilograms (kg) and acceleration in metres per second squared (m/s²).
• 💡Eliminate options that describe action and reaction forces as acting on the same body, because that contradicts the third law.
• 💡When a calculation is embedded, write F = m × a and substitute values before choosing, rather than estimating.
• 💡Identify whether the quantity in the stem is a scalar or a vector before selecting an option, since direction is the deciding feature.
• 💡Match each scalar to its unit: distance in metres (m), speed in metres per second (m/s), mass in kilograms (kg), time in seconds (s) and energy in joules (J).
• 💡For any calculation, write the equation first, such as speed = distance ÷ time, then substitute the values with units.
• 💡Check that your answer's unit matches the quantity asked for, because a correct number with the wrong unit is still wrong.
• 💡Check whether the question asks for distance, displacement, speed or velocity before selecting an answer, because scalar and vector quantities are often offered as distractors.
• 💡For calculation items, write the equation speed = distance ÷ time, substitute the values with units, then state the answer in m s⁻¹.
• 💡If a graph or split times are given, remember that average speed uses total distance ÷ total time, not the speed of the fastest section.
• 💡When asked about stability, refer to both the height of the centre of mass and the width of the base of support, because these are the two key factors.
• 💡Use a sporting example such as a gymnast's tuck or a rugby player's crouched stance to show how the centre of mass shifts.
• 💡Check whether the question asks about the centre of mass itself or about the line of gravity, because the two are related but not identical.
• 💡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.
• 💡For each option, decide which of the four factors it changes and whether that change increases or decreases stability.
• 💡Sketch a simple stick figure with the base and the line of gravity to check whether an option keeps the line inside the base.
• 💡Eliminate options that describe a factor not mentioned in the stem, as these are common distractors.
• 💡Always use specific sporting examples to illustrate biomechanical principles; generic answers without context rarely gain full marks.
• 💡When drawing or describing levers, label the fulcrum, load, and effort clearly and state the class of lever.
• 💡For calculation questions, show all working and include units; marks are awarded for correct substitution and final answer with units.

• Thinking that Newton’s first law means a moving object always needs a continuous force to keep moving; in fact, constant velocity requires no resultant force.
• Confusing Newton’s second and third laws; the second law relates force, mass and acceleration, while the third law describes action–reaction pairs.
• Believing that action and reaction forces cancel out on the same object; they act on different objects.
• Forgetting that Newton’s second law involves the resultant force, not just any single force.
• Thinking the first law means a moving object always needs a continuous force to keep moving; the correction is that constant velocity needs no resultant force, only a change in motion does.
• Believing the third law action and reaction forces cancel on the same body; the correction is that they act on two different bodies, so they do not cancel each other's effect on either one.
• Confusing mass with force or using the wrong unit; the correction is that mass is measured in kilograms (kg) and force in newtons (N), and F = m × a links them.
• Assuming a heavier performer always accelerates faster because they can push harder; the correction is that for the same resultant force a larger mass gives smaller acceleration.
• Treating speed as a vector; the correction is that speed is a scalar with magnitude only, while velocity is the vector that also states direction.
• Using the wrong unit for a scalar, such as giving speed in metres; the correction is that speed is measured in metres per second (m/s).
• Confusing distance with displacement; the correction is that distance is a scalar path length in metres, while displacement is a vector with direction.
• Rearranging speed = distance ÷ time incorrectly, for example multiplying distance by time; the correction is to divide distance by time to find speed.
• Treating distance and displacement as the same quantity. Correction: distance is the total path length (scalar), whereas displacement is the straight-line distance from start to finish with direction (vector).
• Confusing speed with velocity. Correction: speed is scalar and has magnitude only, whereas velocity is a vector and includes direction.
• Using the wrong unit for speed, such as m or m s. Correction: speed is measured in metres per second (m s⁻¹).
• Dividing time by distance instead of distance by time. Correction: speed = distance ÷ time, so a 100 m swim in 80 s gives 100 ÷ 80 = 1.25 m s⁻¹.
• Assuming the centre of mass is always at the body's physical centre. Correction: in a human it lies around the lower abdomen when standing and shifts when limbs or the trunk move.
• Confusing the centre of mass with the base of support. Correction: the centre of mass is a point related to mass distribution, whereas the base of support is the area beneath the body in contact with the ground.
• Believing a lower centre of mass always reduces stability. Correction: a lower centre of mass generally increases stability, especially when combined with a wider base of support.
• Thinking a body topples only when it moves quickly. Correction: a body topples when the line of gravity falls outside the base of support, regardless of speed.
• Thinking that a higher centre of mass increases stability. Correction: 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. Correction: 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. Correction: 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. Correction: in a uniform gravitational field they coincide, and the line of gravity acts vertically through the centre of mass.
• Confusing the centre of mass with the geometric centre of the body. Correction: the centre of mass depends on the distribution of mass, so it shifts when limbs move or when equipment is carried.
• Thinking that the line of gravity must pass through the centre of the base. Correction: it only needs to fall anywhere within the area of the base of support for balance to be maintained.
• Believing that a larger body mass always improves stability. Correction: mass increases inertia and resistance to displacement, but a high centre of mass or a line of gravity outside the base can still cause toppling.
• Treating the base of support as only the feet. Correction: any body part in contact with the ground, such as a hand in a gymnastics support, contributes to the base of support.
• Students often think that a larger base of support always increases stability, but it must be combined with a lower centre of mass and the line of gravity within the base.
• Many believe that third-class levers are inefficient because they require more effort, but they are advantageous for speed and range of motion, which is why they are most common in the body.
• Some confuse mass and weight; mass is the amount of matter (kg) and weight is the force of gravity on that mass (N), which is crucial when applying Newton's second law.

1. 1Day 1-2: Review Newton's laws and create a summary table with definitions and sporting examples for each law.
2. 2Day 3-4: Learn the three classes of levers, draw diagrams for each, and identify examples in the body.
3. 3Day 5-6: Study force summation and stability, using flashcards to memorise key factors and their effects.
4. 4Day 7-8: Practice calculation questions on force, acceleration, and projectile motion, checking units and significant figures.
5. 5Day 9-10: Complete past paper questions on biomechanics, focusing on extended-writing questions and mark schemes.

• 📋Short-answer questions (2-4 marks) asking students to define terms or identify levers and forces in a given sporting action.
• 📋Calculation questions (3-5 marks) requiring the use of F = ma or equations of motion to find force, acceleration, or velocity.
• 📋Extended-writing questions (6-9 marks) where students must analyse a sporting technique using biomechanical principles and suggest improvements.
• 📋Data analysis questions where students interpret graphs or tables related to force, velocity, or stability and explain the implications for performance.