How do Newton's laws and the concepts of force and motion explain sporting movement?
Newton's three laws of motion applied to sport, the definitions of mass, weight, inertia, momentum, force, net force and centre of mass, and the use of free body diagrams and the impulse-momentum relationship.
A focused answer to AQA A-Level PE biomechanics on biomechanical principles, covering Newton's three laws of motion, mass, weight, inertia, momentum, force and net force, free body diagrams and the impulse-momentum relationship.
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What this dot point is asking
AQA wants you to define the key mechanical quantities, apply Newton's three laws of motion to sporting examples, interpret free body diagrams of the forces acting on a performer, and use the impulse-momentum relationship to explain how performers speed up and slow down.
Mechanical quantities
Newton's laws of motion
Free body diagrams
A free body diagram shows all the external forces acting on a performer as arrows drawn from the centre of mass. The four forces normally considered are weight (always acting vertically down from the centre of mass), the normal reaction (acting vertically up from the surface, equal and opposite to weight on a level surface), friction (acting parallel to the surface, opposing the tendency to slide) and air resistance (acting opposite to the direction of motion through the air). The length of each arrow represents the magnitude of the force and its direction the line of action. The net force is the vector sum of these arrows: where opposing forces are balanced the body has no acceleration in that direction, and where they are unbalanced the body accelerates in the direction of the net force. Examiners commonly ask candidates to draw a free body diagram for a sprinter at the moment of foot strike, a cyclist at constant speed, or a gymnast on a beam, and the marks depend on the arrows being correctly labelled, originating from the centre of mass, and proportional in length to the relative force sizes.
Impulse and momentum
For example, a 70 kg sprinter accelerating from rest to 9 m s has a change in momentum of kg m s, which must be supplied by the impulse from the track. AQA frequently presents an impulse graph (force against time) for a foot in contact with the ground: the area under the curve is the impulse. For a sprinter at the start, the net impulse is positive (a large forward area) to accelerate; at constant velocity the positive and negative areas are equal so momentum is unchanged; and during deceleration the net impulse is negative. Reading these graphs is a recurring exam skill.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA 75 kg rugby player accelerates from rest to a velocity of 8 m per second. Calculate the change in momentum, and explain how the player generates the impulse needed to achieve it.Show worked answer →
Worked calculation plus application. Momentum is . At rest . At 8 m per second, kg m per second, so the change in momentum is 600 kg m per second (1 mark formula, 1 mark answer and units). Application: by the impulse-momentum relationship , this change must be supplied by an impulse. The player drives backwards against the ground with a large force over the contact time of each stride, and by Newton's third law the ground returns an equal and opposite forward force (the net positive impulse), accelerating the player. Reward linking impulse to the change in momentum and to Newton's third law.
AQA 20183 marksExplain, using Newton's three laws of motion, what happens when a sprinter drives out of the starting blocks.Show worked answer →
One mark per law applied. First law (inertia): the sprinter is stationary and remains at rest until a net force acts. Second law (acceleration): the sprinter applies a force to the blocks, and the resulting acceleration is proportional to the net force and in its direction (), so a greater driving force produces greater forward acceleration. Third law (action-reaction): the sprinter pushes back and down on the blocks (action) and the blocks push the sprinter forwards and up with an equal and opposite force (reaction), driving them out. Markers reward each law named and correctly applied to the block start, not just stated in the abstract.
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Sources & how we know this
- AQA A-level Physical Education (7582) specification — AQA (2016)