How do drag, lift and the Magnus effect shape performance in air and water?
Fluid mechanics: the factors affecting drag, the Bernoulli principle and lift, and the Magnus effect on a spinning ball, applied to performance in air and water.
A focused answer to Eduqas A-Level PE on fluid mechanics: the factors that increase and reduce drag, the Bernoulli principle and how it generates lift, and the Magnus effect that makes a spinning ball swerve, dip or float, applied to cycling, swimming and ball sports.
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What this dot point is asking
Eduqas wants you to explain the factors affecting drag, the Bernoulli principle and how it generates lift, and the Magnus effect on a spinning ball, applied to sports in air and water.
Drag
The Bernoulli principle and lift
The Magnus effect
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20194 marksState four factors that affect the size of the drag force on a cyclist, and explain one way a cyclist reduces drag.Show worked answer →
A Component 1 fluid-mechanics question. One mark per factor (up to three) and one for a reduction method.
Drag increases with: the velocity of the cyclist (drag rises sharply with speed), the frontal cross-sectional area presented to the air, the shape (a streamlined shape produces less drag than a bluff one), and the roughness or smoothness of the surface (a rough surface increases drag). A cyclist reduces drag by adopting a low, crouched, aerodynamic position to cut the frontal area, wearing smooth tight clothing and a teardrop helmet to streamline the shape and surface, so they meet less air resistance at speed and can go faster for the same power.
A common dropped mark is giving fewer than the required number of factors; list velocity, frontal area, shape and surface.
Eduqas 20226 marksExplain how the Bernoulli principle generates lift, and how the Magnus effect makes a football struck with topspin dip and a ball with backspin float. Use the principle in your answer.Show worked answer →
A Component 1 lift-and-spin question. Markers reward the Bernoulli mechanism and the two spin cases.
Award marks for: the Bernoulli principle states that where a fluid moves faster, its pressure is lower. A shape that makes air travel faster over one surface than the other creates a pressure difference and a net force toward the low-pressure (faster) side, which is lift; a downward-tilted aerofoil (a racing-car spoiler or a discus angled correctly) uses this. The Magnus effect applies the same idea to a spinning ball. With topspin, the top of the ball moves against the airflow and the bottom moves with it, so air travels faster under the ball (lower pressure there); the net Magnus force is downward, adding to gravity, so the ball dips steeply. With backspin, the air travels faster over the top (lower pressure above), so the net force is upward, opposing gravity, and the ball floats and stays in the air longer. So topspin shortens flight (dip) and backspin lengthens it (float).
A top answer states "faster air, lower pressure" and correctly assigns the force direction for topspin (down) and backspin (up).
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Sources & how we know this
- Eduqas A Level Physical Education Specification — Eduqas (2016)