There’s a moment in every sprint start where it feels like the track gives you permission to move. You drive your foot down and back, and somehow you surge forward. In IB SEHS, that “somehow” is one of the simplest ideas in biomechanics, and one of the easiest to misuse in an exam: Newton’s Third Law.
Newton’s Third Law states that for every action force, there is an equal and opposite reaction force. The trick, especially for IB SEHS answers, is to show you understand that these forces act on different objects, and that direction matters just as much as size.
IB SEHS quick checklist: what you must say
Use this mini-checklist when writing about Newton’s Third Law in IB SEHS:
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State the law accurately (equal and opposite)
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Identify the two objects (e.g., foot and ground)
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Describe the action force direction (down/back, down, etc.)
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Link the reaction force to the outcome (propulsion, take-off, braking, injury risk)
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Use the term ground reaction force when the surface is the ground
If you want the syllabus-aligned version of the topic, start with the SEHS subject hub and then drill the same idea using the B.2 Forces, motion and movement Questionbank.
Newton’s Third Law in sport: action-reaction pairs that create movement
In sport, Newton’s Third Law shows up whenever an athlete interacts with something: the floor, starting blocks, a ball, a wall, or even water. In , a high-mark explanation usually names the pair clearly:
