Forces, Movement & Changing Shape
Forces & Motion · 1 question type
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.
What terminal velocity is
- Terminal velocity is the largest steady speed a falling object reaches once the air pushing up on it has grown enough to balance its weight
- At terminal velocity:
- the upward push from matches the downward pull of exactly in size
- the resultant force is zero
- the object stops accelerating and falls at a constant
The four stages of a falling object
- Stage 1, the moment of release. Only weight acts (air resistance is zero because there is no relative motion through the air). The resultant force is the full weight pulling downwards, so the object accelerates downwards at the acceleration of free fall, g
- Stage 2, early fall. The object now has a small downward speed, so air particles begin to collide with it and a small upward air resistance appears. Weight still wins, so the object continues to accelerate downwards, but at a slightly reduced rate
- Stage 3, as the speed climbs, air resistance climbs with it (more air particles hit per second, and harder). The resultant force shrinks, so the acceleration shrinks. The object is still gaining speed, but more slowly
- Stage 4, the speed reaches the point where air resistance equals weight. Resultant force is now zero, acceleration is now zero, and the object falls at a constant
Explaining terminal velocity: "any two from" plus the compulsory mark
What comes up: a multi-mark "explain how a falling object reaches terminal velocity" question (often 3–5 marks), sometimes with a velocity-time or force-time graph to interpret.
Write (two marks from the list, plus one compulsory mark): From the list, pick any two: (1) reference both weight and drag by name; (2) weight is greater than drag initially, so there is a resultant force; (3) drag increases as speed increases. Then the compulsory additional mark: state that at terminal velocity, weight equals drag, so the resultant force is zero and the acceleration is zero.
Watch out: writing "F = ma" as a standalone statement without connecting it to the forces being balanced scores nothing for that mark point. The mark scheme accepts "gravitational force" for weight, and "water resistance" or "water friction" for drag in water contexts — but "gravity" alone on its own is rejected.