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Cognito Newton's laws of motion
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Cognito Terminal velocity
Key facts & equations
Newton's three laws 1st Law : an object stays at rest or constant velocity unless acted on by a resultant force. (Inertia.)2nd Law : resultant force = mass × acceleration. F = ma. Units: F in N, m in kg, a in m/s².3rd Law : for every action there is an equal and opposite reaction. Forces act on DIFFERENT objects.3rd Law pairs: same type, same size, opposite directions, different objects. E.g. rocket expels gas backwards (action) — gas pushes rocket forwards (reaction). Weight and mass Weight = mass × gravitational field strength. W = mg. Units: N.g on Earth ≈ 9.8 N/kg (use 10 N/kg for estimates). Weight is a force (vector). Mass is a scalar (does not change with location). On Moon: g ≈ 1.6 N/kg → weight is ~1/6 of Earth weight. Mass unchanged. Normal force: reaction from surface, perpendicular to surface. F = ma calculations F = ma → a = F/m → m = F/a. Resultant force: the vector sum of all forces. Use free body diagrams. If resultant force = 0: constant velocity (or at rest). If F ≠ 0: acceleration. Example: car mass 1000 kg, engine force 3000 N, friction 1000 N. Resultant F = 3000 - 1000 = 2000 N. a = F/m = 2000/1000 = 2 m/s². Terminal velocity Object falling through fluid: weight acts down, drag acts up. Initially: weight > drag → accelerates. As speed increases: drag increases. Eventually: weight = drag → zero resultant force → terminal velocity (constant). Skydiver: reaches terminal velocity ~55 m/s before parachute opens. On opening parachute: drag suddenly >> weight → decelerates to new lower terminal velocity. Exam questions — 4 questions · 10 marks · AQA 8463 style
⚡ Show all working — the AI marks against the AQA mark scheme. Method marks awarded for correct working even if the final answer is wrong.
1 mark Newton 2nd law AQA 8463 style
A resultant force of 800 N acts on a 200 kg object. What is the acceleration?
A 160,000 m/s²
B 0.25 m/s²
C 4 m/s²
D 600 m/s²
3 marks Force calculation AQA 8463 P1 style 🔢 Calculator
A 1500 kg car accelerates at 3 m/s². There is a friction force of 500 N. Calculate the engine driving force. (3 marks)
Hint: Step 1: F_resultant = ma. Step 2: F_resultant = F_drive - F_friction. Rearrange to find F_drive.
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1 mark Terminal velocity AQA 8463 style
A skydiver reaches terminal velocity. Which statement is correct?
A The skydiver is accelerating upwards
B Weight is greater than air resistance
C Air resistance equals weight — zero resultant force — constant velocity
D Air resistance is zero
4 marks Newton 3rd law pairs AQA 8463 style
A book rests on a table. (a) State the weight of the book. (b) Identify the Newton's 3rd law pair to the weight of the book. (c) Is the normal force from the table a Newton's 3rd law pair to the book's weight? Explain. (3 marks)
Hint: 3rd law pairs: same type, same magnitude, opposite direction, DIFFERENT objects. Weight pairs with the book's pull on Earth — not with the normal force.
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