Forces, Newton's Laws, Equilibrium and Momentum
Using free-body diagrams, Newton's laws, moments, equilibrium, impulse and momentum conservation to analyse interactions.
How to study A-level Physics
Define the system, represent the physics, select a justified model, calculate transparently, then test the result against units, limits and evidence.
Core concepts
Concept 1
The resultant external force determines the rate of change of momentum; for constant mass this becomes F = ma.
Exam cue: Define the object or system and draw only forces acting on it.
Concept 2
Translational equilibrium requires zero resultant force and rotational equilibrium requires zero resultant moment.
Exam cue: Choose a moment point that removes unknown forces when useful.
Concept 3
Momentum is conserved for a defined system when the resultant external impulse is negligible.
Exam cue: For collisions, write total momentum before and after with a consistent sign convention.
Risk pitfalls and guardrails
Placing an action-reaction pair on the same free-body diagram.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Assuming momentum conservation means kinetic energy is conserved.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Using mass rather than weight in a force balance.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Memory anchors
Newton I
With zero resultant force, velocity remains constant.
Newton II
Resultant force equals rate of change of momentum; F = ma for constant mass.
Newton III
Interacting bodies exert equal-magnitude, opposite-direction forces of the same type on each other.
Moment
Moment equals force times perpendicular distance from the pivot.
Impulse
Impulse equals change in momentum and the area under a force-time graph.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Which statement is Newton's first law?
What resultant force accelerates a 1200 kg car at 2.5 m s⁻²?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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