P5 Forces — Section Overview
Forces is the largest section in AQA GCSE Physics. It covers everything from Newton's three laws to momentum, from the forces in space to pressure in fluids. Forces explain both the everyday world and extreme environments.
What this section covers
| Sub-topic | Key ideas |
|---|---|
| P5.1 Forces and their interactions | Contact vs non-contact; resultant force; free-body diagrams |
| P5.2 Work done and energy | W = Fs; energy transfers; power |
| P5.3 Forces and elasticity | Hooke's Law (F = ke); extension; elastic/inelastic deformation |
| P5.4 Forces and motion | v-t and s-t graphs; acceleration; Newton's laws |
| P5.5 Newton's laws | F = ma; weight = mg; action-reaction pairs |
| P5.6 Momentum | p = mv; conservation of momentum; impact forces |
| P5.7 Pressure | p = F/A; pressure in fluids; upthrust; atmospheric pressure |
Newton's three laws
- Inertia: An object stays at rest or moves at constant velocity unless a resultant force acts on it.
- F = ma: Resultant force = mass × acceleration. Force in newtons, mass in kg, acceleration in m/s².
- Action-reaction: Every force has an equal and opposite reaction force on a different object.
Motion graphs
- s-t graph: gradient = velocity. Horizontal line = stationary. Curve = changing velocity.
- v-t graph: gradient = acceleration. Area under = displacement. Horizontal = constant velocity.
Momentum and safety
Momentum p = mv (kg m/s). In a closed system, total momentum is conserved (Newton's 3rd law). Increasing the time of an impact (crumple zones, airbags) reduces the force: F = Δp/Δt.
Pressure in fluids
Pressure = force / area (Pa). In a liquid: p = ρgh. Upthrust equals weight of fluid displaced (Archimedes).
Exam focus
- Always draw a free-body diagram before applying Newton's 2nd law.
- For momentum questions, write p_before = p_after explicitly.
- Distinguish between scalar (speed, distance, mass) and vector (velocity, displacement, force, acceleration, momentum) quantities.
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