Dynamics · revision map · A-Level 9702, Topic 3

TOPIC 3 · DYNAMICS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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DYNAMICSF = ma
Newton's Laws & F = ma

Force is what changes a body's motion.

  • First law: with no resultant force, velocity stays constant.
  • Mass m is the resistance to a change in motion (inertia).
  • Second law: acceleration a is along the resultant force F.
  • Weight is gravity acting on mass.
F = ma W = mg
m F W = mg a a points along the resultant force F
Momentum: the Deeper Second Law

Force is the rate of change of momentum.

  • Linear momentum p = mv is a vector, in kg m s−¹.
  • F = Δp/Δt reduces to F = ma when m is constant.
  • Third law: forces act in equal and opposite pairs.
  • A longer contact time gives a smaller force for the same Δp.
p = mv F = Δp / Δt
A B F on B F on A third-law pair: equal in size, opposite direction
Non-uniform Motion & Terminal Velocity

Resistive forces grow with speed.

  • Friction, viscous drag and air resistance oppose motion.
  • Falling: at first W > drag, so the body accelerates.
  • As v rises, drag rises until drag = weight.
  • Then the resultant force is zero, so a = 0: terminal velocity.
at terminal v: drag = mg, a = 0
v t terminal v
Conservation of Momentum

Total momentum is fixed when there is no external force.

  • For an isolated system: Σp before = Σp after.
  • Resolve and conserve momentum in 1D and 2D.
  • Elastic: kinetic energy is also conserved (approach speed = separation speed).
  • Inelastic: some kinetic energy becomes other forms.
m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂ before after
BUILDS ON · Quantities & Units · Kinematics → DYNAMICS → LEADS TO · Forces · Energy · Circular Motion
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