Forces, density and pressure · revision map · A-Level 9702, Topic 4

TOPIC 4 · FORCES, DENSITY AND PRESSURE
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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FORCES &
PRESSURE4
Turning Effects & Moments

A force acting off the pivot tends to rotate a body.

moment = F × d
d = perpendicular distance
from pivot to the line of F
unit N m
  • A couple: two equal, opposite, parallel forces whose lines of action do not coincide.
  • Torque of a couple = F × d, where d is the perpendicular distance between the forces.
  • A couple gives a pure turning effect with no resultant force.
  • Centre of gravity: the single point where the whole weight is taken to act.
d F pivot moment = F d
Equilibrium of Forces

Balanced forces and balanced moments: no resultant force and no resultant torque.

  • Principle of moments: about any point, sum of clockwise moments = sum of anticlockwise moments.
  • Equilibrium needs both conditions to hold together.
  • Moments may be taken about any point; choosing the pivot removes an unknown.
  • Three coplanar forces in equilibrium form a closed vector triangle.
ΣF = 0 · Σ(moments) = 0 F1 F2 F3 three forces close up
Density & Pressure

Force per unit area; in a fluid it grows with depth.

ρ = m / V p = F / A
ρ density (kg m−³) · p pressure (Pa) · force normal to area A
Δp = ρ g Δh
  • A fluid column presses by its own weight.
  • Hydrostatic pressure rises in proportion to depth h.
depth h p grows with h
Upthrust & Archimedes

A pressure difference lifts a submerged body.

  • The bottom face is deeper, so it is pushed up harder than the top is pushed down.
  • Net upward force = upthrust = weight of fluid displaced (Archimedes' principle).
  • A body floats when upthrust balances weight.
F = ρ g V
ρ fluid density · V = volume of fluid displaced
fluid surface F W
BUILDS ON · Dynamics · IGCSE Forces → FORCES → LEADS TO · Energy · Deformation · Ideal gases
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