Forces
Newton's three laws of motion
First law: An object remains at rest or moves at constant velocity unless acted upon by a resultant force. (Inertia — objects resist changes to their motion.)
Second law: Resultant force = mass × acceleration. F = ma (SI units: N = kg × m/s²). A larger force produces a larger acceleration; a larger mass produces a smaller acceleration for the same force.
Third law: When object A exerts a force on object B, object B exerts an equal and opposite force on object A. These forces act on different objects and are of the same type. Example: a book rests on a table — the book's weight pushes down on the table (gravitational force); the table pushes up on the book with an equal normal contact force. CCEA questions often ask you to identify the Newton's 3rd law pair.
Weight and mass
Weight (W) is the gravitational force on a mass: W = mg where g = 9.8 N/kg (= 9.8 m/s² on Earth). Mass is constant; weight depends on the gravitational field strength.
Free body diagrams
A free body diagram shows all forces acting on a single object as arrows. Correctly label:
- Weight (down, from centre of gravity)
- Normal contact force / reaction (perpendicular to surface)
- Friction (opposing motion)
- Air resistance / drag (opposing motion)
- Tension (along string/rope)
- Applied force
Resultant force = vector sum of all forces. If resultant = 0, the object is in equilibrium.
Terminal velocity (extended)
As an object falls: weight acts down; drag acts up. Initially weight > drag → object accelerates. As speed increases, drag increases. Eventually drag = weight → resultant force = 0 → constant velocity (terminal velocity). A v-t graph shows a curve that levels off at terminal velocity.
Momentum
Momentum (p) = mass × velocity: p = mv (units: kg m/s). Momentum is a vector.
Conservation of momentum: In a closed system (no external forces), total momentum before = total momentum after. p_before = p_after → m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Impulse: Force × time = change in momentum: F × t = Δ(mv) = mv − mu. Crash safety features (crumple zones, air bags) increase collision time → reduce force for same momentum change.
Stopping distances
Stopping distance = thinking distance + braking distance.
- Thinking distance: distance covered during reaction time (typically 0.7 s). Increased by alcohol, drugs, tiredness, distraction, higher speed.
- Braking distance: distance to stop after brakes applied. Depends on speed (quadratic — doubles speed → 4× braking distance), road conditions, tyre condition, brake condition, vehicle mass.
CCEA Higher questions sometimes ask you to calculate stopping distances using v² = u² + 2as.
⚠Common mistakes
- Confusing mass and weight — mass in kg, weight in N. Never say "weight = 70 kg."
- Third law pairs acting on the same object — they always act on different objects.
- Forgetting units for momentum — must be kg m/s.
- Not conserving momentum with sign conventions — choose positive direction and be consistent.
AI-generated · claude-opus-4-7 · v3-ccea-physics