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HOW TO STUDY WITH THIS
Read the 💡 box the whole topic in 1 line
Study the diagram cover it, redraw it
Learn the yellow formulas & keywords
Redo the worked example without looking
THE CHAPTERS
📏
CH 1
Physical Quantities, Units & Measurement
SI units
🏎️
CH 2
Kinematics
speed
🧲
CH 3
Forces, Mass, Weight & Density
W = mg
🚀
CH 4
Dynamics
F = ma
⚖️
CH 5
Turning Effects of Forces
moments
⚡
CH 6
Energy, Work & Power
stores
🫧
CH 7
Kinetic Particle Model of Matter
states
🔥
CH 8
Thermal Processes
conduction
📌 NOTESchools split Sec 3 / Sec 4 physics differently — check the other year if a chapter is missing.
🖨 PRINTCtrl+P → Save as PDF · Scale 100% · Margins None · Background graphics ON
Sec 3 Combined Physics · SEC K326
📏
1
Physical Quantities, Units & Measurement
SI units, prefixes, instruments, scalars & vectors
BIG IDEAEvery physical quantity = number + unit. Use SI units and pick the instrument with the right range and precision.
Base quantity
SI unit
mass
kilogram (kg)
length
metre (m)
time
second (s)
current
ampere (A)
temperature
kelvin (K)
amount of substance
mole (mol)
Prefix
×
T tera
10¹²
G giga
10⁹
M mega
10⁶
k kilo
10³
c centi
10⁻²
m milli
10⁻³
µ micro
10⁻⁶
n nano
10⁻⁹
Instrument
Precision
Use for
metre rule
0.1 cm
lengths 10 cm–1 m
measuring tape
0.1 cm
long distances
stopwatch
0.01 s (but reaction ≈ 0.2 s)
time — time many oscillations
electronic balance
0.01 g
mass
Scalar vs vector
scalar: size only — distance, speed, mass, time, energy vector: size + direction — displacement, velocity, acceleration, force
📏 Orders of magnitude
atom ≈ 10⁻¹⁰ m · cell ≈ 10⁻⁵ m · human ≈ 1 m · Earth ≈ 10⁷ m
⚠ TRAPRead scales with your eye perpendicular to the scale to avoid parallax error; check for zero error.
🔎 WHY IT WORKSWhy time 20 swings, not 1: your reaction time error (~0.2 s) is the same whether you time 1 swing or 20. Spread over 20 swings, the error per swing becomes 20× smaller.
🧠 Worked example — follow the thinking
Convert 450 mA to A and 2.5 km to m.
Step
Why I do this
m = 10⁻³ → 450 × 10⁻³ = 0.45 A
💭 Replace the prefix with its power of 10.
k = 10³ → 2.5 × 10³ = 2500 m
💭 Same idea: kilo means × 1000.
Sec 3 Combined Physics · SEC K326
🏎️
2
Kinematics
speed, velocity, acceleration, motion graphs
BIG IDEAspeed = distance ÷ time · a = (v − u) ÷ t · gradient and area on graphs tell you everything.
distance–time: gradient = speed
speed–time: gradient = acceleration, area = distance
Graph shape
Distance–time
Speed–time
horizontal line
at rest
constant speed
straight slope up
constant speed
constant acceleration
curve getting steeper
speeding up
increasing acceleration
line going down
(moving back)
decelerating
Definitions
speed: distance per unit time (m/s) velocity: speed in a stated direction acceleration: change in velocity per unit time (m/s²)
🪂 Free fall
no air resistance → all objects fall with the same acceleration g ≈ 10 m/s², whatever their mass
🔎 WHY IT WORKSWhy area under v–t = distance: for a constant speed, distance = speed × time — that is the area of a rectangle under the line. Any shape can be split into rectangles and triangles, so the area always gives distance.
🧠 Worked example — follow the thinking
A car speeds up from rest to 20 m/s in 5 s, then moves steadily for 10 s. Find a and total distance.
Step
Why I do this
a = (20 − 0) ÷ 5 = 4 m/s²
💭 Gradient of the first line.
Triangle: ½ × 5 × 20 = 50 m
💭 Area under the accelerating part.
Rectangle: 10 × 20 = 200 m
💭 Area under the steady part.
Total = 250 m
💭 Add the areas.
Sec 3 Combined Physics · SEC K326
🧲
3
Forces, Mass, Weight & Density
W = mg, ρ = m/V, p = F/A
BIG IDEAW = mg · ρ = m ÷ V · p = F ÷ A. Mass never changes; weight depends on g.
Contact
Non-contact
friction, air resistance
gravitational
tension
electrostatic
normal (reaction) force
magnetic
Mass
Weight
Is
amount of matter
gravitational force on it
Unit
kg
N
Changes with place?
no
yes (g on Moon ≈ 1.6 N/kg)
🌍 Gravitational field
region where a mass feels a force · g = gravitational field strength = force per unit mass (N/kg) · Earth: g = 10 N/kg
⚖️ Density
ρ = m ÷ V (kg/m³ or g/cm³) irregular object: displacement in a measuring cylinder ρ object < ρ liquid → floats
📌 Pressure
p = F ÷ A, unit Pa = N/m² · small area → big pressure (knife, nail) · big area → small pressure (snowshoes, tank tracks)
⚠ TRAP1 g/cm³ = 1000 kg/m³. Convert cm² → m²: divide by 10 000, not 100.
🔎 WHY IT WORKSWhy weight changes but mass doesn't: mass counts the matter in you — that is the same everywhere. Weight is the pull of gravity, and the Moon pulls less (smaller g), so W = mg is smaller.
🧠 Worked example — follow the thinking
A 2 kg brick is 0.2 m × 0.1 m × 0.05 m. Find its density and the max pressure it exerts.
Step
Why I do this
V = 0.2 × 0.1 × 0.05 = 0.001 m³
💭 Volume of a cuboid.
ρ = 2 ÷ 0.001 = 2000 kg/m³
💭 ρ = m ÷ V.
W = 2 × 10 = 20 N
💭 Force on the floor = weight.
Max p uses the smallest face: 0.1 × 0.05 = 0.005 m²
💭 Smallest area → largest pressure.
p = 20 ÷ 0.005 = 4000 Pa
💭
Sec 3 Combined Physics · SEC K326
🚀
4
Dynamics
F = ma, free-body diagrams, friction
BIG IDEAF = ma — a resultant force makes a mass accelerate. Balanced forces → no change in motion.
free-body diagram: every force on ONE object
Newton's ideas (apply, no need to state)
1 no resultant force → stays at rest or constant velocity 2 F = ma (resultant force) 3 action–reaction: equal, opposite, act on different objects
reluctance to change motion — depends on mass · why seat belts are needed
⚠ TRAPAction–reaction pairs never cancel — they act on different bodies. Weight and normal force on a book are NOT a pair.
🔎 WHY IT WORKSWhy a heavier object needs more force for the same acceleration: a = F ÷ m. Doubling the mass halves the acceleration for the same force — more matter is harder to speed up.
🧠 Worked example — follow the thinking
A 1200 kg car: engine force 3000 N, friction 600 N. Find its acceleration.
Step
Why I do this
Resultant = 3000 − 600 = 2400 N
💭 F in F = ma is the RESULTANT force, so subtract opposing forces.
a = F ÷ m = 2400 ÷ 1200
💭 Rearrange F = ma.
a = 2 m/s²
💭 Forward, in the direction of the resultant.
Sec 3 Combined Physics · SEC K326
⚖️
5
Turning Effects of Forces
moments, principle of moments, centre of gravity
BIG IDEAmoment = F × d (d = perpendicular distance from pivot). Balanced: clockwise moments = anticlockwise moments.
principle of moments
Turning effect
unit: N m bigger force or longer distance → bigger moment door handle far from hinge · long spanner equilibrium: resultant force = 0 AND resultant moment = 0
🎯 Centre of gravity
point where the whole weight seems to act · find it for a flat lamina: hang from 2 points, draw plumb lines, CG where they cross
🔎 WHY IT WORKSWhy distance matters: pushing far from the pivot means the same force gives more turning. That is why you push a door at the handle, not near the hinge.
🧠 Worked example — follow the thinking
Find the unknown weight in the diagram.
Step
Why I do this
Anticlockwise moment = 40 × 1.5 = 60 N m
💭 Moment = F × perpendicular distance to the pivot.
Clockwise moment = W × 1.2
💭 Unknown on the right.
W × 1.2 = 60 → W = 50 N
💭 Principle of moments: they balance.
Sec 3 Combined Physics · SEC K326
⚡
6
Energy, Work & Power
stores, Ek, Ep, work, power
BIG IDEAEnergy is conserved: it only moves between stores. Ek = ½mv² · Ep = mgh · W = Fs · P = E ÷ t
Store
Example
kinetic
moving object
gravitational potential
raised object
elastic potential
stretched spring
chemical potential
fuel, food, cell
nuclear
uranium
internal
hot object (particles)
Transfer by
Example
mechanical work
force moves an object
electrical work
current in a circuit
heating
hot → cold
waves
light, sound
Work & power
work done = F × distance moved in direction of F (J) power = rate of energy transfer, W = J/s
Falling object
loss in Ep = gain in Ek (no air resistance) mgh = ½mv² → v = √(2gh) — independent of mass
🔎 WHY IT WORKSWhy mass cancels when things fall: both Ep and Ek are proportional to m, so a heavy and light ball dropped together reach the same speed (ignoring air resistance).
🧠 Worked example — follow the thinking
A 0.5 kg ball is dropped from 5 m. Find its speed just before hitting the ground.
Step
Why I do this
Ep lost = mgh = 0.5 × 10 × 5 = 25 J
💭 Energy at the top.
Ek gained = 25 J (conservation)
💭 No air resistance → all Ep becomes Ek.
½ × 0.5 × v² = 25 → v² = 100
💭 Ek = ½mv².
v = 10 m/s
💭
Sec 3 Combined Physics · SEC K326
🫧
7
Kinetic Particle Model of Matter
states, internal energy, changes of state
BIG IDEATemperature measures the average kinetic energy of particles. During a change of state, temperature stays constant.
Solid
Liquid
Gas
Arrangement
closely packed, regular
closely packed, irregular
far apart, random
Motion
vibrate about fixed positions
slide past each other
move fast, randomly
Forces
very strong
strong
very weak
Shape / volume
fixed / fixed
not fixed / fixed
not fixed / not fixed
heating curve: flat parts = change of state
Internal energy
= total kinetic + potential energy of all particles flat part: energy goes into breaking bonds (PE up), KE same → temperature constant gas pressure: particles collide with the walls
🔎 WHY IT WORKSWhy evaporation / sweating cools you: only the fastest particles escape the surface. The ones left have a lower average KE — and lower average KE means lower temperature.
🧠 Worked example — follow the thinking
Explain, using particles, why gas pressure rises when a sealed can is heated.
Step
Why I do this
Particles gain kinetic energy and move faster
💭 Temperature up = average KE up.
They hit the walls more often and harder
💭 More frequent, more forceful collisions.
Greater force per unit area → pressure increases
💭 Pressure = force ÷ area.
Sec 3 Combined Physics · SEC K326
🔥
8
Thermal Processes
conduction, convection, radiation
BIG IDEAThermal energy flows from hot to cold until thermal equilibrium: conduction, convection, radiation.
Process
How
Where
Conduction
particles vibrate and pass energy to neighbours; free electrons in metals carry it fast
solids
Convection
heated fluid expands → less dense → rises; cooler, denser fluid sinks → convection current
liquids, gases
Radiation
infrared electromagnetic waves — no medium needed
everywhere, even vacuum
Radiation depends on
colour & texture: dull black = best absorber and emitter; shiny silver = poorest temperature: hotter emits more surface area: bigger emits more
Applications
vacuum flask (vacuum + silvered walls) · aircon high, heater low · sea & land breezes · cooling fins painted black · copper-bottomed pots · double glazing
🔎 WHY IT WORKSWhy metals conduct so well: besides vibrating particles, metals have free electrons that move quickly through the whole metal, carrying energy far faster than vibrations alone.
🧠 Worked example — follow the thinking
Why is the freezer compartment at the top of an old fridge?
Step
Why I do this
Cold air at the top is denser
💭 Cooling makes air contract.
It sinks; warmer air rises to the top to be cooled
💭 Sets up a convection current.
Whole fridge is cooled by convection
💭 Placing the cooler low would trap cold air at the bottom only.