Physical science cheat sheet

Measurement

A measured value needs a unit and an indication of precision. Unit consistency helps reveal calculation errors.

Speed = distance ÷ time; metres ÷ seconds gives m/s.

Particle model

Solids, liquids and gases differ in particle arrangement and motion. State changes do not change the substance’s identity.

Melting ice is a physical change; forming a new substance is a chemical change.

Energy accounting

Track energy transfers between a defined system and its surroundings. Total energy is conserved in an isolated system.

Stored gravitational energy can become kinetic energy and thermal energy.

Models and evidence

A model simplifies a system. Its assumptions determine where predictions are useful.

A constant-acceleration model is unsuitable when acceleration changes substantially.

Scalar and vector quantities

Scalars have magnitude; vectors also have direction. Add vector components using a defined coordinate system rather than combining magnitudes indiscriminately.

Distance is a scalar; displacement is a vector.

Density

Density relates mass to volume: ρ = m/V. Compare values at stated conditions, since volume can change with temperature or pressure.

A 240 g sample occupying 80 cm³ has density 3 g/cm³.

Conservation in reactions

A balanced chemical equation preserves the number of atoms of each element. Coefficients describe relative amounts of whole chemical entities; subscripts belong to their formulae.

2H₂ + O₂ → 2H₂O balances hydrogen and oxygen atoms.

Wave relationships

For a periodic wave, speed equals frequency times wavelength. Frequency is measured in hertz; choose compatible units for wavelength and speed.

v = fλ; 5 Hz × 2 m = 10 m/s.

Worked example: acceleration

A cart’s velocity changes from 2 m/s to 14 m/s in 4 s along a straight line. Its average acceleration is (14 − 2)/4 = 3 m/s².

Average acceleration does not establish that the instantaneous acceleration was constant.

Common mistake: mass and weight

Mass measures inertia in kilograms. Weight is a gravitational force, approximately W = mg near Earth’s surface, and is measured in newtons.

A 2 kg mass has weight about 19.6 N when g = 9.8 m/s².

Check your understanding

What are the SI units of density?

kg/m³: mass in kilograms divided by volume in cubic metres.

Curriculum connections

Sources