Physical Quantities, SI Units and Vectors
Using SI base and derived units, prefixes, dimensions, estimates, orders of magnitude and vector resolution to express physical relationships precisely.
How to study A-level Physics
Define the system, represent the physics, select a justified model, calculate transparently, then test the result against units, limits and evidence.
Core concepts
Concept 1
A physical quantity is reported as a numerical value with a unit; equations must be dimensionally homogeneous.
Exam cue: Convert every value to a consistent unit system before substitution.
Concept 2
Scalars have magnitude, while vectors also require direction and combine according to vector rules.
Exam cue: Draw and label a vector diagram before resolving components.
Concept 3
Estimation and order-of-magnitude checks expose unit errors and physically impossible answers.
Exam cue: Check dimensions, sign and order of magnitude after calculating.
Risk pitfalls and guardrails
Treating a prefix as though it applies linearly when an area or volume is converted.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Adding vector magnitudes without considering direction.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Writing a number without its required unit or appropriate significant figures.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Memory anchors
Scalar
A scalar has magnitude only.
Vector
A vector has magnitude and direction.
Homogeneous Equation
Every term in a valid physical equation has the same dimensions.
Prefix Check
Apply the prefix conversion to every power of the unit.
Estimate First
An order-of-magnitude estimate gives a scale against which to test the final result.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
Which expression gives the newton in SI base units?
A plate has area 2.5 cm². What is its area in m²?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
What is Pass Harbor?
Completely free exam prep for 247 UK exams.
- Practice questions
- Flashcards
- Study guides
- Mock exams
- No registration
- No paywall
- Start instantly
“No more expensive exam prep. Quality study tools should be accessible to everyone.”