How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: A source of 400 Hz moves at 20 m·s⁻¹ towards a stationary listener. v_sound = 340 m·s⁻¹. f_L?
Before any working — hunt first
Given
Numbers and units already in the sentence: 400 Hz · 20 m·s⁻¹ · 340 m·s⁻¹. Copy each one into a column. A name without a number is not given.
Missing
After you copy the given, the letter that still has no number is missing. The paper is pointing at: f_L. Circle that word — that is the missing variable.
Find
We are required to find: f_L. Write that as a symbol on the left, with a question mark, before you touch a formula.
Calculate
Write f_L = f_s v(v−vs) with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives f_L.
In other words — as if you have never seen this before
If you have never seen this before: f_L = fₛ · v(v − vₛ) when the source moves towards the listener. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
fₛ = 400 Hz, v = 340, vₛ = 20 towards the listener, v_L = 0.
3. Which method — and why this one?
f_L = fₛ · v(v − vₛ) when the source moves towards the listener.
4. Step-by-step working
01 / 05
Doppler
fL=fₛ v(v − vₛ).
fL=fs v(v−vs)Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 05
Substitute
fL=400×340(340−20)=425 Hz.
425 HzWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 05
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
04 / 05
Step F — Box the answer with its unit
425 Hz
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
05 / 05
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
425 Hz
Copy onto your answer sheet — in this order
Formula: fL=fs v(v−vs)
Answer: 425 Hz
6. Check it like a marker
Towards → higher pitch (denominator smaller). Away would be v + vₛ.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”