To explain in Physics, say what changes, how the quantities are related, and why. A formula belongs in the answer only when it supports a sentence.
This page is for students who can solve calculations but freeze on “explain why” questions. It is one of the help pages for SPM Physics.
What does this sound like?
A student might say, “I wrote F = ma and d = v²/2a, because that is what I know, and I still lost the marks.” Or, “I understand it when I calculate, but I do not know what to write.”
The knowledge is there. It has not yet been turned into sentences that answer the question.
What usually causes it?
Three academic causes are common.
- Practice is mostly calculation, so explanation is rehearsed less.
- The “because” link is skipped, so relationships sit next to each other without being joined.
- Answers copy the words of a textbook definition, instead of speaking about the situation in the question.
A worked example
Question: explain why a heavier car needs a longer distance to stop from the same speed, when the braking force is the same.
A formula list: “KE = ½mv², W = Fd.” It has the right ingredients, but no answer.
An explanation: “The heavier car has more kinetic energy at the same speed, because KE = ½mv² increases with mass. The braking force must do work equal to that energy, and work is force times distance. With the same force, more energy needs a longer distance, so the stopping distance is larger.”
Numbers show it. With F = 5000 N and v = 20 m s⁻¹, a 1000 kg car stops in d = mv² ÷ 2F = 1000 × 400 ÷ 10 000 = 40 m. A 1500 kg car stops in 1500 × 400 ÷ 10 000 = 60 m. The distance scales with mass, as the explanation claimed.
What can you do on your own?
Use a three-part frame on every explain question.
| Part | What to write | Example |
|---|---|---|
| Claim | The result in words | The heavier car needs a longer distance |
| Relationship | How the quantities are linked | KE rises with mass, and work equals force times distance |
| Reason | Why the link gives the claim | Same force, more energy, so longer distance |
Take worked solutions you already have and add a “because” under each line. Read explaining the reason for a step for the habit behind it, and use the mistake log and paper error review to note which explain questions lost marks.
When might one-to-one tuition help?
Explanations are hard to mark yourself, because your own writing always looks complete to you. If you cannot tell whether an answer would earn marks, a second reader may help.
A one-to-one teacher can read your answer, point to the missing link, and ask you to rewrite it until the reason is stated. It is a bit like having your answer marked, with the chance to fix it straight away. See online one-to-one Physics tuition or the one-hour trial class (from RM50).
What to read next
If you also struggle to pick the right formula in the first place, read I know the Physics formula but cannot choose one.