A labelled diagram and its explanation are one answer. Every label you draw should appear as a word in a sentence, and every sentence should point to a label.
This lesson is part of the SPM subjects and learning resources section. Practical work and hands-on assessment follow your school’s arrangements; here we practise the drawing and writing.
How do I turn a diagram into an explanation?
- Draw the simplest shape that shows the parts.
- Label each part and mark each force with an arrow and a letter.
- Mark the distance you will refer to.
- Write the explanation using your labels as words.
Worked example: a spanner and a bolt
Picture a spanner lying flat on a bolt head.
Label the bolt centre P (the pivot). Draw a force arrow F at the end of the handle, pointing downward and perpendicular to the handle. Mark the distance from P to F as d.
Question. Explain why a longer spanner makes a bolt easier to loosen.
Explanation using the labels. “The turning effect, or moment, about the pivot P equals force F multiplied by the distance d. If the same force F is applied at a larger distance d, the moment is larger. A longer spanner gives a larger d, so the same hand force produces a larger turning effect, which makes the bolt easier to loosen.”
A worked number shows the idea. With F = 40 N and d = 0.20 m, the moment is 40 × 0.20 = 8 N m. With the same force and d = 0.30 m, the moment is 40 × 0.30 = 12 N m.
The mistake that costs marks
The common slip is a diagram with no labels on the force or distance, followed by text that could be written without the drawing. The explanation then has nothing to point at.
| Feature | Decoration diagram | Working diagram |
|---|---|---|
| Labels | “spanner”, “bolt” | P, F, d marked |
| Force | Not shown | Arrow F, perpendicular to the handle |
| Distance | Not shown | d marked from P to F |
| Writing | General statement | “Moment = F × d, so larger d gives a larger moment” |
Check yourself
A seesaw has its pivot at the middle. A child pushes down at one end with a force. Describe what labels you would add and write one sentence explaining why the child sitting farther from the pivot can balance a heavier load closer to it.
Answer
Labels: pivot P, force F₁ at distance d₁ on the child’s side, force F₂ at distance d₂ on the load’s side.
Sentence: “The seesaw balances when the moments about P are equal, F₁ × d₁ = F₂ × d₂, so a smaller force at a larger distance can balance a larger force at a smaller distance.”
Number check: 200 N at 1.5 m gives 300 N m, which balances 300 N at 1.0 m.
What to study next
Next, connect the diagram to a calculation with units in check units in a conceptual calculation. Then use the Mechanical Engineering original practice set.
The structured answer self-review checks whether each label is used in your writing. A teacher can draw and explain with you in online one-to-one Mechanical Engineering tuition.