Physics 5054/12 — October/November 2016

Walkthrough

The question asks us to select the most suitable instruments to measure two different dimensions of a copper wire: its length (30 m) and its diameter (0.50 mm).

Step 1 — the length (30 m). A rule is typically 30 cm or 1 m long. To measure a 30 m length of wire with a rule would require at least 30 separate measurements, each introducing its own error. A tape measure, in contrast, can span the full 30 m in one go, making it the more suitable and accurate choice. This eliminates options A and B.

Step 2 — the diameter (0.50 mm). This is a very small dimension — half a millimetre. A micrometer measures to the nearest 0.01 mm, which is precise enough to measure 0.50 mm accurately. Calipers typically measure to the nearest 0.1 mm, which is only five times smaller than the diameter itself — not precise enough for an accurate reading. This eliminates option C.

Therefore, the correct answer is D: tape for length and micrometer for diameter.

Key Takeaways

• Choose the measuring instrument appropriate to the size of the quantity being measured.
• A rule is for small lengths up to about 1 m; a tape measure is for longer lengths.
• A micrometer is for very small dimensions (fractions of a millimetre); calipers for dimensions of a few millimetres to a few centimetres.

Common Mistakes

• Choosing calipers for the diameter because they are more common or easier to use — but for 0.50 mm, the micrometer is the only instrument with sufficient precision.
• Assuming a rule can measure 30 m because it can be used repeatedly — each additional measurement adds error.

Things to Be Careful About

• Match the precision of the instrument to the size of the measurement.
• A tape measure is used for long lengths, not just for tailoring.
• The question asks for the instruments that measure "accurately" — this is the key word that points to the micrometer over the calipers for the diameter.