Physics 5054/11 — May/June 2011

Walkthrough

The plumber needs to make two measurements.

The internal diameter is small, about $2\text{ cm}$, and it has to be measured inside the pipe. A vernier calipers is designed to measure small internal and external diameters accurately, usually to $0.1\text{ mm}$, so it is the best choice for the diameter.

The length is much larger, about $80\text{ cm}$. A metre rule can measure a straight length of this size to the nearest millimetre, so it is accurate enough. A tape measure is more useful for longer distances or curved surfaces, and is not the best choice for an accurate straight length here.

Therefore, the best combination is vernier calipers for the internal diameter and a rule for the length, which is option C.

Key Takeaways

• Choose a measuring instrument according to the size and precision of the quantity.
• Vernier calipers are used for small internal diameters and small lengths.
• A metre rule is suitable for measuring lengths up to about $1\text{ m}$.
• A tape measure is usually better for longer or curved distances, not for accurate straight lengths.

Common Mistakes

• Choosing option A because a rule is used for both measurements, but a rule is not accurate enough for the small internal diameter.
• Choosing option B because a tape measure is used for the length, but the diameter is still measured with a rule.
• Choosing option D because the diameter is measured with vernier calipers, but a tape measure is less accurate for the straight length than a rule.

Things to Be Careful About

• The word "internal" means the measurement is inside the pipe, so the instrument must be able to measure an internal diameter.
• "As accurately as possible" means choose the instrument with the best precision, not just one that can measure the quantity.
• In written answers, always include units with measurements, even though this multiple-choice question only asks for the letter.