Which of the following readings cannot be determined with a meter rule?

Analysis

The question asks us to identify which of the given measurements cannot be accurately determined using a standard meter rule. To answer this, we must look at the precision (or the smallest scale division) of a meter rule.

Concept: Precision of a Meter Rule

A standard meter rule is graduated in centimeters (cm) and millimeters (mm).

• The smallest marking on a meter rule is 1 mm, which is equivalent to 0.1 cm.
• When using a meter rule, you can read the value to the nearest millimeter (0.1 cm) with certainty, and you can estimate the next decimal place (the hundredths place) as a fraction of a millimeter (e.g., 0.05 cm).
• However, a meter rule cannot reliably measure to two decimal places (the hundredths place) unless the value ends in .0 or .5 (representing 0 mm or half a millimeter). Any value requiring a precision beyond 0.5 mm is generally considered beyond the limit of a standard meter rule.

Step-by-Step Evaluation

Let's look at the options provided:

• A. 2.56 cm: This requires measuring 0.06 cm (0.6 mm). A standard meter rule cannot accurately distinguish 0.6 mm; it can only reliably measure to the nearest 0.5 mm.
• B. 2.25 cm: This is a value ending in .25. While some might argue it's an estimation, in the context of physics examinations, readings like 2.50 cm or 2.05 cm are standard, but 2.56 cm is physically impossible to read on a standard scale.
• C. 2.50 cm: This is easily read as 2.5 cm on the scale.
• D. 2.05 cm: This is read as 2.0 cm plus a half-millimeter estimation, which is standard practice.

Comparing these, 2.56 cm is the most precise value listed that falls between the smallest readable increments of a meter rule.

Conclusion

This is because a meter rule is limited to a precision of 0.1 cm, and while we can estimate to 0.05 cm, we cannot reliably determine a value like 0.06 cm (the '6' in 2.56).

Exam Tip

Always remember the "Rule of Precision":

• Meter Rule: Precision is 0.1 cm (or 0.05 cm by estimation).
• Vernier Caliper: Precision is 0.01 cm.
• Micrometer Screw Gauge: Precision is 0.001 cm. If a question asks for a measurement that seems "too precise" for a meter rule, look for the option that ends in a digit other than 0 or 5 in the hundredths place!