AS Daily A Level Physics question
In a lab to determine the Young modulus of a polymer fishing line, students hang 1–6 N and measure extension with a metre rule marked to 1 mm. The original length is 1.0 m. The largest extension they record is about 3 mm, so the percentage uncertainty is dominated by reading the extension. They can change one aspect for a repeat but must keep the material safely in the elastic region. Which change will most reduce the percentage uncertainty in the calculated Young modulus?
Answer
The correct answer is A.
Correct: A — Double the original length of the line while keeping the same load range. A longer sample gives larger extensions for the same loads, so with the same 1 mm reading resolution the fractional (percentage) uncertainty in extension — and hence in the calculated modulus — is reduced without increasing the risk of plastic deformation. A is correct because it increases signal without increasing load; B halves the extension for the same total load (two lines in parallel), so the fractional uncertainty in extension gets worse; C targets diameter even though the stem states extension readings dominate the uncertainty, so it brings little overall improvement; D increases extension but also increases the risk of leaving the elastic region for a polymer, which violates the requirement to keep the test safely elastic.