KCSE 2025 Physics P1 Q9 — Calibrating a Spring Balance (which apparatus?)
Published
The Question
“To calibrate a spring balance a student has a spring and a set of known masses. Name one other piece of apparatus needed to calibrate the spring.”
Understand what calibration requires
Hanging the known masses on the spring stretches it by different amounts. Calibration means turning those extensions into a marked scale, so for each load you must know exactly how far the pointer has moved down the spring.
Choose the instrument that measures the extension
The quantity you still cannot measure with just a spring and masses is length — the extension itself. A metre rule provides that measurement, reading off how far the pointer moves for each mass so the student can mark and graduate the calibrated scale on the balance.
Final Result
A metre rule. It measures the extension produced by each known mass so the corresponding force values can be marked onto a graduated scale on the spring balance.
Why this method works
Calibrating a spring balance depends on Hooke's law: within the elastic limit the extension of the spring is directly proportional to the load applied. The known masses supply a series of known forces, but a scale reads positions, so you must convert each force into a length along the spring. A metre rule is the instrument that measures those extensions, letting you pair each known force with a definite mark on the spring. Once several such marks are placed and evenly spaced (as Hooke's law guarantees), the spring becomes a working balance that reads unknown forces directly.
Because extension is proportional to force, equal steps in mass give equally spaced marks on the rule — confirming the scale is linear and correctly graduated.